2134. Key Terms -
alpha ( $\alpha$ ) decay loss of an alpha particle during radioactive decay
alpha particle ( $\alpha$ or $\mathbf{2}_{\mathbf{2}}^{\mathbf{H}} \mathbf{H e}$ or ${ }_{\mathbf{2}}^{\mathbf{4}} \boldsymbol{\alpha}$ ) high-energy helium nucleus; a helium atom that has lost two electrons and contains two protons and two neutrons
antimatter particles with the same mass but opposite properties (such as charge) of ordinary particles
band of stability (also, belt of stability, zone of stability, or valley of stability) region of graph of number of protons versus number of neutrons containing stable (nonradioactive) nuclides
becquerel (Bq) SI unit for rate of radioactive decay; $1 \mathrm{~Bq}=1$ disintegration/s
beta ( $\boldsymbol{\beta}$ ) decay breakdown of a neutron into a proton, which remains in the nucleus, and an electron, which is emitted as a beta particle
beta particle $\left(\boldsymbol{\beta}\right.$ or ${ }_{-\mathbf{1}}^{\mathbf{0}} \mathbf{e}$ or $\left.\underset{-\mathbf{1}}{\mathbf{0}} \boldsymbol{\beta}\right)$ high-energy electron
binding energy per nucleon total binding energy for the nucleus divided by the number of nucleons in the nucleus
chain reaction repeated fission caused when the neutrons released in fission bombard other atoms
chemotherapy similar to internal radiation therapy, but chemical rather than radioactive substances are introduced into the body to kill cancer cells
containment system (also, shield) a three-part structure of materials that protects the exterior of a nuclear fission reactor and operating personnel from the high temperatures, pressures, and radiation levels inside the reactor
control rod material inserted into the fuel assembly that absorbs neutrons and can be raised or lowered to adjust the rate of a fission reaction
critical mass amount of fissionable material that will support a self-sustaining (nuclear fission) chain reaction
curie ( $\mathbf{C i}$ ) larger unit for rate of radioactive decay frequently used in medicine; $1 \mathrm{Ci}=3.7 \times 10^{10}$ disintegrations/s


2134. Key Terms -
curie ( $\mathbf{C i}$ ) larger unit for rate of radioactive decay frequently used in medicine; $1 \mathrm{Ci}=3.7 \times 10^{10}$ disintegrations/s
daughter nuclide nuclide produced by the radioactive decay of another nuclide; may be stable or may decay further
electron capture combination of a core electron with a proton to yield a neutron within the nucleus
electron volt (eV) measurement unit of nuclear binding energies, with 1 eV equaling the amount energy due to the moving an electron across an electric potential difference of 1 volt
external beam radiation therapy radiation delivered by a machine outside the body
fissile (or fissionable) when a material is capable of sustaining a nuclear fission reaction
fission splitting of a heavier nucleus into two or more lighter nuclei, usually accompanied by the conversion of mass into large amounts of energy
fusion combination of very light nuclei into heavier nuclei, accompanied by the conversion of mass into large amounts of energy
fusion reactor nuclear reactor in which fusion reactions of light nuclei are controlled
gamma ( $\boldsymbol{\gamma}$ ) emission decay of an excited-state nuclide accompanied by emission of a gamma ray
gamma ray ( $\boldsymbol{\gamma}$ or $\mathbf{0}_{\mathbf{0}}^{\boldsymbol{0}} \boldsymbol{\gamma}$ ) short wavelength, highenergy electromagnetic radiation that exhibits wave-particle duality
Geiger counter instrument that detects and measures radiation via the ionization produced in a Geiger-Müller tube
gray (Gy) SI unit for measuring radiation dose; 1 Gy = 1 J absorbed $/ \mathrm{kg}$ tissue
half-life ( $\boldsymbol{t}_{\mathbf{1} / \mathbf{2}}$ ) time required for half of the atoms in a radioactive sample to decay
internal radiation therapy (also, brachytherapy) radiation from a radioactive substance introduced into the body to kill cancer cells
ionizing radiation radiation that can cause a molecule to lose an electron and form an ion
magic number nuclei with specific numbers of nucleons that are within the band of stability
mass defect difference between the mass of an atom and the summed mass of its constituent subatomic particles (or the mass "lost" when nucleons are brought together to form a nucleus)


2134. Key Terms -
mass defect difference between the mass of an atom and the summed mass of its constituent subatomic particles (or the mass "lost" when nucleons are brought together to form a nucleus)
mass-energy equivalence equation Albert Einstein's relationship showing that mass and energy are equivalent
millicurie ( $\mathbf{m C i}$ ) larger unit for rate of radioactive decay frequently used in medicine; $1 \mathrm{Ci}=3.7 \times$ $10^{10}$ disintegrations/s
nonionizing radiation radiation that speeds up the movement of atoms and molecules; it is equivalent to heating a sample, but is not energetic enough to cause the ionization of molecules
nuclear binding energy energy lost when an
atom's nucleons are bound together (or the energy needed to break a nucleus into its constituent protons and neutrons)
nuclear chemistry study of the structure of atomic nuclei and processes that change nuclear structure
nuclear fuel fissionable isotope present in sufficient quantities to provide a self-sustaining chain reaction in a nuclear reactor
nuclear moderator substance that slows neutrons to a speed low enough to cause fission
nuclear reaction change to a nucleus resulting in changes in the atomic number, mass number, or energy state
nuclear reactor environment that produces energy via nuclear fission in which the chain reaction is controlled and sustained without explosion
nuclear transmutation conversion of one nuclide into another nuclide
nucleon collective term for protons and neutrons in a nucleus
nuclide nucleus of a particular isotope
parent nuclide unstable nuclide that changes spontaneously into another (daughter) nuclide
particle accelerator device that uses electric and magnetic fields to increase the kinetic energy of nuclei used in transmutation reactions
positron $\left({ }_{+1}^{0} \beta\right.$ or $\left.{ }_{+1}^{0} \mathrm{e}\right)$ antiparticle to the electron; it has identical properties to an electron, except for having the opposite (positive) charge
positron emission (also, $\beta^{+}$decay) conversion of a proton into a neutron, which remains in the nucleus, and a positron, which is emitted
radiation absorbed dose (rad) SI unit for measuring radiation dose, frequently used in medical applications; $1 \mathrm{rad}=0.01$ Gy


2134. Key Terms -
radiation absorbed dose (rad) SI unit for measuring radiation dose, frequently used in medical applications; $1 \mathrm{rad}=0.01$ Gy
radiation dosimeter device that measures ionizing radiation and is used to determine personal radiation exposure
radiation therapy use of high-energy radiation to damage the DNA of cancer cells, which kills them or keeps them from dividing
radioactive decay spontaneous decay of an unstable nuclide into another nuclide
radioactive decay series chains of successive disintegrations (radioactive decays) that ultimately lead to a stable end-product
radioactive tracer (also, radioactive label)
radioisotope used to track or follow a substance by monitoring its radioactive emissions
radioactivity phenomenon exhibited by an unstable nucleon that spontaneously undergoes change into a nucleon that is more stable; an unstable nucleon is said to be radioactive
radiocarbon dating highly accurate means of dating objects 30,000-50,000 years old that were derived from once-living matter; achieved by calculating the ratio of ${ }_{6}^{14} \mathrm{C}:{ }_{6}^{12} \mathrm{C}$ in the object vs. the ratio of ${ }_{6}^{14} \mathrm{C}$ : ${ }_{6}^{12} \mathrm{C}$ in the present-day atmosphere
radioisotope isotope that is unstable and undergoes conversion into a different, more stable isotope
radiometric dating use of radioisotopes and their properties to date the formation of objects such as archeological artifacts, formerly living organisms, or geological formations
reactor coolant assembly used to carry the heat produced by fission in a reactor to an external boiler and turbine where it is transformed into electricity
relative biological effectiveness (RBE) measure of the relative damage done by radiation
roentgen equivalent man (rem) unit for radiation damage, frequently used in medicine; 100 rem = 1 Sv
scintillation counter instrument that uses a scintillator-a material that emits light when excited by ionizing radiation-to detect and measure radiation
sievert (Sv) SI unit measuring tissue damage caused by radiation; takes into account energy and biological effects of radiation
strong nuclear force force of attraction between nucleons that holds a nucleus together
subcritical mass amount of fissionable material that cannot sustain a chain reaction; less than a critical mass


2134. Key Terms -
strong nuclear force force of attraction between nucleons that holds a nucleus together
subcritical mass amount of fissionable material that cannot sustain a chain reaction; less than a critical mass
supercritical mass amount of material in which there is an increasing rate of fission
transmutation reaction bombardment of one type of nuclei with other nuclei or neutrons
transuranium element element with an atomic number greater than 92; these elements do not occur in nature


2135. Key Equations -
$E=m c^{2}$
decay rate $=\lambda N$
$t_{1 / 2}=\frac{\ln 2}{\lambda}=\frac{0.693}{\lambda}$
rem $=\mathrm{RBE} \times \mathrm{rad}$
$\mathrm{Sv}=\mathrm{RBE} \times \mathrm{Gy}$


2136. Summary - 2136.1. Nuclear Structure and Stability
An atomic nucleus consists of protons and neutrons, collectively called nucleons. Although protons repel each other, the nucleus is held tightly together by a short-range, but very strong, force called the strong nuclear force. A nucleus has less mass than the total mass of its constituent nucleons. This "missing" mass is the mass defect, which has been converted into the binding energy that holds the nucleus together according to Einstein's mass-energy equivalence equation, $E=m c^{2}$. Of the many nuclides that exist, only a small number are stable. Nuclides with even numbers of protons or neutrons, or those with magic numbers of nucleons, are especially likely to be stable. These stable nuclides occupy a narrow band of stability on a graph of number of protons versus number of neutrons. The binding energy per nucleon is largest for the elements with mass numbers near 56 ; these are the most stable nuclei.


2136. Summary - 2136.2. Nuclear Equations
Nuclei can undergo reactions that change their number of protons, number of neutrons, or energy state. Many different particles can be involved in nuclear reactions. The most common are protons, neutrons, positrons (which are positively charged electrons), alpha ( $\alpha$ ) particles (which are highenergy helium nuclei), beta ( $\beta$ ) particles (which are high-energy electrons), and gamma ( $\gamma$ ) rays (which compose high-energy electromagnetic radiation). As with chemical reactions, nuclear reactions are always balanced. When a nuclear reaction occurs, the total mass (number) and the total charge remain unchanged.


2136. Summary - 2136.3. Radioactive Decay
Nuclei that have unstable n:p ratios undergo spontaneous radioactive decay. The most common types of radioactivity are $\alpha$ decay, $\beta$ decay, $\gamma$ emission, positron emission, and electron capture. Nuclear reactions also often involve $\gamma$ rays, and some nuclei decay by electron capture. Each of these modes of decay leads to the formation of a new nucleus with a more stable n:p ratio. Some
substances undergo radioactive decay series, proceeding through multiple decays before ending in a stable isotope. All nuclear decay processes follow first-order kinetics, and each radioisotope has its own characteristic half-life, the time that is required for half of its atoms to decay. Because of the large differences in stability among nuclides, there is a very wide range of half-lives of radioactive substances. Many of these substances have found useful applications in medical diagnosis and treatment, determining the age of archaeological and geological objects, and more.


2136. Summary - 2136.4. Transmutation and Nuclear Energy
It is possible to produce new atoms by bombarding other atoms with nuclei or high-speed particles. The products of these transmutation reactions can be stable or radioactive. A number of artificial elements, including technetium, astatine, and the transuranium elements, have been produced in this way.

Nuclear power as well as nuclear weapon detonations can be generated through fission (reactions in which a heavy nucleus is split into two or more lighter nuclei and several neutrons). Because the neutrons may induce additional fission reactions when they combine with other heavy nuclei, a chain reaction can result. Useful power is obtained if the fission process is carried out in a nuclear reactor. The conversion of light nuclei into heavier nuclei (fusion) also produces energy. At present, this energy has not been contained adequately and is too expensive to be feasible for commercial energy production.


2136. Summary - 2136.5. Uses of Radioisotopes
Compounds known as radioactive tracers can be used to follow reactions, track the distribution of a substance, diagnose and treat medical conditions, and much more. Other radioactive substances are helpful for controlling pests, visualizing structures, providing fire warnings, and for many other applications. Hundreds of millions of nuclear medicine tests and procedures, using a wide variety of radioisotopes with relatively short half-lives, are
performed every year in the US. Most of these radioisotopes have relatively short half-lives; some are short enough that the radioisotope must be made on-site at medical facilities. Radiation therapy uses high-energy radiation to kill cancer cells by damaging their DNA. The radiation used for this treatment may be delivered externally or internally.


2136. Summary - 2136.6. Biological Effects of Radiation
We are constantly exposed to radiation from a variety of naturally occurring and human-produced sources. This radiation can affect living organisms. Ionizing radiation is the most harmful because it can ionize molecules or break chemical bonds, which damages the molecule and causes malfunctions in cell processes. It can also create reactive hydroxyl radicals that damage biological molecules and disrupt physiological processes. Radiation can cause somatic or genetic damage, and is most harmful to
rapidly reproducing cells. Types of radiation differ in their ability to penetrate material and damage tissue, with alpha particles the least penetrating but potentially most damaging and gamma rays the most penetrating.

Various devices, including Geiger counters, scintillators, and dosimeters, are used to detect and measure radiation, and monitor radiation exposure. We use several units to measure radiation: becquerels or curies for rates of radioactive decay; gray or rads for energy absorbed; and rems or sieverts for biological effects of radiation. Exposure to radiation can cause a wide range of health effects, from minor to severe, and including death. We can minimize the effects of radiation by shielding with dense materials such as lead, moving away from the source, and limiting time of exposure.


2137. Exercises - 2137.1. Nuclear Structure and Stability
1. Write the following isotopes in hyphenated form (e.g., "carbon-14")
(a) ${ }_{11}^{24} \mathrm{Na}$
(b) ${ }_{13}^{29} \mathrm{Al}$
(c) ${ }_{36}^{73} \mathrm{Kr}$
(d) ${ }_{77}^{194} \mathrm{Ir}$
2. Write the following isotopes in nuclide notation (e.g., " ${ }_{6}^{14} \mathrm{C}$ ")
(a) oxygen-14
(b) copper-70
(c) tantalum-175
(d) francium-217
3. For the following isotopes that have missing information, fill in the missing information to complete the notation
(a) ${ }_{14}^{34} \mathrm{X}$
(b) ${ }_{X}^{36} \mathrm{P}$
(c) ${ }_{\mathrm{X}}{ }^{57} \mathrm{Mn}$
(d) ${ }_{56}^{121} \mathrm{X}$
4. For each of the isotopes in Exercise 20.1, determine the numbers of protons, neutrons, and electrons in a neutral atom of the isotope.
5. Write the nuclide notation, including charge if applicable, for atoms with the following characteristics:
(a) 25 protons, 20 neutrons, 24 electrons
(b) 45 protons, 24 neutrons, 43 electrons
(c) 53 protons, 89 neutrons, 54 electrons
(d) 97 protons, 146 neutrons, 97 electrons
6. Calculate the density of the ${ }_{12}^{24} \mathrm{Mg}$ nucleus in $\mathrm{g} / \mathrm{mL}$, assuming that it has the typical nuclear diameter of $1 \times$ $10^{-13} \mathrm{~cm}$ and is spherical in shape.
7. What are the two principal differences between nuclear reactions and ordinary chemical changes?
8. The mass of the atom ${ }_{11}^{23} \mathrm{Na}$ is 22.9898 amu .
(a) Calculate its binding energy per atom in millions of electron volts.
(b) Calculate its binding energy per nucleon.


2137. Exercises - 2137.1. Nuclear Structure and Stability
(a) Calculate its binding energy per atom in millions of electron volts.
(b) Calculate its binding energy per nucleon.
9. Which of the following nuclei lie within the band of stability shown in Figure 20.2?
(a) chlorine-37
(b) calcium-40
(c) ${ }^{204} \mathrm{Bi}$
(d) ${ }^{56} \mathrm{Fe}$
(e) ${ }^{206} \mathrm{~Pb}$
(f) ${ }^{211} \mathrm{~Pb}$
(g) ${ }^{222} \mathrm{Rn}$
(h) carbon-14
10. Which of the following nuclei lie within the band of stability shown in Figure 20.2?
(a) argon-40
(b) oxygen-16
(c) ${ }^{122} \mathrm{Ba}$
(d) ${ }^{58} \mathrm{Ni}$
(e) ${ }^{205} \mathrm{Tl}$
(f) ${ }^{210} \mathrm{Tl}$
(g) ${ }^{226} \mathrm{Ra}$
(h) magnesium-24


2137. Exercises - 2137.2. Nuclear Equations
11. Write a brief description or definition of each of the following:
(a) nucleon
(b) $\alpha$ particle
(c) $\beta$ particle
(d) positron
(e) $\gamma$ ray
(f) nuclide
(g) mass number
(h) atomic number
12. Which of the various particles ( $\alpha$ particles, $\beta$ particles, and so on) that may be produced in a nuclear reaction are actually nuclei?
13. Complete each of the following equations by adding the missing species:
(a) ${ }_{13}^{27} \mathrm{Al}+{ }_{2}^{4} \mathrm{He} \longrightarrow$ ? $+{ }_{0}^{1} \mathrm{n}$
(b) ${ }_{94}^{239} \mathrm{Pu}+$ ? $\longrightarrow{ }_{96}^{242} \mathrm{Cm}+{ }_{0}^{1} \mathrm{n}$
(c) ${ }_{7}^{14} \mathrm{~N}+{ }_{2}^{4} \mathrm{He} \longrightarrow$ ? $+{ }_{1}^{1} \mathrm{H}$
(d) ${ }_{92}^{235} \mathrm{U} \longrightarrow ?+{ }_{55}^{135} \mathrm{Cs}+4{ }_{0}^{1} \mathrm{n}$
14. Complete each of the following equations:
(a) ${ }_{3}^{7} \mathrm{Li}+? \longrightarrow 2{ }_{2}^{4} \mathrm{He}$
(b) ${ }_{6}^{14} \mathrm{C} \longrightarrow{ }_{7}^{14} \mathrm{~N}+$ ?
(c) ${ }_{13}^{27} \mathrm{Al}+{ }_{2}^{4} \mathrm{He} \longrightarrow ?+{ }_{0}^{1} \mathrm{n}$
(d) ${ }_{96}^{250} \mathrm{Cm} \longrightarrow ?+{ }_{38}^{98} \mathrm{Sr}+4{ }_{0}^{1} \mathrm{n}$
15. Write a balanced equation for each of the following nuclear reactions:


2137. Exercises - 2137.2. Nuclear Equations
15. Write a balanced equation for each of the following nuclear reactions:
(a) the production of ${ }^{17} \mathrm{O}$ from ${ }^{14} \mathrm{~N}$ by $\alpha$ particle bombardment
(b) the production of ${ }^{14} \mathrm{C}$ from ${ }^{14} \mathrm{~N}$ by neutron bombardment
(c) the production of ${ }^{233} \mathrm{Th}$ from ${ }^{232} \mathrm{Th}$ by neutron bombardment
(d) the production of ${ }^{239} \mathrm{U}$ from ${ }^{238} \mathrm{U}$ by ${ }_{1}^{2} \mathrm{H}$ bombardment
16. Technetium-99 is prepared from ${ }^{98} \mathrm{Mo}$. Molybdenum- 98 combines with a neutron to give molybdenum-99, an unstable isotope that emits a $\beta$ particle to yield an excited form of technetium-99, represented as ${ }^{99} \mathrm{Tc}$ ". This excited nucleus relaxes to the ground state, represented as ${ }^{99} \mathrm{Tc}$, by emitting a $\gamma$ ray. The ground state of ${ }^{99} \mathrm{Tc}$ then emits a $\beta$ particle. Write the equations for each of these nuclear reactions.
17. The mass of the atom ${ }_{9}^{19} \mathrm{~F}$ is 18.99840 amu .
(a) Calculate its binding energy per atom in millions of electron volts.
(b) Calculate its binding energy per nucleon.
18. For the reaction ${ }_{6}^{14} \mathrm{C} \longrightarrow{ }_{7}^{14} \mathrm{~N}+$ ?, if 100.0 g of carbon reacts, what volume of nitrogen gas $\left(\mathrm{N}_{2}\right)$ is produced at 273 K and 1 atm ?


2137. Exercises - 2137.3. Radioactive Decay
19. What are the types of radiation emitted by the nuclei of radioactive elements?
20. What changes occur to the atomic number and mass of a nucleus during each of the following decay scenarios?
(a) an $\alpha$ particle is emitted
(b) a $\beta$ particle is emitted
(c) $\gamma$ radiation is emitted
(d) a positron is emitted
(e) an electron is captured
21. What is the change in the nucleus that results from the following decay scenarios?
(a) emission of a $\beta$ particle
(b) emission of a $\beta^{+}$particle
(c) capture of an electron
22. Many nuclides with atomic numbers greater than 83 decay by processes such as electron emission. Explain the observation that the emissions from these unstable nuclides also normally include $\alpha$ particles.
23. Why is electron capture accompanied by the emission of an X-ray?
24. Explain, in terms of Figure 20.2, how unstable heavy nuclides (atomic number $>83$ ) may decompose to form nuclides of greater stability (a) if they are below the band of stability and (b) if they are above the band of stability.
25. Which of the following nuclei is most likely to decay by positron emission? Explain your choice.
(a) chromium-53
(b) manganese-51
(c) iron-59
26. The following nuclei do not lie in the band of stability. How would they be expected to decay? Explain your answer.
(a) ${ }_{15}^{34} \mathrm{P}$
(b) ${ }_{92}^{239} \mathrm{U}$
(c) ${ }_{20}^{38} \mathrm{Ca}$
(d) ${ }_{1}^{3} \mathrm{H}$
(e) ${ }_{94}^{245} \mathrm{Pu}$
27. The following nuclei do not lie in the band of stability. How would they be expected to decay?
(a) ${ }_{15}^{28} \mathrm{P}$
(b) ${ }_{92}^{235} \mathrm{U}$
(c) ${ }_{20}^{37} \mathrm{Ca}$
(d) ${ }_{3}^{9} \mathrm{Li}$


2137. Exercises - 2137.3. Radioactive Decay
(b) ${ }_{92}^{235} \mathrm{U}$
(c) ${ }_{20}^{37} \mathrm{Ca}$
(d) ${ }_{3}^{9} \mathrm{Li}$
(e) ${ }_{96}^{245} \mathrm{Cm}$
28. Predict by what mode(s) of spontaneous radioactive decay each of the following unstable isotopes might proceed:
(a) ${ }_{2}^{6} \mathrm{He}$
(b) ${ }_{30}^{60} \mathrm{Zn}$
(c) ${ }_{91}^{235} \mathrm{~Pa}$
(d) ${ }_{94}^{241} \mathrm{~Np}$
(e) ${ }^{18} \mathrm{~F}$
(f) ${ }^{129} \mathrm{Ba}$
(g) ${ }^{237} \mathrm{Pu}$
29. Write a nuclear reaction for each step in the formation of ${ }_{84}^{218} \operatorname{Po}$ from ${ }_{98}^{238} \mathrm{U}$, which proceeds by a series of decay reactions involving the step-wise emission of $\alpha, \beta, \beta, \alpha, \alpha, \alpha$ particles, in that order.
30. Write a nuclear reaction for each step in the formation of ${ }_{82}^{208} \mathrm{~Pb}$ from ${ }_{90}^{228} \mathrm{Th}$, which proceeds by a series of decay reactions involving the step-wise emission of $\alpha, \alpha, \alpha, \alpha, \beta, \beta, \alpha$ particles, in that order.
31. Define the term half-life and illustrate it with an example.
32. A $1.00 \times 10^{-6}$-g sample of nobelium, ${ }_{102}^{254} \mathrm{No}$, has a half-life of 55 seconds after it is formed. What is the percentage of ${ }_{102}^{254}$ No remaining at the following times?
(a) 5.0 min after it forms
(b) 1.0 h after it forms


2137. Exercises - 2137.3. Radioactive Decay
(a) 5.0 min after it forms
(b) 1.0 h after it forms
33. ${ }^{239} \mathrm{Pu}$ is a nuclear waste byproduct with a half-life of $24,000 \mathrm{y}$. What fraction of the ${ }^{239} \mathrm{Pu}$ present today will be present in 1000 y ?
34. The isotope ${ }^{208} \mathrm{Tl}$ undergoes $\beta$ decay with a half-life of 3.1 min .
(a) What isotope is produced by the decay?
(b) How long will it take for $99.0 \%$ of a sample of pure ${ }^{208} \mathrm{Tl}$ to decay?
(c) What percentage of a sample of pure ${ }^{208} \mathrm{Tl}$ remains un-decayed after 1.0 h ?
35. If 1.000 g of ${ }_{88}^{226} \mathrm{Ra}$ produces 0.0001 mL of the gas ${ }_{86}^{222} \mathrm{Rn}$ at STP (standard temperature and pressure) in 24 h , what is the half-life of ${ }^{226} \mathrm{Ra}$ in years?
36. The isotope ${ }_{38}^{90} \mathrm{Sr}$ is one of the extremely hazardous species in the residues from nuclear power generation. The strontium in a $0.500-\mathrm{g}$ sample diminishes to 0.393 g in 10.0 y . Calculate the half-life.
37. Technetium-99 is often used for assessing heart, liver, and lung damage because certain technetium compounds are absorbed by damaged tissues. It has a half-life of 6.0 h . Calculate the rate constant for the decay of ${ }_{43}^{99} \mathrm{Tc}$.
38. What is the age of mummified primate skin that contains $8.25 \%$ of the original quantity of ${ }^{14} \mathrm{C}$ ?
39. A sample of rock was found to contain 8.23 mg of rubidium -87 and 0.47 mg of strontium- 87 .


2137. Exercises - 2137.3. Radioactive Decay
39. A sample of rock was found to contain 8.23 mg of rubidium -87 and 0.47 mg of strontium- 87 .
(a) Calculate the age of the rock if the half-life of the decay of rubidium by $\beta$ emission is $4.7 \times 10^{10} \mathrm{y}$.
(b) If some ${ }_{38}^{87} \mathrm{Sr}$ was initially present in the rock, would the rock be younger, older, or the same age as the age calculated in (a)? Explain your answer.
40. A laboratory investigation shows that a sample of uranium ore contains 5.37 mg of ${ }_{92}^{238} \mathrm{U}$ and 2.52 mg of ${ }_{82}^{206} \mathrm{~Pb}$. Calculate the age of the ore. The half-life of ${ }_{92}^{238} \mathrm{U}$ is $4.5 \times 10^{9} \mathrm{yr}$.
41. Plutonium was detected in trace amounts in natural uranium deposits by Glenn Seaborg and his associates in 1941. They proposed that the source of this ${ }^{239} \mathrm{Pu}$ was the capture of neutrons by ${ }^{238} \mathrm{U}$ nuclei. Why is this plutonium not likely to have been trapped at the time the solar system formed $4.7 \times 10^{9}$ years ago?
42. $\mathrm{A}{ }_{4}^{7} \mathrm{Be}$ atom (mass $=7.0169 \mathrm{amu}$ ) decays into a ${ }_{3}^{7} \mathrm{Li}$ atom (mass $=7.0160 \mathrm{amu}$ ) by electron capture. How much energy (in millions of electron volts, MeV ) is produced by this reaction?


2137. Exercises - 2137.3. Radioactive Decay
43. $\mathrm{A}{ }_{5}^{8} \mathrm{~B}$ atom (mass $=8.0246 \mathrm{amu}$ ) decays into a ${ }_{4}^{8} \mathrm{~B}$ atom (mass $=8.0053 \mathrm{amu}$ ) by loss of a $\beta^{+}$particle (mass $=0.00055 \mathrm{amu}$ ) or by electron capture. How much energy (in millions of electron volts) is produced by this reaction?
44. Isotopes such as ${ }^{26} \mathrm{Al}$ (half-life: $7.2 \times 10^{5}$ years) are believed to have been present in our solar system as it formed, but have since decayed and are now called extinct nuclides.
(a) ${ }^{26} \mathrm{Al}$ decays by $\beta^{+}$emission or electron capture. Write the equations for these two nuclear transformations.
(b) The earth was formed about $4.7 \times 10^{9}$ ( 4.7 billion) years ago. How old was the earth when $99.999999 \%$ of the ${ }^{26} \mathrm{Al}$ originally present had decayed?
45. Write a balanced equation for each of the following nuclear reactions:
(a) bismuth-212 decays into polonium-212
(b) beryllium-8 and a positron are produced by the decay of an unstable nucleus
(c) neptunium-239 forms from the reaction of uranium-238 with a neutron and then spontaneously converts into plutonium-239
(d) strontium-90 decays into yttrium-90
46. Write a balanced equation for each of the following nuclear reactions:
(a) mercury-180 decays into platinum-176
(b) zirconium-90 and an electron are produced by the decay of an unstable nucleus
(c) thorium-232 decays and produces an alpha particle and a radium-228 nucleus, which decays into actinium- 228 by beta decay
(d) neon-19 decays into fluorine-19


2137. Exercises - 2137.4. Transmutation and Nuclear Energy
47. Write the balanced nuclear equation for the production of the following transuranium elements:
(a) berkelium-244, made by the reaction of Am-241 and $\mathrm{He}-4$
(b) fermium-254, made by the reaction of $\mathrm{Pu}-239$ with a large number of neutrons
(c) lawrencium-257, made by the reaction of Cf-250 and B-11
(d) dubnium-260, made by the reaction of Cf-249 and N-15
48. How does nuclear fission differ from nuclear fusion? Why are both of these processes exothermic?
49. Both fusion and fission are nuclear reactions. Why is a very high temperature required for fusion, but not for fission?
50. Cite the conditions necessary for a nuclear chain reaction to take place. Explain how it can be controlled to produce energy, but not produce an explosion.
51. Describe the components of a nuclear reactor.
52. In usual practice, both a moderator and control rods are necessary to operate a nuclear chain reaction safely for the purpose of energy production. Cite the function of each and explain why both are necessary.
53. Describe how the potential energy of uranium is converted into electrical energy in a nuclear power plant.
54. The mass of a hydrogen atom $\left({ }_{1}^{1} \mathrm{H}\right)$ is 1.007825 amu ; that of a tritium atom $\left({ }_{1}^{3} \mathrm{H}\right)$ is 3.01605 amu ; and that of an $\alpha$ particle is 4.00150 amu . How much energy in kilojoules per mole of ${ }_{2}^{4} \mathrm{He}$ produced is released by the following fusion reaction: ${ }_{1}^{1} \mathrm{H}+{ }_{1}^{3} \mathrm{H} \longrightarrow{ }_{2}^{4} \mathrm{He}$.


2137. Exercises - 2137.5. Uses of Radioisotopes
55. How can a radioactive nuclide be used to show that the equilibrium: $\mathrm{AgCl}(s) \rightleftharpoons \mathrm{Ag}^{+}(a q)+\mathrm{Cl}^{-}(a q)$ is a dynamic equilibrium?
56. Technetium-99m has a half-life of 6.01 hours. If a patient injected with technetium- 99 m is safe to leave the hospital once $75 \%$ of the dose has decayed, when is the patient allowed to leave?
57. Iodine that enters the body is stored in the thyroid gland from which it is released to control growth and metabolism. The thyroid can be imaged if iodine-131 is injected into the body. In larger doses, I-133 is also used as a means of treating cancer of the thyroid. I- 131 has a half-life of 8.70 days and decays by $\beta^{-}$ emission.
(a) Write an equation for the decay.
(b) How long will it take for $95.0 \%$ of a dose of I-131 to decay?


2137. Exercises - 2137.6. Biological Effects of Radiation
58. If a hospital were storing radioisotopes, what is the minimum containment needed to protect against:
(a) cobalt-60 (a strong $\gamma$ emitter used for irradiation)
(b) molybdenum-99 (a beta emitter used to produce technetium-99 for imaging)
59. Based on what is known about Radon- 222 's primary decay method, why is inhalation so dangerous?
60. Given specimens uranium-232 ( $t_{1 / 2}=68.9 \mathrm{y}$ ) and uranium-233 ( $\left.t_{1 / 2}=159,200 \mathrm{y}\right)$ of equal mass, which one would have greater activity and why?
61. A scientist is studying a 2.234 g sample of thorium-229 ( $\left.t_{1 / 2}=7340 \mathrm{y}\right)$ in a laboratory.
(a) What is its activity in Bq?
(b) What is its activity in Ci ?
62. Given specimens neon-24 ( $t_{1 / 2}=3.38 \mathrm{~min}$ ) and bismuth-211 ( $\left.t_{1 / 2}=2.14 \mathrm{~min}\right)$ of equal mass, which one would have greater activity and why?

Figure 21.1 All organic compounds contain carbon and most are formed by living things, although they are also formed by geological and artificial processes. (credit left: modification of work by Jon Sullivan; credit left middle: modification of work by Deb Tremper; credit right middle: modification of work by "annszyp"/Wikimedia Commons; credit right: modification of work by George Shuklin)


2138. CHAPTER OUTLINE - 2138.1. Hydrocarbons
21.2 Alcohols and Ethers
21.3 Aldehydes, Ketones, Carboxylic Acids, and Esters
21.4 Amines and Amides

INTRODUCTION All living things on earth are formed mostly of carbon compounds. The prevalence of carbon compounds in living things has led to the epithet "carbon-based" life. The truth is we know of no other kind of life. Early chemists regarded substances isolated from organisms (plants and animals) as a different type of matter that could not be synthesized artificially, and these substances were thus known as organic compounds. The widespread belief called vitalism held that organic compounds were formed by a vital force present only in living organisms. The German chemist Friedrich Wohler was one of the early chemists to refute this aspect of vitalism, when, in 1828, he reported the synthesis of urea, a component of many body fluids, from nonliving materials. Since then, it has been recognized that organic molecules obey the same natural laws as inorganic substances, and the category of organic compounds has evolved to include both natural and synthetic compounds that contain carbon. Some carbon-containing compounds are not classified as organic, for example, carbonates and cyanides, and simple oxides, such as CO and $\mathrm{CO}_{2}$. Although a single, precise definition has yet to be identified by the chemistry community, most agree that a defining trait of organic molecules is the presence of carbon as the principal element, bonded to hydrogen and other carbon atoms.

Today, organic compounds are key components of plastics, soaps, perfumes, sweeteners, fabrics, pharmaceuticals, and many other substances that we use every day. The value to us of organic compounds ensures that organic chemistry is an important discipline within the general field of chemistry. In this chapter, we discuss why the element carbon gives rise to a vast number and variety of compounds, how those compounds are classified, and the role of organic compounds in representative biological and industrial settings.


2139. LEARNING OBJECTIVES -
By the end of this section, you will be able to:

- Explain the importance of hydrocarbons and the reason for their diversity
- Name saturated and unsaturated hydrocarbons, and molecules derived from them
- Describe the reactions characteristic of saturated and unsaturated hydrocarbons
- Identify structural and geometric isomers of hydrocarbons

The largest database ${ }^{\frac{1}{1}}$ of organic compounds lists about 10 million substances, which include compounds originating from living organisms and those synthesized by chemists. The number of potential organic compounds has been estimated ${ }^{2}$ at $10^{60}$-an astronomically high number. The existence of so many organic molecules is a consequence of the ability of carbon atoms to form up to four strong bonds to other carbon atoms, resulting in chains and rings of many different sizes, shapes, and complexities.

The simplest organic compounds contain only the elements carbon and hydrogen, and are called hydrocarbons. Even though they are composed of only two types of atoms, there is a wide variety of hydrocarbons because they may consist of varying lengths of chains, branched chains, and rings of carbon atoms, or combinations of these structures. In addition, hydrocarbons may differ in the types of carbon-carbon bonds present in their molecules. Many hydrocarbons are found in plants, animals, and their fossils; other hydrocarbons have been prepared in the laboratory. We use hydrocarbons every day, mainly as fuels, such as natural gas, acetylene, propane, butane, and the principal components of gasoline, diesel fuel, and heating oil. The familiar plastics polyethylene, polypropylene, and polystyrene are also hydrocarbons. We can distinguish several types of hydrocarbons by differences in the bonding between carbon atoms. This leads to differences in geometries and in the hybridization of the carbon orbitals.


2140. Alkanes -
Alkanes, or saturated hydrocarbons, contain only single covalent bonds between carbon atoms. Each of the carbon atoms in an alkane has $s p^{3}$ hybrid orbitals and is bonded to four other atoms, each of which is either carbon or hydrogen. The Lewis structures and models of methane, ethane, and pentane are illustrated in Figure 21.2. Carbon chains are usually drawn as straight lines in Lewis structures, but one has to remember that Lewis structures are not intended to indicate the geometry of molecules. Notice that the carbon atoms in the structural models (the ball-and-stick and space-filling models) of the pentane molecule do not lie in a straight line. Because of the $s p^{3}$ hybridization, the bond angles in carbon chains are close to $109.5^{\circ}$, giving such chains in an alkane a zigzag shape.

The structures of alkanes and other organic molecules may also be represented in a less detailed manner by condensed structural formulas (or simply, condensed formulas). Instead of the usual format for chemical formulas in which each element symbol appears just once, a condensed formula is written to suggest the bonding in the molecule. These formulas have the appearance of a Lewis structure from which most or all of the bond symbols have been removed. Condensed structural formulas for ethane and pentane are shown at the bottom of Figure 21.2, and several additional examples are provided in the exercises at the end of this chapter.

[^13]

methane
$\mathrm{CH}_{4}$

ethane
$\mathrm{CH}_{3} \mathrm{CH}_{3}$ or $\mathrm{C}_{2} \mathrm{H}_{6}$

pentane
$\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}$ or $\mathrm{C}_{5} \mathrm{H}_{12}$

FIGURE 21.2 Pictured are the Lewis structures, ball-and-stick models, and space-filling models for molecules of methane, ethane, and pentane.


2140. Alkanes -
FIGURE 21.2 Pictured are the Lewis structures, ball-and-stick models, and space-filling models for molecules of methane, ethane, and pentane.

A common method used by organic chemists to simplify the drawings of larger molecules is to use a skeletal structure (also called a line-angle structure). In this type of structure, carbon atoms are not symbolized with a C, but represented by each end of a line or bend in a line. Hydrogen atoms are not drawn if they are attached to a carbon. Other atoms besides carbon and hydrogen are represented by their elemental symbols. Figure 21.3 shows three different ways to draw the same structure.

Expanded formula

Condensed formula
Skeletal structure
FIGURE 21.3 The same structure can be represented three different ways: an expanded formula, a condensed formula, and a skeletal structure.


2142. Drawing Skeletal Structures -
Draw the skeletal structures for these two molecules:

(a)

(b)


2143. Solution -
Each carbon atom is converted into the end of a line or the place where lines intersect. All hydrogen atoms attached to the carbon atoms are left out of the structure (although we still need to recognize they are there):

(a)

(b)


2144. Check Your Learning -
Draw the skeletal structures for these two molecules:

(a)

$$
\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}
$$

(b)

Answer:

(a)

(b)


2146. Interpreting Skeletal Structures -
Identify the chemical formula of the molecule represented here:


2147. Solution -
There are eight places where lines intersect or end, meaning that there are eight carbon atoms in the molecule. Since we know that carbon atoms tend to make four bonds, each carbon atom will have the number of hydrogen atoms that are required for four bonds. This compound contains 16 hydrogen atoms for a molecular formula of $\mathrm{C}_{8} \mathrm{H}_{16}$.

Location of the hydrogen atoms:

Check Your Learning
Identify the chemical formula of the molecule represented here:


2148. Answer: -
$\mathrm{C}_{9} \mathrm{H}_{20}$

All alkanes are composed of carbon and hydrogen atoms, and have similar bonds, structures, and formulas; noncyclic alkanes all have a formula of $\mathrm{C}_{\mathrm{n}} \mathrm{H}_{2 \mathrm{n}+2}$. The number of carbon atoms present in an alkane has no limit. Greater numbers of atoms in the molecules will lead to stronger intermolecular attractions (dispersion forces) and correspondingly different physical properties of the molecules. Properties such as melting point and boiling point (Table 21.1) usually change smoothly and predictably as the number of carbon and hydrogen atoms in the molecules change.


2149. Properties of Some Alkanes ${ }^{\text {³ }}$ -
| Alkane | Molecular
 Formula | Melting Point
 $\left({ }^{\circ} \mathrm{C}\right)$ | Boiling Point
 $\left({ }^{\circ} \mathrm{C}\right)$ | Phase at
 STP | Number of Structural
 Isomers |
| :--- | :--- | :--- | :--- | :--- | :--- |
| methane | $\mathrm{CH}_{4}$ | -182.5 | -161.5 | gas | 1 |
| ethane | $\mathrm{C}_{2} \mathrm{H}_{6}$ | -183.3 | -88.6 | gas | 1 |
| propane | $\mathrm{C}_{3} \mathrm{H}_{8}$ | -187.7 | -42.1 | gas | 1 |
| putane | $\mathrm{C}_{4} \mathrm{H}_{10}$ | -138.3 | -0.5 | gas | 2 |
| pentane | $\mathrm{C}_{5} \mathrm{H}_{12}$ | -129.7 | 36.1 | liquid | 3 |
| hepane | $\mathrm{C}_{6} \mathrm{H}_{14}$ | -95.3 | 68.7 | liquid | 5 |
| octane | $\mathrm{C}_{7} \mathrm{H}_{16} \mathrm{H}_{18}$ | -90.6 | 98.4 | liquid | 9 |
| nonane | $\mathrm{C}_{9} \mathrm{H}_{20}$ | -56.8 | 125.7 | liquid | 18 |
| decane | $\mathrm{C}_{10} \mathrm{H}_{22}$ | -53.6 | -29.7 | 150.8 | liquid |
| tetradecane | $\mathrm{C}_{14} \mathrm{H}_{30}$ | 5.9 | liquid | 75 |  |
| octadecane | $\mathrm{C}_{18} \mathrm{H}_{38}$ | 28.2 | solid | 1858 |  |


2149. Properties of Some Alkanes ${ }^{\text {³ }}$ -
[^14]Hydrocarbons with the same formula, including alkanes, can have different structures. For example, two alkanes have the formula $\mathrm{C}_{4} \mathrm{H}_{10}$ : They are called $n$-butane and 2-methylpropane (or isobutane), and have the following Lewis structures:

$n$-butane

2-methylpropane

The compounds $n$-butane and 2-methylpropane are structural isomers (the term constitutional isomers is also commonly used). Constitutional isomers have the same molecular formula but different spatial arrangements of the atoms in their molecules. The $n$-butane molecule contains an unbranched chain, meaning that no carbon atom is bonded to more than two other carbon atoms. We use the term normal, or the prefix $n$, to refer to a chain of carbon atoms without branching. The compound 2 -methylpropane has a branched chain (the carbon atom in the center of the Lewis structure is bonded to three other carbon atoms)

Identifying isomers from Lewis structures is not as easy as it looks. Lewis structures that look different may actually represent the same isomers. For example, the three structures in Figure 21.4 all represent the same molecule, $n$-butane, and hence are not different isomers. They are identical because each contains an unbranched chain of four carbon atoms.

[^15]

FIGURE 21.4 These three representations of the structure of $n$-butane are not isomers because they all contain the same arrangement of atoms and bonds.


2150. The Basics of Organic Nomenclature: Naming Alkanes -
The International Union of Pure and Applied Chemistry (IUPAC) has devised a system of nomenclature that begins with the names of the alkanes and can be adjusted from there to account for more complicated structures. The nomenclature for alkanes is based on two rules:

1. To name an alkane, first identify the longest chain of carbon atoms in its structure. A two-carbon chain is called ethane; a three-carbon chain, propane; and a four-carbon chain, butane. Longer chains are named as follows: pentane (five-carbon chain), hexane (6), heptane (7), octane (8), nonane (9), and decane (10). These prefixes can be seen in the names of the alkanes described in Table 21.1.
2. Add prefixes to the name of the longest chain to indicate the positions and names of substituents. Substituents are branches or functional groups that replace hydrogen atoms on a chain. The position of a substituent or branch is identified by the number of the carbon atom it is bonded to in the chain. We number the carbon atoms in the chain by counting from the end of the chain nearest the substituents. Multiple substituents are named individually and placed in alphabetical order at the front of the name.

propane

2-chloropropane

2-methylpropane

2,4-difluorohexane

1-bromo-3-chlorohexane

When more than one substituent is present, either on the same carbon atom or on different carbon atoms, the substituents are listed alphabetically. Because the carbon atom numbering begins at the end closest to a substituent, the longest chain of carbon atoms is numbered in such a way as to produce the lowest number for the substituents. The ending -o replaces -ide at the end of the name of an electronegative substituent (in ionic compounds, the negatively charged ion ends with -ide like chloride; in organic compounds, such atoms are treated as substituents and the -o ending is used). The number of substituents of the same type is indicated by the prefixes di-(two), tri- (three), tetra-(four), and so on (for example, difluoro- indicates two fluoride substituents).


2152. Naming Halogen-substituted Alkanes -
Name the molecule whose structure is shown here:


2153. Solution -
The four-carbon chain is numbered from the end with the chlorine atom. This puts the substituents on positions 1 and 2 (numbering from the other end would put the substituents on positions 3 and 4). Four carbon atoms means that the base name of this compound will be butane. The bromine at position 2 will be described by adding 2 -bromo-; this will come at the beginning of the name, since bromo- comes before chloroalphabetically. The chlorine at position 1 will be described by adding 1-chloro-, resulting in the name of the molecule being 2-bromo-1-chlorobutane.


2154. Check Your Learning -
Name the following molecule:


2155. Answer: -
3,3-dibromo-2-iodopentane

We call a substituent that contains one less hydrogen than the corresponding alkane an alkyl group. The name of an alkyl group is obtained by dropping the suffix -ane of the alkane name and adding -yl:

methane

methyl group

ethane

ethyl group

The open bonds in the methyl and ethyl groups indicate that these alkyl groups are bonded to another atom.


2157. Naming Substituted Alkanes -
Name the molecule whose structure is shown here:


2158. Solution -
The longest carbon chain runs horizontally across the page and contains six carbon atoms (this makes the base of the name hexane, but we will also need to incorporate the name of the branch). In this case, we want to number from right to left (as shown by the blue numbers) so the branch is connected to carbon 3 (imagine the numbers from left to right-this would put the branch on carbon 4, violating our rules). The branch attached to position 3 of our chain contains two carbon atoms (numbered in red)-so we take our name for two carbons eth- and attach $-y l$ at the end to signify we are describing a branch. Putting all the pieces together, this molecule is 3-ethylhexane.


2159. Check Your Learning -
Name the following molecule:


2160. Answer: -
4-propyloctane

Some hydrocarbons can form more than one type of alkyl group when the hydrogen atoms that would be removed have different "environments" in the molecule. This diversity of possible alkyl groups can be identified in the following way: The four hydrogen atoms in a methane molecule are equivalent; they all have the same environment. They are equivalent because each is bonded to a carbon atom (the same carbon atom) that is bonded to three hydrogen atoms. (It may be easier to see the equivalency in the ball and stick models in Figure 21.2. Removal of any one of the four hydrogen atoms from methane forms a methyl group. Likewise, the six hydrogen atoms in ethane are equivalent (Figure 21.2) and removing any one of these hydrogen atoms produces an ethyl group. Each of the six hydrogen atoms is bonded to a carbon atom that is bonded to two other hydrogen atoms and a carbon atom. However, in both propane and $2-$ methylpropane, there are hydrogen atoms in two different environments, distinguished by the adjacent atoms or groups of atoms:

propane

2-methylpropane

Each of the six equivalent hydrogen atoms of the first type in propane and each of the nine equivalent hydrogen atoms of that type in 2-methylpropane (all shown in black) are bonded to a carbon atom that is bonded to only one other carbon atom. The two purple hydrogen atoms in propane are of a second type. They differ from the six hydrogen atoms of the first type in that they are bonded to a carbon atom bonded to two other carbon atoms. The green hydrogen atom in 2-methylpropane differs from the other nine hydrogen atoms in that molecule and from the purple hydrogen atoms in propane. The green hydrogen atom in 2 -methylpropane is bonded to a carbon atom bonded to three other carbon atoms. Two different alkyl groups can be formed from each of these molecules, depending on which hydrogen atom is removed. The names and structures of these and several other alkyl groups are listed in Figure 21.5.


2160. Answer: -
| Alkyl Group | Structure |
| :---: | :---: |
| methyl | $\mathrm{CH}_{3}$ - |
| ethyl | $\mathrm{CH}_{3} \mathrm{CH}_{2}$ - |
| $n$-propyl | $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2}$ - |
| isopropyl | <smiles>CC(C)I</smiles> |
| $n$-butyl | $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2}$ - |
| sec-butyl | <smiles>CCC(C)I</smiles> |
| isobutyl | <smiles>CCCCC</smiles> |
| tert-butyl | <smiles>CC(C)(C)C</smiles> |

FIGURE 21.5 This listing gives the names and formulas for various alkyl groups formed by the removal of hydrogen atoms from different locations.

Note that alkyl groups do not exist as stable independent entities. They are always a part of some larger molecule. The location of an alkyl group on a hydrocarbon chain is indicated in the same way as any other substituent:

Alkanes are relatively stable molecules, but heat or light will activate reactions that involve the breaking of $\mathrm{C}-\mathrm{H}$ or $\mathrm{C}-\mathrm{C}$ single bonds. Combustion is one such reaction:

$$
\mathrm{CH}_{4}(g)+2 \mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{~g})
$$


2160. Answer: -
Alkanes burn in the presence of oxygen, a highly exothermic oxidation-reduction reaction that produces carbon dioxide and water. As a consequence, alkanes are excellent fuels. For example, methane, $\mathrm{CH}_{4}$, is the principal component of natural gas. Butane, $\mathrm{C}_{4} \mathrm{H}_{10}$, used in camping stoves and lighters is an alkane. Gasoline is a liquid mixture of continuous- and branched-chain alkanes, each containing from five to nine carbon atoms, plus various additives to improve its performance as a fuel. Kerosene, diesel oil, and fuel oil are primarily mixtures of alkanes with higher molecular masses. The main source of these liquid alkane fuels is crude oil, a complex mixture that is separated by fractional distillation. Fractional distillation takes advantage of differences in the boiling points of the components of the mixture (see Figure 21.6). You may recall that boiling point is a function of intermolecular interactions, which was discussed in the chapter on solutions and colloids.

Small molecules:

- Low boiling point
- Very volatile
- Flows easily
- Ignites easily

Large molecules:

- High boiling point
- Not very volatile
- Does not flow easily
- Does not ignite easily

FIGURE 21.6 In a column for the fractional distillation of crude oil, oil heated to about $425^{\circ} \mathrm{C}$ in the furnace vaporizes when it enters the base of the tower. The vapors rise through bubble caps in a series of trays in the tower. As the vapors gradually cool, fractions of higher, then of lower, boiling points condense to liquids and are drawn off. (credit left: modification of work by Luigi Chiesa)

In a substitution reaction, another typical reaction of alkanes, one or more of the alkane's hydrogen atoms is replaced with a different atom or group of atoms. No carbon-carbon bonds are broken in these reactions, and
the hybridization of the carbon atoms does not change. For example, the reaction between ethane and molecular chlorine depicted here is a substitution reaction:


2160. Answer: -
The $\mathrm{C}-\mathrm{Cl}$ portion of the chloroethane molecule is an example of a functional group, the part or moiety of a molecule that imparts a specific chemical reactivity. The types of functional groups present in an organic molecule are major determinants of its chemical properties and are used as a means of classifying organic compounds as detailed in the remaining sections of this chapter.


2161. (C) LINK TO LEARNING -
Want more practice naming alkanes? Watch this brief video tutorial (http://openstax.org/l/16alkanes) to review the nomenclature process.


2162. Alkenes -
Organic compounds that contain one or more double or triple bonds between carbon atoms are described as unsaturated. You have likely heard of unsaturated fats. These are complex organic molecules with long chains of carbon atoms, which contain at least one double bond between carbon atoms. Unsaturated hydrocarbon molecules that contain one or more double bonds are called alkenes. Carbon atoms linked by a double bond are bound together by two bonds, one $\sigma$ bond and one $\pi$ bond. Double and triple bonds give rise to a different geometry around the carbon atom that participates in them, leading to important differences in molecular shape and properties. The differing geometries are responsible for the different properties of unsaturated versus saturated fats.

Ethene, $\mathrm{C}_{2} \mathrm{H}_{4}$, is the simplest alkene. Each carbon atom in ethene, commonly called ethylene, has a trigonal planar structure. The second member of the series is propene (propylene) (Figure 21.7); the butene isomers follow in the series. Four carbon atoms in the chain of butene allows for the formation of isomers based on the position of the double bond, as well as a new form of isomerism.

FIGURE 21.7 Expanded structures, ball-and-stick structures, and space-filling models for the alkenes ethene, propene, and 1-butene are shown.

Ethylene (the common industrial name for ethene) is a basic raw material in the production of polyethylene and other important compounds. Over 135 million tons of ethylene were produced worldwide in 2010 for use in the polymer, petrochemical, and plastic industries. Ethylene is produced industrially in a process called cracking, in which the long hydrocarbon chains in a petroleum mixture are broken into smaller molecules.


2164. Recycling Plastics -
Polymers (from Greek words poly meaning "many" and mer meaning "parts") are large molecules made up of repeating units, referred to as monomers. Polymers can be natural (starch is a polymer of sugar residues and proteins are polymers of amino acids) or synthetic [like polyethylene, polyvinyl chloride (PVC), and polystyrene]. The variety of structures of polymers translates into a broad range of properties and uses that make them integral parts of our everyday lives. Adding functional groups to the structure of a polymer can result in significantly different properties (see the discussion about Kevlar later in this chapter).

An example of a polymerization reaction is shown in Figure 21.8. The monomer ethylene $\left(\mathrm{C}_{2} \mathrm{H}_{4}\right)$ is a gas at room temperature, but when polymerized, using a transition metal catalyst, it is transformed into a solid material made up of long chains of $-\mathrm{CH}_{2}$ - units called polyethylene. Polyethylene is a commodity plastic used primarily for packaging (bags and films).

FIGURE 21.8 The reaction for the polymerization of ethylene to polyethylene is shown.
Polyethylene is a member of one subset of synthetic polymers classified as plastics. Plastics are synthetic organic solids that can be molded; they are typically organic polymers with high molecular masses. Most of the monomers that go into common plastics (ethylene, propylene, vinyl chloride, styrene, and ethylene terephthalate) are derived from petrochemicals and are not very biodegradable, making them candidate materials for recycling. Recycling plastics helps minimize the need for using more of the petrochemical supplies and also minimizes the environmental damage caused by throwing away these nonbiodegradable materials.


2164. Recycling Plastics -
Plastic recycling is the process of recovering waste, scrap, or used plastics, and reprocessing the material into useful products. For example, polyethylene terephthalate (soft drink bottles) can be melted down and used for plastic furniture, in carpets, or for other applications. Other plastics, like polyethylene (bags) and polypropylene (cups, plastic food containers), can be recycled or reprocessed to be used again. Many areas of the country have recycling programs that focus on one or more of the commodity plastics that have been assigned a recycling code (see Figure 21.9). These operations have been in effect since the 1970s and have made the production of some plastics among the most efficient industrial operations today.

| polyethylene terephthalate (PETE) | Soda bottles and oven-ready food trays |
| :---: | :---: |
| high-density polyethylene (HDPE) | Bottles for milk and dishwashing liquids |
| polyvinyl chloride (PVC)
 V | Food trays, plastic wrap, bottles for mineral water and shampoo |
| low density polyethylene (LDPE) | Shopping bags and garbage bags |
| polypropylene (PP)
 PP | Margarine tubs, microwaveable food trays |
| polystyrene (PS) | Yogurt tubs, foam meat trays, egg cartons, vending cups, plastic cutlery, packaging for electronics and toys |
| any other plastics (OTHER) | Plastics that do not fall into any of the above categories One example is melamine resin (plastic plates, plastic cups) |

FIGURE 21.9 Each type of recyclable plastic is imprinted with a code for easy identification.

The name of an alkene is derived from the name of the alkane with the same number of carbon atoms. The presence of the double bond is signified by replacing the suffix -ane with the suffix -ene. The location of the double bond is identified by naming the smaller of the numbers of the carbon atoms participating in the double bond:

ethene (ethylene)

propene (propylene)

1-butene

2-butene


2165. Isomers of Alkenes -
Molecules of 1-butene and 2-butene are structural isomers; the arrangement of the atoms in these two molecules differs. As an example of arrangement differences, the first carbon atom in 1-butene is bonded to two hydrogen atoms; the first carbon atom in 2-butene is bonded to three hydrogen atoms.

The compound 2-butene and some other alkenes also form a second type of isomer called a geometric isomer. In a set of geometric isomers, the same types of atoms are attached to each other in the same order, but the geometries of the two molecules differ. Geometric isomers of alkenes differ in the orientation of the groups on either side of a $\mathbf{C}=\mathbf{C}$ bond.

Carbon atoms are free to rotate around a single bond but not around a double bond; a double bond is rigid.

This makes it possible to have two isomers of 2-butene, one with both methyl groups on the same side of the double bond and one with the methyl groups on opposite sides. When structures of butene are drawn with $120^{\circ}$ bond angles around the $s p^{2}$-hybridized carbon atoms participating in the double bond, the isomers are apparent. The 2-butene isomer in which the two methyl groups are on the same side is called a cis-isomer; the one in which the two methyl groups are on opposite sides is called a trans-isomer (Figure 21.10). The different geometries produce different physical properties, such as boiling point, that may make separation of the isomers possible:

1-butene

cis isomer

trans isomer


2165. Isomers of Alkenes -
1-butene

cis isomer

trans isomer

FIGURE 21.10 These molecular models show the structural and geometric isomers of butene.
Alkenes are much more reactive than alkanes because the $\mathrm{C}=\mathrm{C}$ moiety is a reactive functional group. $\mathrm{A} \pi$ bond, being a weaker bond, is disrupted much more easily than a $\sigma$ bond. Thus, alkenes undergo a characteristic reaction in which the $\pi$ bond is broken and replaced by two $\sigma$ bonds. This reaction is called an addition reaction. The hybridization of the carbon atoms in the double bond in an alkene changes from $s p^{2}$ to $s p^{3}$ during an addition reaction. For example, halogens add to the double bond in an alkene instead of replacing hydrogen, as occurs in an alkane:


2167. Alkene Reactivity and Naming -
Provide the IUPAC names for the reactant and product of the halogenation reaction shown here:


2168. Solution -
The reactant is a five-carbon chain that contains a carbon-carbon double bond, so the base name will be
pentene. We begin counting at the end of the chain closest to the double bond-in this case, from the left-the double bond spans carbons 2 and 3, so the name becomes 2-pentene. Since there are two carbon-containing groups attached to the two carbon atoms in the double bond-and they are on the same side of the double bond-this molecule is the cis-isomer, making the name of the starting alkene cis-2-pentene. The product of the halogenation reaction will have two chlorine atoms attached to the carbon atoms that were a part of the carbon-carbon double bond:

This molecule is now a substituted alkane and will be named as such. The base of the name will be pentane. We will count from the end that numbers the carbon atoms where the chlorine atoms are attached as 2 and 3, making the name of the product 2,3-dichloropentane.


2169. Check Your Learning -
Provide names for the reactant and product of the reaction shown:


2170. Answer: -
reactant: cis-3-hexene product: 3,4-dichlorohexane


2171. Alkynes -
Hydrocarbon molecules with one or more triple bonds are called alkynes; they make up another series of unsaturated hydrocarbons. Two carbon atoms joined by a triple bond are bound together by one $\sigma$ bond and two $\pi$ bonds. The $s p$-hybridized carbons involved in the triple bond have bond angles of $180^{\circ}$, giving these types of bonds a linear, rod-like shape.

The simplest member of the alkyne series is ethyne, $\mathrm{C}_{2} \mathrm{H}_{2}$, commonly called acetylene. The Lewis structure for ethyne, a linear molecule, is:

```
H—C三C—H
ethyne (acetylene)
```

The IUPAC nomenclature for alkynes is similar to that for alkenes except that the suffix -yne is used to indicate a triple bond in the chain. For example, $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CH}$ is called 1-butyne.


2173. Structure of Alkynes -
Describe the geometry and hybridization of the carbon atoms in the following molecule:


2174. Solution -
Carbon atoms 1 and 4 have four single bonds and are thus tetrahedral with $s p^{3}$ hybridization. Carbon atoms 2 and 3 are involved in the triple bond, so they have linear geometries and would be classified as $s p$ hybrids.


2175. Check Your Learning -
Identify the hybridization and bond angles at the carbon atoms in the molecule shown:


2176. Answer: -
carbon 1: $s p, 180^{\circ}$; carbon 2: $s p, 180^{\circ}$; carbon 3: $s p^{2}, 120^{\circ}$; carbon 4: $s p^{2}, 120^{\circ}$; carbon 5: $s p^{3}, 109.5^{\circ}$

Chemically, the alkynes are similar to the alkenes. Since the $\mathrm{C} \equiv \mathrm{C}$ functional group has two $\pi$ bonds, alkynes typically react even more readily, and react with twice as much reagent in addition reactions. The reaction of acetylene with bromine is a typical example:

Acetylene and the other alkynes also burn readily. An acetylene torch takes advantage of the high heat of combustion for acetylene.


2177. Aromatic Hydrocarbons -
Benzene, $\mathrm{C}_{6} \mathrm{H}_{6}$, is the simplest member of a large family of hydrocarbons, called aromatic hydrocarbons. These compounds contain ring structures and exhibit bonding that must be described using the resonance hybrid concept of valence bond theory or the delocalization concept of molecular orbital theory. (To review these concepts, refer to the earlier chapters on chemical bonding). The resonance structures for benzene, $\mathrm{C}_{6} \mathrm{H}_{6}$, are:

Valence bond theory describes the benzene molecule and other planar aromatic hydrocarbon molecules as hexagonal rings of $s p^{2}$-hybridized carbon atoms with the unhybridized $p$ orbital of each carbon atom perpendicular to the plane of the ring. Three valence electrons in the $s p^{2}$ hybrid orbitals of each carbon atom and the valence electron of each hydrogen atom form the framework of $\sigma$ bonds in the benzene molecule. The fourth valence electron of each carbon atom is shared with an adjacent carbon atom in their unhybridized $p$ orbitals to yield the $\pi$ bonds. Benzene does not, however, exhibit the characteristics typical of an alkene. Each of the six bonds between its carbon atoms is equivalent and exhibits properties that are intermediate between those of a $\mathrm{C}-\mathrm{C}$ single bond and a $\mathrm{C}=\mathrm{C}$ double bond. To represent this unique bonding, structural formulas for benzene and its derivatives are typically drawn with single bonds between the carbon atoms and a circle within the ring as shown in Figure 21.11.

FIGURE 21.11 This condensed formula shows the unique bonding structure of benzene.
There are many derivatives of benzene. The hydrogen atoms can be replaced by many different substituents. Aromatic compounds more readily undergo substitution reactions than addition reactions; replacement of one
of the hydrogen atoms with another substituent will leave the delocalized double bonds intact. The following are typical examples of substituted benzene derivatives:

toluene

xylene

styrene

Toluene and xylene are important solvents and raw materials in the chemical industry. Styrene is used to produce the polymer polystyrene.


2179. Structure of Aromatic Hydrocarbons -
One possible isomer created by a substitution reaction that replaces a hydrogen atom attached to the aromatic ring of toluene with a chlorine atom is shown here. Draw two other possible isomers in which the chlorine atom replaces a different hydrogen atom attached to the aromatic ring:


2180. Solution -
Since the six-carbon ring with alternating double bonds is necessary for the molecule to be classified as aromatic, appropriate isomers can be produced only by changing the positions of the chloro-substituent relative to the methyl-substituent:

Check Your Learning
Draw three isomers of a six-membered aromatic ring compound substituted with two bromines.

Answer:


2181. LEARNING OBJECTIVES -
By the end of this section, you will be able to:

- Describe the structure and properties of alcohols
- Describe the structure and properties of ethers
- Name and draw structures for alcohols and ethers

In this section, we will learn about alcohols and ethers.


2182. Alcohols -
Incorporation of an oxygen atom into carbon- and hydrogen-containing molecules leads to new functional groups and new families of compounds. When the oxygen atom is attached by single bonds, the molecule is either an alcohol or ether.

Alcohols are derivatives of hydrocarbons in which an - OH group has replaced a hydrogen atom. Although all alcohols have one or more hydroxyl (-OH) functional groups, they do not behave like bases such as NaOH and $\mathrm{KOH} . \mathrm{NaOH}$ and KOH are ionic compounds that contain $\mathrm{OH}^{-}$ions. Alcohols are covalent molecules; the - OH group in an alcohol molecule is attached to a carbon atom by a covalent bond.

Ethanol, $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}$, also called ethyl alcohol, is a particularly important alcohol for human use. Ethanol is the alcohol produced by some species of yeast that is found in wine, beer, and distilled drinks. It has long been prepared by humans harnessing the metabolic efforts of yeasts in fermenting various sugars:

Large quantities of ethanol are synthesized from the addition reaction of water with ethylene using an acid as a catalyst:

Alcohols containing two or more hydroxyl groups can be made. Examples include 1,2-ethanediol (ethylene glycol, used in antifreeze) and 1,2,3-propanetriol (glycerine, used as a solvent for cosmetics and medicines):

1,2-ethanediol

1,2,3-propanetriol

Naming Alcohols
The name of an alcohol comes from the hydrocarbon from which it was derived. The final -e in the name of the hydrocarbon is replaced by -ol, and the carbon atom to which the -OH group is bonded is indicated by a number placed before the name. ${ }^{-5}$


2184. Naming Alcohols -
Consider the following example. How should it be named?


2185. Solution -
The carbon chain contains five carbon atoms. If the hydroxyl group was not present, we would have named this molecule pentane. To address the fact that the hydroxyl group is present, we change the ending of the name to -ol. In this case, since the - OH is attached to carbon 2 in the chain, we would name this molecule 2-pentanol.


2186. Check Your Learning -
Name the following molecule:

Answer:
2-methyl-2-pentanol


2187. Ethers -
Ethers are compounds that contain the functional group -O-. Ethers do not have a designated suffix like the other types of molecules we have named so far. In the IUPAC system, the oxygen atom and the smaller carbon branch are named as an alkoxy substituent and the remainder of the molecule as the base chain, as in alkanes. As shown in the following compound, the red symbols represent the smaller alkyl group and the oxygen atom, which would be named "methoxy." The larger carbon branch would be ethane, making the molecule methoxyethane. Many ethers are referred to with common names instead of the IUPAC system names. For common names, the two branches connected to the oxygen atom are named separately and followed by "ether." The common name for the compound shown in Example 21.9 is ethylmethyl ether:

[^16]

EXAMPLE 21.9


2188. Naming Ethers -
Provide the IUPAC and common name for the ether shown here:


2189. Solution -
IUPAC: The molecule is made up of an ethoxy group attached to an ethane chain, so the IUPAC name would be ethoxyethane.

Common: The groups attached to the oxygen atom are both ethyl groups, so the common name would be diethyl ether.


2190. Check Your Learning -
Provide the IUPAC and common name for the ether shown:


2191. Answer: -
IUPAC: 2-methoxypropane; common: isopropylmethyl ether

Ethers can be obtained from alcohols by the elimination of a molecule of water from two molecules of the alcohol. For example, when ethanol is treated with a limited amount of sulfuric acid and heated to $140^{\circ} \mathrm{C}$, diethyl ether and water are formed:

In the general formula for ethers, $R-\mathbf{O}-\mathrm{R}$, the hydrocarbon groups ( R ) may be the same or different. Diethyl ether, the most widely used compound of this class, is a colorless, volatile liquid that is highly flammable. It was first used in 1846 as an anesthetic, but better anesthetics have now largely taken its place. Diethyl ether and other ethers are presently used primarily as solvents for gums, fats, waxes, and resins. Tertiary-butyl methyl ether, $\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{OCH}_{3}$ (abbreviated MTBE-italicized portions of names are not counted when ranking the groups alphabetically-so butyl comes before methyl in the common name), is used as an additive for gasoline. MTBE belongs to a group of chemicals known as oxygenates due to their capacity to increase the oxygen content of gasoline.


2192. LINK TO LEARNING -
Want more practice naming ethers? This brief video review (http://openstax.org/l/16ethers) summarizes the nomenclature for ethers.


2194. Carbohydrates and Diabetes -
Carbohydrates are large biomolecules made up of carbon, hydrogen, and oxygen. The dietary forms of carbohydrates are foods rich in these types of molecules, like pastas, bread, and candy. The name "carbohydrate" comes from the formula of the molecules, which can be described by the general formula $\mathrm{C}_{\mathrm{m}}\left(\mathrm{H}_{2} \mathrm{O}\right)_{\mathrm{n}}$, which shows that they are in a sense "carbon and water" or "hydrates of carbon." In many cases, $m$ and $n$ have the same value, but they can be different. The smaller carbohydrates are generally referred to as "sugars," the biochemical term for this group of molecules is "saccharide" from the Greek word for sugar (Figure 21.12). Depending on the number of sugar units joined together, they may be classified as monosaccharides (one sugar unit), disaccharides (two sugar units), oligosaccharides (a few sugars), or polysaccharides (the polymeric version of sugars-polymers were described in the feature box earlier in this chapter on recycling plastics). The scientific names of sugars can be recognized by the suffix -ose at the end of the name (for instance, fruit sugar is a monosaccharide called "fructose" and milk sugar is a disaccharide called lactose composed of two monosaccharides, glucose and galactose, connected together). Sugars contain some of the functional groups we have discussed: Note the alcohol groups present in the structures and how monosaccharide units are linked to form a disaccharide by formation of an ether.

fructos

FIGURE 21.12 The illustrations show the molecular structures of fructose, a five-carbon monosaccharide, and of lactose, a disaccharide composed of two isomeric, six-carbon sugars.


2194. Carbohydrates and Diabetes -
FIGURE 21.12 The illustrations show the molecular structures of fructose, a five-carbon monosaccharide, and of lactose, a disaccharide composed of two isomeric, six-carbon sugars.

Organisms use carbohydrates for a variety of functions. Carbohydrates can store energy, such as the polysaccharides glycogen in animals or starch in plants. They also provide structural support, such as the polysaccharide cellulose in plants and the modified polysaccharide chitin in fungi and animals. The sugars ribose and deoxyribose are components of the backbones of RNA and DNA, respectively. Other sugars play key roles in the function of the immune system, in cell-cell recognition, and in many other biological roles.

Diabetes is a group of metabolic diseases in which a person has a high sugar concentration in their blood (Figure 21.13). Diabetes may be caused by insufficient insulin production by the pancreas or by the body's cells not responding properly to the insulin that is produced. In a healthy person, insulin is produced when it is needed and functions to transport glucose from the blood into the cells where it can be used for energy.

The long-term complications of diabetes can include loss of eyesight, heart disease, and kidney failure.
In 2013, it was estimated that approximately $3.3 \%$ of the world's population ( $\sim 380$ million people) suffered from diabetes, resulting in over a million deaths annually. Prevention involves eating a healthy diet, getting plenty of exercise, and maintaining a normal body weight. Treatment involves all of these lifestyle practices and may require injections of insulin.


2194. Carbohydrates and Diabetes -
Even after treatment protocols were introduced, the need to continually monitor their glucose levels posed a challenge for people with diabetes. The first tests required a doctor or lab, and therefore limited access and frequency. Eventually, researchers developed small tablets that would react to the presence of glucose in urine, but these still required a relatively complex process. Chemist Helen Free, who was working on improvements to the tablets, conceived a simpler device: a small test strip. With her husband and research partner, Alfred Free, she produced the first such product for measuring glucose; soon after, she expanded the technology to provide test strips for other compounds and conditions. While very recent advances (such as breath tests, discussed earlier in the text) have shown promise in replacing test strips, they have been widely used for decades and remain a primary method today.

FIGURE 21.13 Diabetes is a disease characterized by high concentrations of glucose in the blood. Treating diabetes involves making lifestyle changes, monitoring blood-sugar levels, and sometimes insulin injections. (credit: "Blausen Medical Communications"/Wikimedia Commons)


2195. LEARNING OBJECTIVES -
By the end of this section, you will be able to:

- Describe the structure and properties of aldehydes, ketones, carboxylic acids and esters

Another class of organic molecules contains a carbon atom connected to an oxygen atom by a double bond, commonly called a carbonyl group. The trigonal planar carbon in the carbonyl group can attach to two other substituents leading to several subfamilies (aldehydes, ketones, carboxylic acids and esters) described in this section.


2196. Aldehydes and Ketones -
Both aldehydes and ketones contain a carbonyl group, a functional group with a carbon-oxygen double bond. The names for aldehyde and ketone compounds are derived using similar nomenclature rules as for alkanes and alcohols, and include the class-identifying suffixes -al and -one, respectively:

In an aldehyde, the carbonyl group is bonded to at least one hydrogen atom. In a ketone, the carbonyl group is bonded to two carbon atoms:

Functional group of an aldehyde

An aldehyde ethanal (acetaldehyde)

Functional group of a ketone

A ketone
butanone

As text, an aldehyde group is represented as -CHO ; a ketone is represented as $-\mathrm{C}(\mathrm{O})-$ or $-\mathrm{CO}-$.
In both aldehydes and ketones, the geometry around the carbon atom in the carbonyl group is trigonal planar; the carbon atom exhibits $s p^{2}$ hybridization. Two of the $s p^{2}$ orbitals on the carbon atom in the carbonyl group are used to form $\sigma$ bonds to the other carbon or hydrogen atoms in a molecule. The remaining $s p^{2}$ hybrid orbital forms a $\sigma$ bond to the oxygen atom. The unhybridized $p$ orbital on the carbon atom in the carbonyl group overlaps a $p$ orbital on the oxygen atom to form the $\pi$ bond in the double bond.

Like the $\mathrm{C}=\mathrm{O}$ bond in carbon dioxide, the $\mathrm{C}=\mathrm{O}$ bond of a carbonyl group is polar (recall that oxygen is significantly more electronegative than carbon, and the shared electrons are pulled toward the oxygen atom and away from the carbon atom). Many of the reactions of aldehydes and ketones start with the reaction between a Lewis base and the carbon atom at the positive end of the polar $\mathrm{C}=\mathrm{O}$ bond to yield an unstable intermediate that subsequently undergoes one or more structural rearrangements to form the final product (Figure 21.14).

FIGURE 21.14 The carbonyl group is polar, and the geometry of the bonds around the central carbon is trigonal planar.


2196. Aldehydes and Ketones -
FIGURE 21.14 The carbonyl group is polar, and the geometry of the bonds around the central carbon is trigonal planar.

The importance of molecular structure in the reactivity of organic compounds is illustrated by the reactions that produce aldehydes and ketones. We can prepare a carbonyl group by oxidation of an alcohol-for organic molecules, oxidation of a carbon atom is said to occur when a carbon-hydrogen bond is replaced by a carbonoxygen bond. The reverse reaction-replacing a carbon-oxygen bond by a carbon-hydrogen bond-is a reduction of that carbon atom. Recall that oxygen is generally assigned a -2 oxidation number unless it is elemental or attached to a fluorine. Hydrogen is generally assigned an oxidation number of +1 unless it is attached to a metal. Since carbon does not have a specific rule, its oxidation number is determined algebraically by factoring the atoms it is attached to and the overall charge of the molecule or ion. In general, a carbon atom attached to an oxygen atom will have a more positive oxidation number and a carbon atom attached to a hydrogen atom will have a more negative oxidation number. This should fit nicely with your understanding of the polarity of $\mathrm{C}-\mathrm{O}$ and $\mathrm{C}-\mathrm{H}$ bonds. The other reagents and possible products of these reactions are beyond the scope of this chapter, so we will focus only on the changes to the carbon atoms:

EXAMPLE 21.10


2197. Oxidation and Reduction in Organic Chemistry -
Methane represents the completely reduced form of an organic molecule that contains one carbon atom. Sequentially replacing each of the carbon-hydrogen bonds with a carbon-oxygen bond would lead to an alcohol, then an aldehyde, then a carboxylic acid (discussed later), and, finally, carbon dioxide:

$$
\mathrm{CH}_{4} \longrightarrow \mathrm{CH}_{3} \mathrm{OH} \longrightarrow \mathrm{CH}_{2} \mathrm{O} \longrightarrow \mathrm{HCO}_{2} \mathrm{H} \longrightarrow \mathrm{CO}_{2}
$$

What are the oxidation numbers for the carbon atoms in the molecules shown here?


2198. Solution -
In this example, we can calculate the oxidation number (review the chapter on oxidation-reduction reactions if necessary) for the carbon atom in each case (note how this would become difficult for larger molecules with additional carbon atoms and hydrogen atoms, which is why organic chemists use the definition dealing with replacing $\mathrm{C}-\mathrm{H}$ bonds with $\mathrm{C}-\mathrm{O}$ bonds described). For $\mathrm{CH}_{4}$, the carbon atom carries a -4 oxidation number (the hydrogen atoms are assigned oxidation numbers of +1 and the carbon atom balances that by having an oxidation number of -4 ). For the alcohol (in this case, methanol), the carbon atom has an oxidation number of -2 (the oxygen atom is assigned -2 , the four hydrogen atoms each are assigned +1 , and the carbon atom balances the sum by having an oxidation number of -2 ; note that compared to the carbon atom in $\mathrm{CH}_{4}$, this carbon atom has lost two electrons so it was oxidized); for the aldehyde, the carbon atom's oxidation number is 0 ( -2 for the oxygen atom and +1 for each hydrogen atom already balances to 0 , so the oxidation number for the carbon atom is 0 ); for the carboxylic acid, the carbon atom's oxidation number is +2 (two oxygen atoms each at -2 and two hydrogen atoms at +1 ); and for carbon dioxide, the carbon atom's oxidation number is +4 (here, the carbon atom needs to balance the -4 sum from the two oxygen atoms).


2199. Check Your Learning -
Indicate whether the marked carbon atoms in the three molecules here are oxidized or reduced relative to the marked carbon atom in ethanol:

There is no need to calculate oxidation states in this case; instead, just compare the types of atoms bonded to the marked carbon atoms:

(a)

(b)

(c)


2200. Answer: -
(a) reduced (bond to oxygen atom replaced by bond to hydrogen atom); (b) oxidized (one bond to hydrogen atom replaced by one bond to oxygen atom); (c) oxidized ( 2 bonds to hydrogen atoms have been replaced by bonds to an oxygen atom)

Aldehydes are commonly prepared by the oxidation of alcohols whose - OH functional group is located on the carbon atom at the end of the chain of carbon atoms in the alcohol:

Alcohols that have their -OH groups in the middle of the chain are necessary to synthesize a ketone, which requires the carbonyl group to be bonded to two other carbon atoms:

An alcohol with its -OH group bonded to a carbon atom that is bonded to no or one other carbon atom will form an aldehyde. An alcohol with its -OH group attached to two other carbon atoms will form a ketone. If three carbons are attached to the carbon bonded to the -OH , the molecule will not have a $\mathrm{C}-\mathrm{H}$ bond to be replaced, so it will not be susceptible to oxidation.

Formaldehyde, an aldehyde with the formula HCHO , is a colorless gas with a pungent and irritating odor. It is sold in an aqueous solution called formalin, which contains about $37 \%$ formaldehyde by weight. Formaldehyde causes coagulation of proteins, so it kills bacteria (and any other living organism) and stops many of the biological processes that cause tissue to decay. Thus, formaldehyde is used for preserving tissue specimens and embalming bodies. It is also used to sterilize soil or other materials. Formaldehyde is used in the manufacture of Bakelite, a hard plastic having high chemical and electrical resistance.

Dimethyl ketone, $\mathrm{CH}_{3} \mathrm{COCH}_{3}$, commonly called acetone, is the simplest ketone. It is made commercially by fermenting corn or molasses, or by oxidation of 2-propanol. Acetone is a colorless liquid. Among its many uses are as a solvent for lacquer (including fingernail polish), cellulose acetate, cellulose nitrate, acetylene, plastics, and varnishes; as a paint and varnish remover; and as a solvent in the manufacture of pharmaceuticals and chemicals.


2201. Carboxylic Acids and Esters -
The odor of vinegar is caused by the presence of acetic acid, a carboxylic acid, in the vinegar. The odor of ripe bananas and many other fruits is due to the presence of esters, compounds that can be prepared by the reaction of a carboxylic acid with an alcohol. Because esters do not have hydrogen bonds between molecules, they have lower vapor pressures than the alcohols and carboxylic acids from which they are derived (see Figure 21.15).

Wintergreen methyl salicylate

Honey methyl phenylacetate

Strawberry ethyl methylphenylglycidate

FIGURE 21.15 Esters are responsible for the odors associated with various plants and their fruits.
Both carboxylic acids and esters contain a carbonyl group with a second oxygen atom bonded to the carbon atom in the carbonyl group by a single bond. In a carboxylic acid, the second oxygen atom also bonds to a hydrogen atom. In an ester, the second oxygen atom bonds to another carbon atom. The names for carboxylic acids and esters include prefixes that denote the lengths of the carbon chains in the molecules and are derived following nomenclature rules similar to those for inorganic acids and salts (see these examples):

ethanoic acid
(acetic acid)

methyl ethanoate
(methyl acetate)

The functional groups for an acid and for an ester are shown in red in these formulas.
The hydrogen atom in the functional group of a carboxylic acid will react with a base to form an ionic salt:

Carboxylic acids are weak acids (see the chapter on acids and bases), meaning they are not 100\% ionized in water. Generally only about $1 \%$ of the molecules of a carboxylic acid dissolved in water are ionized at any given time. The remaining molecules are undissociated in solution.

We prepare carboxylic acids by the oxidation of aldehydes or alcohols whose - OH functional group is located on the carbon atom at the end of the chain of carbon atoms in the alcohol:


2201. Carboxylic Acids and Esters -
We prepare carboxylic acids by the oxidation of aldehydes or alcohols whose - OH functional group is located on the carbon atom at the end of the chain of carbon atoms in the alcohol:

Esters are produced by the reaction of acids with alcohols. For example, the ester ethyl acetate, $\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}$, is formed when acetic acid reacts with ethanol:

The simplest carboxylic acid is formic acid, $\mathrm{HCO}_{2} \mathrm{H}$, known since 1670. Its name comes from the Latin word formicus, which means "ant"; it was first isolated by the distillation of red ants. It is partially responsible for the pain and irritation of ant and wasp stings, and is responsible for a characteristic odor of ants that can be sometimes detected in their nests.

Acetic acid, $\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}$, constitutes $3-6 \%$ vinegar. Cider vinegar is produced by allowing apple juice to ferment without oxygen present. Yeast cells present in the juice carry out the fermentation reactions. The fermentation reactions change the sugar present in the juice to ethanol, then to acetic acid. Pure acetic acid has a penetrating odor and produces painful burns. It is an excellent solvent for many organic and some inorganic compounds, and it is essential in the production of cellulose acetate, a component of many synthetic fibers such as rayon.


2201. Carboxylic Acids and Esters -
The distinctive and attractive odors and flavors of many flowers, perfumes, and ripe fruits are due to the presence of one or more esters (Figure 21.16). Among the most important of the natural esters are fats (such as lard, tallow, and butter) and oils (such as linseed, cottonseed, and olive oils), which are esters of the trihydroxyl alcohol glycerine, $\mathrm{C}_{3} \mathrm{H}_{5}(\mathrm{OH})_{3}$, with large carboxylic acids, such as palmitic acid, $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{14} \mathrm{CO}_{2} \mathrm{H}$, stearic acid, $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{16} \mathrm{CO}_{2} \mathrm{H}$, and oleic acid, $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{7} \mathrm{CH}=\mathrm{CH}\left(\mathrm{CH}_{2}\right)_{7} \mathrm{CO}_{2} \mathrm{H}$. Oleic acid is an unsaturated acid; it contains a $\mathrm{C}=\mathrm{C}$ double bond. Palmitic and stearic acids are saturated acids that contain no double or triple bonds.

FIGURE 21.16 Over 350 different volatile molecules (many members of the ester family) have been identified in strawberries. (credit: Rebecca Siegel)


2202. LEARNING OBJECTIVES -
By the end of this section, you will be able to:

- Describe the structure and properties of an amine
- Describe the structure and properties of an amide

Amines are molecules that contain carbon-nitrogen bonds. The nitrogen atom in an amine has a lone pair of electrons and three bonds to other atoms, either carbon or hydrogen. Various nomenclatures are used to derive names for amines, but all involve the class-identifying suffix -ine as illustrated here for a few simple examples:

In some amines, the nitrogen atom replaces a carbon atom in an aromatic hydrocarbon. Pyridine (Figure 21.17) is one such heterocyclic amine. A heterocyclic compound contains atoms of two or more different elements in its ring structure.

FIGURE 21.17 The illustration shows one of the resonance structures of pyridine.


2204. DNA in Forensics and Paternity -
The genetic material for all living things is a polymer of four different molecules, which are themselves a combination of three subunits. The genetic information, the code for developing an organism, is contained in the specific sequence of the four molecules, similar to the way the letters of the alphabet can be sequenced to form words that convey information. The information in a DNA sequence is used to form two other types of polymers, one of which are proteins. The proteins interact to form a specific type of organism with individual characteristics.

A genetic molecule is called DNA, which stands for deoxyribonucleic acid. The four molecules that make up DNA are called nucleotides. Each nucleotide consists of a single- or double-ringed molecule containing nitrogen, carbon, oxygen, and hydrogen called a nitrogenous base. Each base is bonded to a five-carbon sugar called deoxyribose. The sugar is in turn bonded to a phosphate group ( $-\mathrm{PO}_{4}{ }^{3-}$ ) When new DNA is made, a polymerization reaction occurs that binds the phosphate group of one nucleotide to the sugar group of a second nucleotide. The nitrogenous bases of each nucleotide stick out from this sugar-phosphate backbone. DNA is actually formed from two such polymers coiled around each other and held together by hydrogen bonds between the nitrogenous bases. Thus, the two backbones are on the outside of the coiled pair of strands, and the bases are on the inside. The shape of the two strands wound around each other is called a double helix (see Figure 21.18).

It probably makes sense that the sequence of nucleotides in the DNA of a cat differs from those of a dog. But it is also true that the sequences of the DNA in the cells of two individual pugs differ. Likewise, the sequences of DNA in you and a sibling differ (unless your sibling is an identical twin), as do those between you and an unrelated individual. However, the DNA sequences of two related individuals are more similar than the sequences of two unrelated individuals, and these similarities in sequence can be observed in various ways.

This is the principle behind DNA fingerprinting, which is a method used to determine whether two DNA samples came from related (or the same) individuals or unrelated individuals.

(a)

(b)

(c)


2204. DNA in Forensics and Paternity -
This is the principle behind DNA fingerprinting, which is a method used to determine whether two DNA samples came from related (or the same) individuals or unrelated individuals.

(a)

(b)

(c)

FIGURE 21.18 DNA is an organic molecule and the genetic material for all living organisms. (a) DNA is a double helix consisting of two single DNA strands hydrogen bonded together at each nitrogenous base. (b) This detail shows the hydrogen bonding (dotted lines) between nitrogenous bases on each DNA strand and the way in which each nucleotide is joined to the next, forming a backbone of sugars and phosphate groups along each strand. (c) This detail shows the structure of one of the four nucleotides that makes up the DNA polymer. Each nucleotide consists of a nitrogenous base (a double-ring molecule, in this case), a five-carbon sugar (deoxyribose), and a phosphate group.

Using similarities in sequences, technicians can determine whether a man is the father of a child (the identity of the mother is rarely in doubt, except in the case of an adopted child and a potential birth mother). Likewise, forensic geneticists can determine whether a crime scene sample of human tissue, such as blood or skin cells, contains DNA that matches exactly the DNA of a suspect.


2205. LINK TO LEARNING -
Watch this video animation (http://openstax.org/l/16dnapackaging) of how DNA is packaged for a visual lesson
in its structure.

Like ammonia, amines are weak bases due to the lone pair of electrons on their nitrogen atoms:

The basicity of an amine's nitrogen atom plays an important role in much of the compound's chemistry. Amine functional groups are found in a wide variety of compounds, including natural and synthetic dyes, polymers, vitamins, and medications such as penicillin and codeine. They are also found in many molecules essential to life, such as amino acids, hormones, neurotransmitters, and DNA.


2207. Addictive Alkaloids -
Since ancient times, plants have been used for medicinal purposes. One class of substances, called alkaloids, found in many of these plants has been isolated and found to contain cyclic molecules with an amine functional group. These amines are bases. They can react with $\mathrm{H}_{3} \mathrm{O}^{+}$in a dilute acid to form an ammonium salt, and this property is used to extract them from the plant:

$$
\mathrm{R}_{3} \mathrm{~N}+\mathrm{H}_{3} \mathrm{O}^{+}+\mathrm{Cl}^{-} \longrightarrow\left[\mathrm{R}_{3} \mathrm{NH}^{+}\right] \mathrm{Cl}^{-}+\mathrm{H}_{2} \mathrm{O}
$$

The name alkaloid means "like an alkali." Thus, an alkaloid reacts with acid. The free compound can be recovered after extraction by reaction with a base:

$$
\left[\mathrm{R}_{3} \mathrm{NH}^{+}\right] \mathrm{Cl}^{-}+\mathrm{OH}^{-} \longrightarrow \mathrm{R}_{3} \mathrm{~N}+\mathrm{H}_{2} \mathrm{O}+\mathrm{Cl}^{-}
$$

The structures of many naturally occurring alkaloids have profound physiological and psychotropic effects in humans. Examples of these drugs include nicotine, morphine, codeine, and heroin. The plant produces these substances, collectively called secondary plant compounds, as chemical defenses against the numerous pests that attempt to feed on the plant:

nicotine

codeine

morphine

heroin


2207. Addictive Alkaloids -
nicotine

codeine

morphine

heroin

In these diagrams, as is common in representing structures of large organic compounds, carbon atoms in the rings and the hydrogen atoms bonded to them have been omitted for clarity. The solid wedges indicate bonds that extend out of the page. The dashed wedges indicate bonds that extend into the page. Notice that small changes to a part of the molecule change the properties of morphine, codeine, and heroin. Morphine, a strong narcotic used to relieve pain, contains two hydroxyl functional groups, located at the bottom of the molecule in this structural formula. Changing one of these hydroxyl groups to a methyl ether group forms codeine, a less potent drug used as a local anesthetic. If both hydroxyl groups are converted to esters of acetic acid, the powerfully addictive drug heroin results (Figure 21.19).

FIGURE 21.19 Poppies can be used in the production of opium, a plant latex that contains morphine from which other opiates, such as heroin, can be synthesized. (credit: Karen Roe)

Amides are molecules that contain nitrogen atoms connected to the carbon atom of a carbonyl group. Like amines, various nomenclature rules may be used to name amides, but all include use of the class-specific
suffix -amide:

acetamide

Amides can be produced when carboxylic acids react with amines or ammonia in a process called amidation. A water molecule is eliminated from the reaction, and the amide is formed from the remaining pieces of the carboxylic acid and the amine (note the similarity to formation of an ester from a carboxylic acid and an alcohol discussed in the previous section):

The reaction between amines and carboxylic acids to form amides is biologically important. It is through this reaction that amino acids (molecules containing both amine and carboxylic acid substituents) link together in a polymer to form proteins.