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CnH2n+2 + (n + ) O2 → (n + 1) H2O + n CO2 |
or CnH2n+2 + () O2 → (n + 1) H2O + n CO2 |
In the absence of sufficient oxygen, carbon monoxide or even soot can be formed, as shown below: |
CnH2n+2 + (n + ) O2 → (n + 1) H2O + n CO |
CnH2n+2 + (n + ) O2 → (n + 1) H2O + n C |
For example, methane: |
2 CH4 + 3 O2 → 4 H2O + 2 CO |
CH4 + O2 → 2 H2O + C |
See the alkane heat of formation table for detailed data. |
The standard enthalpy change of combustion, ΔcH⊖, for alkanes increases by about 650 kJ/mol per CH2 group. Branched-chain alkanes have lower values of ΔcH⊖ than straight-chain alkanes of the same number of carbon atoms, and so can be seen to be somewhat more stable. |
Reactions with halogens |
Alkanes react with halogens in a so-called free radical halogenation reaction. The hydrogen atoms of the alkane are progressively replaced by halogen atoms. Free radicals are the reactive species that participate in the reaction, which usually leads to a mixture of products. The reaction is highly exothermic with halog... |
These reactions are an important industrial route to halogenated hydrocarbons. There are three steps: |
Initiation the halogen radicals form by homolysis. Usually, energy in the form of heat or light is required. |
Chain reaction or Propagation then takes place—the halogen radical abstracts a hydrogen from the alkane to give an alkyl radical. This reacts further. |
Chain termination where the radicals recombine. |
Experiments have shown that all halogenation produces a mixture of all possible isomers, indicating that all hydrogen atoms are susceptible to reaction. The mixture produced, however, is not a statistical mixture: Secondary and tertiary hydrogen atoms are preferentially replaced due to the greater stability of secondar... |
Cracking |
Cracking breaks larger molecules into smaller ones. This can be done with a thermal or catalytic method. The thermal cracking process follows a homolytic mechanism with formation of free radicals. The catalytic cracking process involves the presence of acid catalysts (usually solid acids such as silica-alumina and zeol... |
Isomerization and reformation |
Dragan and his colleague were the first to report about isomerization in alkanes. Isomerization and reformation are processes in which straight-chain alkanes are heated in the presence of a platinum catalyst. In isomerization, the alkanes become branched-chain isomers. In other words, it does not lose any carbons or hy... |
Other reactions |
Alkanes will react with steam in the presence of a nickel catalyst to give hydrogen. Alkanes can be chlorosulfonated and nitrated, although both reactions require special conditions. The fermentation of alkanes to carboxylic acids is of some technical importance. In the Reed reaction, sulfur dioxide, chlorine and light... |
Occurrence |
Occurrence of alkanes in the Universe |
Alkanes form a small portion of the atmospheres of the outer gas planets such as Jupiter (0.1% methane, 2 ppm ethane), Saturn (0.2% methane, 5 ppm ethane), Uranus (1.99% methane, 2.5 ppm ethane) and Neptune (1.5% methane, 1.5 ppm ethane). Titan (1.6% methane), a satellite of Saturn, was examined by the Huygens probe, w... |
Occurrence of alkanes on Earth |
Traces of methane gas (about 0.0002% or 1745 ppb) occur in the Earth's atmosphere, produced primarily by methanogenic microorganisms, such as Archaea in the gut of ruminants. |
The most important commercial sources for alkanes are natural gas and oil. Natural gas contains primarily methane and ethane, with some propane and butane: oil is a mixture of liquid alkanes and other hydrocarbons. These hydrocarbons were formed when marine animals and plants (zooplankton and phytoplankton) died and sa... |
C6H12O6 → 3 CH4 + 3 CO2 |
These hydrocarbon deposits, collected in porous rocks trapped beneath impermeable cap rocks, comprise commercial oil fields. They have formed over millions of years and once exhausted cannot be readily replaced. The depletion of these hydrocarbons reserves is the basis for what is known as the energy crisis. |
Methane is also present in what is called biogas, produced by animals and decaying matter, which is a possible renewable energy source. |
Alkanes have a low solubility in water, so the content in the oceans is negligible; however, at high pressures and low temperatures (such as at the bottom of the oceans), methane can co-crystallize with water to form a solid methane clathrate (methane hydrate). Although this cannot be commercially exploited at the pres... |
Biological occurrence |
Acyclic alkanes occur in nature in various ways. |
Bacteria and archaea |
Certain types of bacteria can metabolize alkanes: they prefer even-numbered carbon chains as they are easier to degrade than odd-numbered chains. |
On the other hand, certain archaea, the methanogens, produce large quantities of methane by the metabolism of carbon dioxide or other oxidized organic compounds. The energy is released by the oxidation of hydrogen: |
CO2 + 4 H2 → CH4 + 2 H2O |
Methanogens are also the producers of marsh gas in wetlands. The methane output of cattle and other herbivores, which can release 30 to 50 gallons per day, and of termites, is also due to methanogens. They also produce this simplest of all alkanes in the intestines of humans. Methanogenic archaea are, hence, at the end... |
Fungi and plants |
Alkanes also play a role, if a minor role, in the biology of the three eukaryotic groups of organisms: fungi, plants and animals. Some specialized yeasts, e.g., Candida tropicale, Pichia sp., Rhodotorula sp., can use alkanes as a source of carbon or energy. The fungus Amorphotheca resinae prefers the longer-chain alkan... |
In plants, the solid long-chain alkanes are found in the plant cuticle and epicuticular wax of many species, but are only rarely major constituents. They protect the plant against water loss, prevent the leaching of important minerals by the rain, and protect against bacteria, fungi, and harmful insects. The carbon cha... |
More volatile short-chain alkanes are also produced by and found in plant tissues. The Jeffrey pine is noted for producing exceptionally high levels of n-heptane in its resin, for which reason its distillate was designated as the zero point for one octane rating. Floral scents have also long been known to contain volat... |
Edible vegetable oils also typically contain small fractions of biogenic alkanes with a wide spectrum of carbon numbers, mainly 8 to 35, usually peaking in the low to upper 20s, with concentrations up to dozens of milligrams per kilogram (parts per million by weight) and sometimes over a hundred for the total alkane fr... |
Animals |
Alkanes are found in animal products, although they are less important than unsaturated hydrocarbons. One example is the shark liver oil, which is approximately 14% pristane (2,6,10,14-tetramethylpentadecane, C19H40). They are important as pheromones, chemical messenger materials, on which insects depend for communicat... |
Ecological relations |
One example, in which both plant and animal alkanes play a role, is the ecological relationship between the sand bee (Andrena nigroaenea) and the early spider orchid (Ophrys sphegodes); the latter is dependent for pollination on the former. Sand bees use pheromones in order to identify a mate; in the case of A. nigroae... |
which will be dispersed after the departure of the frustrated male to other blooms. |
Production |
Petroleum refining |
As stated earlier, the most important source of alkanes is natural gas and crude oil. Alkanes are separated in an oil refinery by fractional distillation and processed into many products. |
Fischer–Tropsch |
The Fischer–Tropsch process is a method to synthesize liquid hydrocarbons, including alkanes, from carbon monoxide and hydrogen. This method is used to produce substitutes for petroleum distillates. |
Laboratory preparation |
There is usually little need for alkanes to be synthesized in the laboratory, since they are usually commercially available. Also, alkanes are generally unreactive chemically or biologically, and do not undergo functional group interconversions cleanly. When alkanes are produced in the laboratory, it is often a side-pr... |
C4H9Li + H2O → C4H10 + LiOH |
However, at times it may be desirable to make a section of a molecule into an alkane-like functionality (alkyl group) using the above or similar methods. For example, an ethyl group is an alkyl group; when this is attached to a hydroxy group, it gives ethanol, which is not an alkane. To do so, the best-known methods ar... |
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