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Chemistry
Part II
Textbook for Class XI
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chemistry
ALL RIGHTS RESERVED
No part of this publication may be reproduced, stored in a retrieval system or
transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise wi... |
We must recognise that, given space, time and freedom, children generate new knowledge by engaging with the information passed on to them by adults. Treating the prescribed textbook as the sole basis of examination is one of the key reasons why other resources and sites of learning are ignored. Inculcating creativity a... |
Narlikar and the Chief Advisor for this book, Professor B.
L. Khandelwal for guiding the work of this committee. Several teachers contributed to the development of this textbook; we are grateful to their principals for making this possible. We are indebted to the institutions and organisations which have generously pe... |
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textbook develoPment Committee
ChairPerson , advisory GrouP For textbooks in sCienCe and mathematiCs
J.V. Narlikar, Emeritus Professor, Chairman, Advisory Committee, Inter
University Centre f... |
Gupta, Reader, School of Studies in Chemistry, Jiwaji University, GwaliorSadhna Bhargava, PGT (Chemistry), Sardar Patel Vidyalaya, Lodhi Estate, New DelhiShubha Keshwan, Headmistress, Demonstration School, Regional Institute of
Education, NCERT, Mysore
Sukhvir Singh, Reader, DESM, NCERT, New DelhiSunita Malhotra, Prof... |
We are thankful to them. We express gratitude to R.S. Sindhu, Professor (Retd. ), DESM, NCERT, New
Delhi, for editing, reviewing and refining the textbook right from the initial stage. We are also grateful to Ruchi Verma, Associate Professor, DESM, NCERT, New
Delhi; Pramila Tanwar, Associate Professor, DESM, NCERT, N... |
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Contents
Foreword iii
rationalisation oF Content in the textbook v
Unit 7 Redox Reactions 235
7.1 Classical Idea of Redox Reactions-Oxidation and Reduction Reactions 235
7.2 Redox Reactions in Terms of Electron Transfer Reactions 237
7.3 Oxidation Nu... |
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In reactions (7.1) and (7.2), the elements
magnesium and sulphur are oxidised on
account of addition of oxygen to them. Similarly, methane is oxidised owing to the addition of oxygen to it. CH
4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l) (7.3)
A careful examin... |
However, the term reduction has been broadened these days to include removal
of oxygen/electronegative element from a substance or addition of hydrogen/electropositive element to a substance. According to the definition given above,
the following are the examples of reduction processes:
2 HgO (s)
2 Hg (l) + O2 (g)... |
Simultaneously, chlorine, oxygen and sulphur are reduced because to each of these, the electropositive element sodium has been added. From our knowledge of chemical bonding we also know that sodium chloride, sodium oxide and sodium sulphide are ionic compounds and perhaps better written as Na
+Cl– (s), (Na+)2O2–(s), an... |
To summarise, we may mention that
Oxidation : Loss of electron(s) by any species.Reduction : Gain of electron(s) by any species. Oxidising agent : Acceptor of electron(s).Reducing agent : Donor of electron(s). Problem 7.2 Justify that the reaction:2 Na(s) + H
2(g) → 2 NaH (s) is a redox
change.SolutionSince in the... |
Copper ion is reduced by gaining electrons from the zinc. Reaction (7.15) may be rewritten as : At this stage we may investigate the state
of equilibrium for the reaction represented by equation (7.15). For this purpose, let us place a strip of metallic copper in a zinc sulphate solution. No visible reaction is notice... |
(7.16)
Here, Cu(s) is oxidised to Cu2+(aq) and
Ag+(aq) is reduced to Ag(s). Equilibrium greatly
favours the products Cu2+ (aq) and Ag(s). By way of contrast, let us also compare
the reaction of metallic cobalt placed in nickel sulphate solution. The reaction that occurs here is :
(7.17)
Unit 7.indd 238 11/11/202... |
By comparison we have come to know that zinc releases electrons to copper and copper releases electrons to silver and, therefore, the electron releasing tendency of the metals is in the order: Zn>Cu>Ag. We would love to make our list more vast and design a metal activity series or electrochemical series. The competitio... |
For example, we rewrite equations (7.18 to 7.20) to show charge on each of the atoms forming part of the reaction :
0 0 +1 –2
2H2(g) + O2(g) → 2H2O (l) (7.21)
0 0 +1 –1
H2 (s) + Cl2(g) → 2HCl(g) (7.22)
–4+1 0 +4 –1 +1 –1
CH4(g) + 4Cl2(g) → ... |
For ions composed of only one atom, the oxidation number is equal to the charge on the ion. Thus Na
+ ion has an oxidation
number of +1, Mg2+ ion, +2, Fe3+ ion, +3,
Cl– ion, –1, O2– ion, –2; and so on. In their
compounds all alkali metals have oxidation number of +1, and all alkaline earth metals have an oxidation n... |
when oxygen is bonded to fluorine. In such compounds e.g., oxygen difluoride (OF
2) and dioxygen difluoride (O2F2), the
oxygen is assigned an oxidation number of +2 and +1, respectively. The number assigned to oxygen will depend upon the bonding state of oxygen but this number
would now be a positive figure only. 4. ... |
6. The algebraic sum of the oxidation number of all the atoms in a compound must be zero. In polyatomic ion, the algebraic sum
of all the oxidation numbers of atoms of the ion must equal the charge on the ion. Thus, the sum of oxidation number of three oxygen atoms and one carbon atom in the carbonate ion, (CO
3)2– m... |
A term that is often used interchangeably
with the oxidation number is the oxidation state. Thus in CO
2, the oxidation state of
carbon is +4, that is also its oxidation number and similarly the oxidation state as well as oxidation number of oxygen is – 2. This implies that the oxidation number denotes the
oxidation... |
Problem 7.3
Using Stock notation, represent the
following compounds :HAuCl4, Tl2O, FeO,
Fe2O3, CuI, CuO, MnO and MnO2. Solution
By applying various rules of calculating
the oxidation number of the desired element in a compound, the oxidation number of each metallic element in its compound is as follows:
HAuCl
4 ... |
Solution
Let us assign oxidation number to each
of the species in the reaction under examination. This results into:
+1 –2 +1 –2 0 +4 –2
2Cu2O(s) + Cu2S(s) → 6Cu(s) + SO2
We therefore, conclude that in this reaction copper is reduced from +1 state
to zero oxidation state and sulphur is ... |
Examples of this class of reactions are:
+1 –2 0 0
2H2O (l) 2H2 (g) + O2(g) (7.26)
+1 –1 0 0
2NaH (s) 2Na (s) + H2(g) (7.27)
+1 +5 –2 +1 –1 0
2KClO3 (s) 2KCl (s) + 3O2(g) (7.28)
It may carefully be noted that there is no
change in t... |
(b) Non-metal displacement: The non-
metal displacement redox reactions include hydrogen displacement and a rarely occurring reaction involving oxygen displacement. Unit 7.indd 242 10/10/2022 10:37:04 AM
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redox reactions
All alkali metals and some alkaline earth
metals (Ca, Sr, and Ba) which are very go... |
The power of these elements as oxidising agents decreases as we move down from fluorine to iodine in group 17 of the periodic table. This implies that fluorine is so reactive that it can replace chloride, bromide and iodide ions in solution. In fact, fluorine is so reactive that it attacks water and displaces the oxyge... |
Disproportionation reactions Disproportionation reactions are a special type
of redox reactions. In a disproportionation reaction an element in one oxidation state is simultaneously oxidised and reduced. One of the reacting substances in a disproportionation reaction always contains an element that can exist in at le... |
This departure shown by fluorine is not surprising for us as we know the limitation of fluorine that, being the most electronegative element, it cannot exhibit any positive oxidation state. This means that among halogens, fluorine does not show a disproportionation tendency. Problem 7.5
Which of the following species,... |
Actually this fractional oxidation state is the average
oxidation state of the element under examination and the structural parameters reveal that the element for whom fractional oxidation state is realised is present in different oxidation states. Structure of the species C
3O2, Br3O8 and S4O62– reveal the following... |
In reality (revealed by structures only), the element in that particular species is present in more than one whole number oxidation states. Fe
3O4, Mn3O4,
Pb3O4 are some of the other examples of the compounds, which are mixed oxides, where we come
across with fractional oxidation states of the metal atom. However, th... |
PbO
2
thus can act as an oxidant (oxidising agent) and, therefore, can oxidise Cl
– ion
of HCl into chlorine. We may also keep in mind that PbO is a basic oxide. Therefore, the reaction
Pb
3O4 + 8HCl → 3PbCl2 + Cl2 + 4H2O
can be splitted into two reactions namely:2PbO + 4HCl → 2PbCl
2 + 2H2O
(acid-base reactio... |
Either the formulas of reactants or products are wrong or the oxidation numbers have not been assigned properly). Step 4: Ascertain the involvement of ions if
the reaction is taking place in water, add H
+ or
OH– ions to the expression on the appropriate
side so that the total ionic charges of reactants and products... |
Step 3: Calculate the increase and
decrease of oxidation number, and make them equal: from step-2 we can notice that there is change in oxidation state of chromium and sulphur. Oxidation state of chromium changes form +6 to +3. There is decrease of +3 in oxidation state of chromium on right hand side of the equation. ... |
Step 3: Calculate the increase and
decrease of oxidation number, and make
the increase equal to the decrease. +7 –1 +4 +5
2MnO4–(aq)+Br –(aq) → 2MnO2(s)+BrO3–(aq)
Step 4: As the reaction occurs in the
basic medium, and the ionic charges are not equal on both sides, add 2 OH
– ions
... |
Cr2O7 2–(aq) → 2 Cr3+(aq) (7.53)
Step 4: For reactions occurring in acidic
medium, add H2O to balance O atoms and H+
to balance H atoms. Thus, we get :
Cr2O7 2– (aq) + 14H+ (aq) → 2 Cr3+(aq) + 7H2O (l)
(7.54)
Step 5: Add electrons to one side of the half
reaction to balance the charges. If need be,
make
the n... |
Where
H+ and OH– appear on the same side of the
equation, combine these to give H2O. Problem 7.10
Permanganate(VII) ion, MnO4– in basic
solution oxidises iodide ion, I– to produce
molecular iodine (I2) and manganese
(IV) oxide (MnO2). Write a balanced ionic
equation to represent this redox reaction. SolutionStep ... |
The visible end point in this case is achieved after the last of the reductant (Fe
2+ or C2O42–) is oxidised and the first
lasting tinge of pink colour appears at MnO
4– concentration as low as 10–6 mol dm–3
(10–6 mol L–1). This ensures a minimal
‘overshoot’ in colour beyond the equivalence point, the point where t... |
This process of evolution is continuing. In fact, in recent past the oxidation process is visualised as a decrease in electron density and reduction process as an increase in electron density around the atom(s) involved in the reaction. 7.4
REDOX REACTIONS AND ELECTRODE
PROCESSES
The experiment corresponding to re... |
These represent the species in the reduction and oxidation half reactions. A redox couple is defined as having together the oxidised and reduced forms of a substance taking part in an oxidation or reduction half reaction. This is represented by separating the
oxidised form from the reduced form by a vertical line or a... |
The zinc and copper rods are connected by a metallic wire with a provision for an ammeter and a switch. The set-up as shown in Fig.7.3 is known as Daniell cell. When the switch is in the off position, no reaction takes place in either of the beakers and no current flows through the metallic wire. As soon as the switch ... |
By convention, the standard electrode potential (E
) of hydrogen electrode is 0.00 volts. The
electrode potential value for each electrode process is a measure of the relative tendency of the active species in the process to remain in the oxidised/reduced form. A negative E
means that the redox couple is a stronge... |
Table 7.1 The Standard Electrode Potentials at 298 K
Ions are present as aqueous species and H2O as liquid; gases and
solids are shown by g and s respectively.reducing agent than the H+/H2 couple. A
positive E means that the redox couple is a
weaker reducing agent than the H+/H2 couple. The standard electrode pote... |
Redox reactions are classified into four categories: combination, decomposition displacement and disproportionation reactions. The concept of redox couple and electrode processes is introduced here. The redox reactions find wide applications in the study of electrode processes and cells. EXERCISES
7.1 Assign oxidation ... |
Why ? 7.9 Consider the reactions:
(a) 6 CO2(g) + 6H2O(l) → C6 H12 O6(aq) + 6O2(g)
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redox reactions
(b) O3(g) + H2O2(l) → H2O(l) + 2O2(g)
Why it is more appropriate to write these reactions as :
(a) 6CO2(g) + 12H2O(l) → C6 H12 O6(aq) + 6H2O(l) + 6O2(g... |
Write a balanced redox equation for the reaction. (b) When concentrated sulphuric acid is added to an inorganic mixture containing chloride, we get colourless pungent smelling gas HCl, but if the mixture contains
bromide then we get red vapour of bromine. Why ? 7.13 Identify the substance oxidised reduced, oxidising ... |
(a) P4(s) + OH–(aq) → PH3(g) + HPO2– (aq)
(b) N2H4(l) + ClO3 –(aq) → NO(g) + Cl–(g)
(c) Cl2O7 (g) + H2O2(aq) → ClO2–(aq) + O2(g) + H+
7.20 What sorts of informations can you draw from the following reaction ? (CN)2(g) + 2OH–(aq) → CN–(aq) + CNO–(aq) + H2O(l)
7.21 The Mn3+ ion is unstable in solution and undergo... |
Present a balanced equation
for this redox change taking place in water. 7.24 Refer to the periodic table given in your book and now answer the following questions:
(a) Select the possible non metals that can show disproportionation reaction. (b) Select three metals that can show disproportionation reaction. 7.25 In... |
Cr3+/Cr = –0.74V
arrange these metals in their increasing order of reducing power
. 7.30 Depict the galvanic cell in which the reaction Zn(s) + 2Ag+(aq) → Zn2+(aq) +2Ag(s)
takes place, Further show:
(i) which of the electrode is negatively charged,
(ii) the carriers of the current in the cell, and
(iii) individual... |
Around the year 1780, chemists began to distinguish between organic compounds obtained from plants and animals and inorganic compounds prepared from mineral sources. Berzilius, a Swedish chemist proposed that a ‘vital force’ was responsible for the formation of organic compounds. However, this notion was rejected in 18... |
The sp hybrid orbital contains more s
character and hence it is closer to its nucleus and forms shorter and stronger bonds than the sp
3 hybrid orbital. The sp2 hybrid orbital
is intermediate in s character between sp
and sp3 and, hence, the length and enthalpy
of the bonds it forms, are also intermediate between... |
The electron charge cloud of the π bond is located above and below the plane of bonding atoms. This results in the electrons being easily available to the attacking
reagents. In general, π bonds provide the most
reactive centres in the molecules containing
multiple bonds. Problem 8.1
How many σ and π bonds are pre... |
The Lewis
structure or dot structure, dash structure,
condensed structure and bond line structural formulas are some of the specific types. The Lewis structures, however, can be simplified by representing the two-electron covalent bond by a dash (–). Such a structural formula focuses on the electrons involved in bond... |
The only atoms specifically written are oxygen, chlorine, nitrogen etc. The terminals denote methyl
(–CH
3) groups (unless indicated otherwise by
a functional group), while the line junctions denote carbon atoms bonded to appropriate number of hydrogens required to satisfy the valency of the carbon atoms. Some of th... |
The dashed-wedge is used to depict the bond projecting out of the plane of the paper and away from the observer. Wedges are shown in such a way that the broad end of the wedge is towards the observer. The bonds lying in plane of the paper are depicted by using a normal line (—). 3-D representation of methane molecule o... |
In the ball-and-stick model, both
the atoms and the bonds are shown. Balls represent atoms and the stick denotes a bond. Compounds containing C=C (e.g., ethene) can best be represented by using springs in place of sticks. These models are referred to as ball-and-spring model. The space-filling model emphasises the rel... |
Cyclohexane Cyclohexene Cyclopropane
Sometimes atoms other than carbon are also present in the ring (heterocylic). Tetrahydrofuran given below is an example of this type of compound:
Tetrahydrofuran
These exhibit some of the properties similar to those of aliphatic compounds. (b)
aromatic compounds
Aromatic compounds... |
8.4.2 Homologous
series
A group or a series of organic compounds each
containing
a characteristic functional group
forms a homologous series and the members of the series are called homologues . The members
of a homologous series can be represented by general molecular formula and the successive members differ fro... |
For instance, citric acid is named so because it is found in citrus fruits and the acid found in red ant is named formic acid since the Latin word for ant is formica. These names are traditional and are considered as trivial or common names. Some common
names are followed even today. For example, Buckminsterfullerene... |
The IUPAC name for a homologous series of such compounds is alkane. Paraffin (Latin: little affinity) was the earlier name given to these compounds. Unsaturated hydrocarbons are those, which contain at least one carbon-carbon double or triple bond. 8.5.2
iu Pac nomenclature of alkanes
Straight chain hydrocarbons:
Th... |
Some alkyl
groups are listed in Table 8.3.
table 8.3 some alkyl groups
table
8.2 iu Pac names of some unbranched
Saturated Hydrocarbons
Branched chain hydrocarbons: In a
branched chain compound small chains of
carbon atoms are attached at one or more carbon atoms of the parent chain. The small carbon chains (bra... |
Nomenclature of branched chain alkanes:
We encounter a number of branched chain alkanes. The rules for naming them are given
below. Unit 8.indd 263 10/10/2022 10:37:31 AM
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chemistry
separated from the groups by hyphens
and there is no break between methyl
and nonane.] 4. If two or more identical su... |
Thus, the following compound is 3-ethyl-6-methyloctane and not 6-ethyl-3-methyloctane. 1 2 3 4 5 6 7 8
CH3 — CH2—CH—CH2—CH2—CH—CH2 —CH3
CH2CH3 CH3
6. The branched alkyl groups can be named by following the above mentioned procedur
es. However, the carbon atom
of the branch that attaches to the root alka... |
The numbering is done in such a way that the branched carbon atoms get the lowest possible numbers. Thus, the numbering in the
above example should be from left to right (branching at carbon atoms 2 and 6) and not from right to left (giving numbers 4 and 8 to the carbon atoms at which
branches are attached). 1 2 3 4... |
In multi-substituted compounds, the following rules may aso be remembered:
• If
there happens to be two chains of equal
size, then that chain is to be selected which
contains more number of side chains. • After selection of the chain, numbering
is to be done from the end closer to the substituent. 5-(2,2-Dimethy... |
Examples of some functional groups with their prefixes and suffixes along with some examples of organic compounds possessing these are given in Table 8.4. First of all, the functional group present
in the molecule is identified which determines the choice of appropriate suffix. The longest
chain of carbon atoms conta... |
Thus, a compound containing both an alcohol and a keto group is named as hydroxyalkanone since the keto group is preferred to the hydroxyl group. For example, HOCH
2(CH2)3CH2COCH3 will
be named as 7-hydroxyheptan-2-one and not as 2-oxoheptan -7-ol. Similarly, BrCH
2CH=CH2
is named as 3-bromoprop-1-ene and not 1-brom... |
Hence the suf
fix is ‘-ol’. • The longest chain containing -OH has eight carbon atoms. Hence the
corresponding saturated hydr
ocar-
bon is octane. • The -OH is on carbon atom 3. In
addition, a methyl group is attached at 6
th carbon. Hence, the systematic name of this compound is 6-Methyloctan-3-ol. solution
The fu... |
Thus, the systematic name is Hexane-2,4-dione. Unit 8.indd 266 10/10/2022 10:37:32 AM
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organic chemistry – some basic principles and techniques
table 8.4 some Functional groups and classes of Organic compounds
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solution
Here, two functiona... |
The name of compound, therefore, is Hexa-1,3-dien-5-yne. Problem 8.9Derive the structure of (i) 2-Chlorohexane,
(ii) Pent-4-en-2-ol, (iii) 3- Nitrocyclohexene, (iv) Cyclohex-2-en-1-ol, (v) 6-Hydroxy-heptanal. solution
(i) ‘hexane’ indicates the presence of
6 carbon atoms in the chain. The functional group chloro is... |
Thus, complete structural formula of the compound is (II). Double bond is suffixed functional group whereas NO
2 is prefixed
functional group therefore double bond gets preference over –NO
2 group:
(iv) ‘1-ol’ means that a -OH group is present at C-1. OH is suffixed functional group and gets preference over C=C bond. ... |
Substituent of the base compound is
ass
igned number1 and then the direction
of numbering is chosen such that the next substituent gets the lowest number. The substituents appear in the name in alphabetical order. Some examples are given below. Methylbenzene Methoxybenzene Aminobenzene (Toluene) ... |
Such compounds are called as isomers. The following flow chart shows different types of isomerism. 8.6.1
structural isomerism
Compounds having the sam
e molecular
formula but different structures (manners
in which atoms are linked) are classified as structural isomers. Some typical examples of different types of str... |
8.7
FUNDAMENTAL CONCEPTS IN
ORGANIC REACTION MECHANISM
In an organic reaction, the or
ganic molecule
(also referred as a substrate) reacts with an appropriate attacking reagent and leads to the formation of one or more intermediate(s) and finally product(s)
The general reaction is depicted as follows : in understand... |
Carbocations are highly unstable and reactive species. Alkyl groups directly attached to the positively charged carbon stabilise the carbocations due to inductive and hyperconjugation effects, which you will be studying in the sections 8.7.5 and 8.7.9. The observed order of carbocation stability is: C
+H3 < CH3C+
H2 < ... |
The remaining
carbon orbital is perpendicular to the molecular plane and contains no electrons. [Fig. 8.3(a)]. Fig. 8.3 (a) Shape of methyl carbocation
The heterolytic cleavage can also give a
species in which carbon gets the shared pair of electrons. For example, when group Z attached to the carbon leaves without ... |
8.3 (b) Shape of methyl carbanion
Carbanions are also unstable and reactive species. The organic reactions which proceed through heterolytic bond cleavage are called ionic or heteropolar or just polar reactions. In homolytic cleavage, one of the electrons
of the shared pair in a covalent bond goes with each of the ... |
8.7.2
substrate and reagent
Ions are generally not formed in the r
eactions
of organic compounds. Molecules as such
participate in the reaction. It is convenient to name one reagent as substrate and other as reagent. In general, a molecule whose carbon is involved in new bond formation is called substrate and the o... |
A reagent that takes away
an electron pair from reactive site is called electrophile (E
+) i.e., electron seeking and the
reaction is called electrophilic. During a polar organic reaction, a
nucleophile attacks an electrophilic centre of the substrate which is that specific atom or part of the substrate which is ele... |
(a) CH
3–SCH3, (b) CH3–CN, (c) CH3–Cu
Solution
Problem 8.12Giving justification, categorise the
following molecules/ions as nucleophile or electrophile:
Solution
Nucleophiles: HS–,C2H5O–,(CH3)3N:H2N:–
These species have unshared pair of
electrons, which can be donated and shared with an electrophile. Electrophiles:... |
For example, in transfer of hydroxide ion giving ethanol and in the dissociation of chloromethane, the movement of electron using curved arrows can be depicted as follows:
8.7.4 electron displacement effects in
covalent Bonds
The electron displacement in an organic
molecule may take place either in the ground
st... |
The fractional electronic charges on the two atoms in a polar covalent bond are denoted by symbol δ (delta) and the shift of electron density is shown by an arrow that points from δ
+ to δ– end of the polar bond. δδ+ δ+ δ−
CH3→CH2→Cl
2 1
In turn carbon-1, which has developed
partial positive charge (
δ+) draws so... |
are electron-withdrawing groups. On the other hand, the alkyl groups like methyl (–CH
3) and
ethyl (–CH2–CH3) are usually considered as
electron donating groups. Unit 8.indd 274 10/10/2022 10:37:34 AM
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organic chemistry – some basic principles and techniques
Problem 8.14
Which bond is more polar in th... |
Thus, the structure of benzene cannot be represented adequately by the above structure. Further, benzene can be represented equally well by the energetically identical structures I and II. benzene cannot be adequately represented
by any of these structures, rather it is a hybrid of the two structures (I and II) called... |
The difference
in energy between the actual structure and the lowest energy resonance structure is called the resonance stabilisation energy or simply the resonance energy . The more the number
of important contributing structures, the more is the resonance energy. Resonance is particularly important when the contrib... |
solutionsolution
The two structures are less important
contributors as they involve charge
separation. Additionally, structure I contains a carbon atom with an
incomplete octe
t.
8.7.7 resonance effect
The resonance effect is defined as ‘the polarity
pr
oduced in the molecule by the interaction
of two π -bonds o... |
These systems often show abnormal behaviour. The examples are 1,3- butadiene, aniline and nitrobenzene etc. In such systems, the π-electrons are delocalised and the system
develops polarity. 8.7.8
electromeric effect ( e effect)
It is a temporary effect. The or
ganic compounds
having a multiple bond (a double or tri... |
For example: system or to an atom with an unshared
p orbital. The σ electrons of C—H bond of the
alkyl group enter into partial conjugation with the attached unsaturated system or with the unshared p orbital. Hyperconjugation is a
permanent effect. To understand hyperconjugation effect, let
us take an example of C... |
8.4(a) Orbital diagram showing
hyperconjugation in ethyl cation
This type of overlap stabilises the
carbocation because electron density from the adjacent σ bond helps in dispersing the positive charge. In general, greater the number of alkyl
groups attached to a positively charged carbon
atom, the greater is the ... |
Various methods used for the purification of organic compounds are based on the nature of the compound and the impurity present in it. The common techniques used for
purification are as follows :
(i)
Sublimation
(ii) Crystallisation
(iii) Distillation
(iv) Differential extraction and
(v) Chromatography
Finally, th... |
8.8.2
crystallisation
This is one of the most commonly used
techniques for the purification of solid organic
compounds. It is based on the difference in the solubilities of the compound and the impurities in a suitable solvent. The impure compound is dissolved in a solvent in which it is sparingly soluble at room t... |
Liquids having different boiling points vaporise at different temperatures. The vapours are cooled and the liquids so formed are collected separately. Chloroform (b.p 334 K) and aniline (b.p. 457 K) are easily separated by the technique of distillation (Fig 8.5). The liquid mixture is taken in a round bottom flask and ... |
The technique of fractional distillation is used in such cases. In this technique, vapours of a liquid mixture are passed through a fractionating column before condensation. The fractionating column is fitted over the mouth of the round bottom flask (Fig.8.6, page 280). Vapours of the liquid with higher boiling
point ... |
Some of the condensing liquid in the fractionating column obtains heat from the ascending vapours and revaporises. The vapours thus become richer in low boiling component. The vapours of low boiling component ascend to the top of the column. On reaching the top, the vapours become pure in low boiling component and pass... |
Such liquids are made to boil at a temperature lower than their normal boiling points by reducing the pressure on their surface. A liquid boils at a temperature at which its vapour pressure is equal to the external pressure. The pressure is reduced with the help of a water pump or vacuum pump (Fig.8.8). Glycerol can be... |
In steam distillation, the liquid boils when the sum of vapour pressures due to the organic liquid (p
1)
and that due to water (p2) becomes equal to
the atmospheric pressure (p), i.e. p =p1+ p2. Since p1 is lower than p, the organic liquid
vaporises at lower temperature than its boiling point. Thus, if one of the su... |
If the organic compound is less soluble in the organic solvent, a very large quantity of solvent would be required to extract even a very small quantity of the compound. The technique of continuous extraction is employed in such cases. In this technique same solvent is repeatedly used for extraction of the compound. 8.... |
Two of these are:
(a)
Adsorption chromatography, and
(b) Partition chromatography. a)
Adsorption Chromatography : Adsor-
ption chromatography is based on the fact
that different compounds are adsorbed on an adsorbent to different degrees. Commonly used adsorbents are silica gel and alumina. When a mobile phase is a... |
8.11). The mixture adsorbed on adsorbent is placed on the top of the adsorbent column packed in a glass tube. An appropriate eluant which is a liquid or a mixture of liquids is allowed to flow down the column slowly. Depending upon the degree to which the compounds are adsorbed, complete separation takes place. The mos... |
Differ ent
stages of separation of components of a mixture. Thin Layer Chromatography: Thin layer
chromatography (TLC) is another type of adsorption chromatography, which involves separation of substances of a mixture over a thin layer of an adsorbent coated on glass plate. A thin layer (about 0.2mm thick) of an adso... |
For example, amino acids may be detected by spraying the plate with ninhydrin solution (Fig.8.12b). Fig.8.12 (b) Developed chromatogram.Fig.8.12 (a) Thin layer chromatography. Chromatogram being developed. Unit 8.indd 283 11/10/2022 15:22:47
2024-25
284
chemistry
spot on the chromatogram. The spots of the
sepa... |
The elements pr
esent
in the compound are converted from covalent form into the ionic form by fusing the compound with sodium metal. Following reactions take place:
Na + C + N NaCN
2Na + S Na2S
Na + X Na X
(X = Cl, Br or I)
C, N,
S and X come from organic compound. Cyanide, sulphide and halide of sodi... |
This solvent acts as the mobile phase. The solvent rises up the paper by capillary action and flows over the spot. The paper selectively retains different components according to their differing partition in the two phases. The paper strip so developed is known as a chromatogram. The spots of the separated coloured com... |
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