Kalsi p s organic reactions and their mechanisms 5 e

Organic Reaction

1. About the Book:
In the new fifth edition the text in almost all the chapters has been updated by adding new material and deleting the old
ones. The book is designed to provide a comprehensive coverage in the area of organic reaction mechanisms for
chemistry undergraduate and postgraduate students. Now-a-days the practice of medicine increasingly demands deep
knowledge of the behaviour of molecules. Therefore, the future biologists will have to be more of organic chemists,
among other things.
Key Features:
ŸIn writing this fifth edition the major goal has been to integrate the information about many fundamental organic
reactions.
ŸBased on the feedback given to me by hundreds of students and my learned colleagues, I have made changes by
completely rewriting the book at different places.
ŸNew reagents, new organic reactions and solved exercises have been added.
ŸThe study of organic reactions and their mechanisms is an enormously broad subject. A full analysis of reaction
mechanism requires a good knowledge about molecular structure, stereochemistry and thermodynamics. These
topics are, therefore, further developed by laying more emphasis in the fifth edition.
About the Author:
P S Kalsi obtained his PhD degree from Pune University, Pune under the guidance of Professor
S C Bhattacharya at National Chemical Laboratory, Pune in 1964. He has published over 150 research papers in national and
international journals of repute in the area of chemistry of natural products. Prof. Kalsi was honoured by the Punjab
Agricultural University in 1969 in recognition of his merit as a teacher. Prof. Kalsi was invited by the Swedish Royal Academy
of Sciences to submit proposals for the award of the Nobel Prize for Chemistry, 1985. Indian Chemical Society in 2003
conferred on him S C Ameta medal for his outstanding research contributions. In 2011, Indian Chemical Society conferred on
him Lifetime Achievement Award for his outstanding contributions to chemical education on the eve of International Year of
Chemistry. He was honoured as the best teacher of chemistry in India at 28th Gujarat Science Congress held at North Gujarat
University, Patan on 22nd–23rd February, 2014 deliberating on Excellence in Science Education in India–A challenge ahead.
He is actively involved in teaching in different universities/postgraduate colleges and serves as a UGC resource person to
deliver lectures in refresher courses. In an academic year he visits about 12 different universities and delivers about 200
lectures.
ORGANIC REACTIONS
AND THEIR MECHANISMS
ISBN : 978 93 89802 08 5
Price : 499.00
Pub Date : 2021
Format : Paperback
Extent : 724 Pages
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•
LONDON • NEW DELHI • NAIROBI
NEW AGE INTERNATIONAL (P) LIMITED, PUBLISHERS
Committed to Educate the Nation
IN INDIA
FIFTH EDITION
Contents:
Ÿ Basic Concepts
Ÿ Delocalized Chemical
Bonding
Ÿ Organic Acids and Bases
Ÿ Organic Reactions and the
Determination of their
Mechanisms
Ÿ Aliphatic Nucleophilic
Substitution and its
Synthetic Applications
Ÿ Common Organic Reactions
and their Mechanisms
Ÿ Reagents in Organic
Synthesis and Relevant
Name Reactions
Ÿ Electrophilic Aromatic
Substitution
Ÿ Aromatic Nucleophilic
Substitution
Ÿ Photochemistry
Ÿ Addition to Carbon-Carbon
and Carbon-Hetero
Multiple Bonds
Ÿ Elimination Reactions
Ÿ Oxidation Methods
Ÿ Reduction Methods
Ÿ Molecular Rearrangements
Ÿ Free Radical Reactions
Ÿ Chapterwise Review
Problems with Solutions
P S KALSI
Professor of Eminence, Shoolini University, Solan,
Visiting Professor, Gujarat Forensic Sciences University, Gandhinagar
Visiting Professor, Kanoria PG Mahila Mahavidyalaya, Jaipur
Former Visiting Professor of Chemistry, Indira Gandhi National Open University (IGNOU), New Delhi
Former Dean of Colleges, Punjab Technical University, Jalandhar
Former Professor and Head, Department of Chemistry, College of Basic Sciences & Humanities
Punjab Agricultural University, Ludhiana
Readership and Market Potential: Undergraduate/postgraduate students and faculties of Chemistry • University and college libraries • Biochemists, Medical and
Pharmacology students and other related professionals.

2. NEW AGE
P S KALSI
+ L Pd Ph
Ph
H
H
H
L Pd
Ph
ORGANIC REACTIONS
AND THEIR MECHANISMS
TH
5
Edition
(In two colours)

3. Click Here to Buy the Book Online
ORGANIC REACTIONS
AND THEIR MECHANISMS

4. Click Here to Buy the Book Online
•
LONDON • NEW DELHI • NAIROBI
NEW AGE INTERNATIONAL (P) LIMITED, PUBLISHERS
FIFTH EDITION
ORGANIC REACTIONS
AND THEIR MECHANISMS
P S Kalsi
Professor of Eminence
Shoolini University, Solan
Visiting Professor
Gujarat Forensic Sciences University, Gandhinagar
Visiting Professor
Kanoria PG Mahila Mahavidyalaya, Jaipur
Former Visiting Professor of Chemistry
Indira Gandhi National Open University (IGNOU), New Delhi
Former Dean of Colleges
Punjab Technical University, Jalandhar
Former Professor and Head
Department of Chemistry
College of Basic Sciences & Humanities
Punjab Agricultural University, Ludhiana
(In two colours)

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Copyright © 2020, 2017, 2010, 2000, 1996, New Age International (P) Ltd., Publishers
Published by New Age International (P) Ltd., Publishers
First Edition: 1996
Fifth Edition (in two colours): 2020
All rights reserved.
No part of this book may be reproduced in any form, by photostat, microfilm, xerography, or any other means, or
incorporated into any information retrieval system, electronic or mechanical, without the written permission of
the publisher.

6. Based on the feedback from students and colleagues alike who used the fourth edition of
this text, I have made changes that are meant to make the material better organized.
A number of more specialized new reagents and chemical reactions have been added.
Almost all the chapters have been modified by incorporating the strategies of new reagents
and chemical reactions.
Chapter 1
More materials on polarigability of atoms and polarity of bonds have been added. Most
of the materials have been shifted to other chapters at relevant places.
Chapter 2
New material on conjugation and stability has been added. The aromatic system in
cyclobutadienyl dianion generated via complex formation with Fe(O) has been discussed.
Chapter 4
The portion on generation and properties of free radicals has been completely rewritten.
Chapter 5
Summary of S
N
1 and S
N
2 reactions has been presented in the beginning itself along with
the product formation from tertiary substrates depending on strength of base. The
comparison with primary and secondary haloalkanes has been elaborated.
Stereochemical outcome of S
N
1 and S
N
2 reactions has been elaborated. Examples of
intramolecular substitutions have been added.
Chapter 6
New material on Mukaiyama, Henry and Nef reactions has been added and the
mechanisms of these reactions have been presented for better understanding.
The material on Michael reaction (conjugate addition) has been completely rewritten.
Chapter 7
The mechanism and utility of Heck, Suzuki, Sonogashira and Stille reactions in organic
synthesis has been rewritten and new reactions and examples are added.
Preface to the Fifth Edition
v
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7. viContents
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Titanium–Mc Murry Coupling reaction has been added. The mechanism of Tebbe reaction
has been included with new reactions.
The material on alkene methathesis has been completely rewritten. Corey-Winter reaction
for the synthesis of olefins has been added.
A detailed new discussion with new examples on the synthetic utility of organolithium
compounds has been added including allyllithium and alkenyllithium compounds.
Chapter 10
Photoreduction of , -unsaturated compounds has been added.
Chapter 11
More material on bridged ions has been added.
Chapter 12
More examples on anti and syn eliminations defining conformation and reactivity have
been added. Examples of -eliminations on acyl chlorides and alkenyl ethers are added.
Chapter 13
New material on hydroxyl directed epoxidation of alkenes has been added. Formation
of epoxides from bromohydrins has been included. New problems on predicting the
stereochemistry of sharpless epoxidation have been added. The mechanism of the reaction
of alkenes with Mn (Salen*) Cl has been provided. There is more on Baeyer Villiger oxidation.
Chapter 14
A full account on increasing alkyl substitution on carbons of a double bond in relation to
stability has been added.
The material on hydrogenolysis has been completely rewritten.
The role of zinc in synthesis is further elaborated, and the mechanism of Clemmenson
reduction has been made more understandable.
Reduction of alkynes using lithium aluminium hydride has been added along with the
reduction with (R) and (S) BiNAL-H.
Chapter 15
Some more examples have been added. Tiffeneau-Demjavov carbocation rearrangement
has been given.
Chapter 16
More material has been added on free radical cyclisations to generate five membered
rings. The mechanism of Hunsdiecker-Reaction has been modified giving new examples.
Chapter 17
This is a new Chapter added to the fifth edition and replaces the original chapter 17
devoted to Pericyclic reaction in fourth edition. A full account on Pericyclic reactions is
given in the book on “Stereochemistry” by the same author. The problems have been carefully
drawn so that the student can thread through the material read in previous chapters.
These problems are also aimed to provide a great way to the students to face and solve
the problems in such competitive examination like NET.
Ludhiana
P. S. KALSI
viPreface to the Fifth Edition

8. Foreword (vii)
Preface to the Fifth Edition (ix)
Preface to the First Edition (xi)
Important Reactions and Rearrangements (xxi)
Some More Important Reagents (xxiii)
1. Basic Concepts 1–24
1.1Introduction 1
1.2Electronegativity—Dipole Moment 2
1.3Inductive and Field Effects 5
1.4Hydrogen Bond 9
1.5Other Weaker Bonds 12
1.6Bond Dissociation Energy 13
1.7The Hammett Equation—Linear Free Energy Relationship 15
1.8Taft Equation 17
1.9Steric Effects, Strain and Bredt Rule 18
Problems 23
Answers to the Problems 23
2. Delocalized Chemical Bonding 25–79
2.11, 3-butadiene—A Typical Conjugated System 25
2.2Resonance 29
2.3Aromaticity 41
2.4The Terms Aromatic, Antiaromatic and Nonaromatic 45
2.5Annulenes 48
Contents
vii
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9. viiiContents
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2.6The Frost Circle—Molecular Orbital Description of
Aromaticity and Antiaromaticity 50
2.7Aromatic and Antiaromatic Ions 51
2.8Other Non-benzenoid Aromatic Compounds 56
2.9Heterocyclic Aromatic Compounds 58
2.10Metallocenes and Related Compounds 59
2.11Fused Benzenoids and Fullerenes 60
2.12Homoaromatic Compounds 65
2.13Hyperconjugation 65
2.14Tautomerism 67
Problems 72
Answers to the Problems 75
3. Organic Acids and Bases 80–114
3.1The Bronsted–Lowry Concepts of Acids and Bases 80
3.2The Lewis Definition of Acids and Bases 83
3.3The Relation between Structure and Acidity 84
3.4Bases 99
3.5Relation between Structure and Basicity 99
3.6Synthetic Applications of Lithium Diisopropylamide (LDA) 107
3.7Acid-Base Reactions 109
3.8The Effects of the Solvent on Acid and Base Strength 110
3.9Leveling Effect 111
3.10Hard and Soft Acids and Bases 111
Problems 112
Answers to the Problems 113
4. Organic Reactions and the Determination 115–175
of their Mechanisms
4.1Mechanistic Classification 115
4.2Nucleophiles and Electrophiles 123
4.3Electron Movement 124
4.4Equilibria and Free Energy 124
4.5Free Energy Change in Relation to Bond Strengths and Degree of
Order in a System – Exothermic and Endothermic Reactions 125
4.6Reaction Rates 127
4.7The Transition State—Activation Energy—Exergonic and
Endergonic Reactions 128
4.8Transition State Theory—Measurement of Activation Energy 129
4.9Reaction Profile Diagrams 130

10. Contentsix
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4.10The Rate Determining Step, Intermediate and Transition State 132
4.11Thermodynamic and Kinetic Control 134
4.12Applications of Kinetic Principles 137
4.13The Curtin-Hammett Principle—Importance of
Transition State 142
4.14Microscopic Reversibility 143
4.15Methods of Determining Mechanisms 143
4.16Reactive Intermediates 151
Problems 174
Answers to the Problems 175
5. Aliphatic Nucleophilic Substitution and 176–217
its Synthetic Applications
5.1Introduction 176
5.2Synchronous Substitution—S
N
2 Process 182
5.3Substitution by Ionization — S
N
1 Mechanism 196
5.4S
N
1 Versus S
N
2 Reactions 198
5.5Other Aliphatic Substitution Pathways 201
5.6The Role of Ion Pairs 203
5.7Intramolecular Substitution
Reactions Give Cyclic Products 205
5.8Neighbouring Group Participation
and Nonclassical Carbocations 206
5.9Nucleophilic Substitution at Silicon 214
Problems 214
Answers to the Problems 216
6. Common Organic Reactions and their Mechanisms 218–261
6.1Base Catalysed Reactions (Formation of Carbon-Carbon Bonds) 218
6.2Stork Enamine Reactions (Formation of Carbon-Carbon Bonds)—
Reaction of an Enamine with Reactive Electrophiles 244
6.3Acid Catalysed Reactions (Formation of Carbon-Carbon Bonds) 251
6.4Reactions of Carboxylic Acids and their Derivatives 255
Problems 257
Answers to the Problems 258
7. Reagents in Organic Synthesis and Relevant 262–353
Name Reactions
7.1Organotransition Metal Reagents and Catalysis 262
7.2Some Transition Metal Organometallic Reactions 265
7.3Phosphorus Containing Reagents 290

11. xContents
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7.4Organosulphur Compounds: Sulphur Ylides 302
7.5Silicon Reagents 309
7.6Boron Containing Reagents 318
7.7Organometallic Reagents—Metal Insertion—Haloge—Metal Exchange 331
Problems 349
Answers to the Problems 351
8. Electrophilic Aromatic Substitution 354–397
8.1General View—The Arenium Ion—The Arenium Ion
Mechanism—S
E
2 Reaction 355
8.2Electrophilic Substitution on Monosubstituted Benzenes—
Orientation and Reactivity 365
8.3Electrophilic Substitution in Naphthalene and Larger Polycyclic
Aromatic Hydrocarbons 378
8.4Attack of the Electrophile at a Carbon already Bearing a Substituent
(ipso Position)—ipso Substitution 379
8.5Aromatic Rearrangements and Name Reactions 381
8.6Electrophilic Substitution on Heteroaromatic Compounds 384
8.7Diazonium Coupling 387
Problems 389
Answers to the Problems 392
9. Aromatic Nucleophilic Substitution 398–412
9.1The S
N
Ar Mechanism—The Addition—Elimination Mechanism—
The General Nucleophilic Aromatic ipso Substitution 399
9.2The S
N
1 Mechanism in Nucleophilic Aromatic Substitution—
The Aryl Cation Mechanism—Diazonium Salts 401
9.3Nucleophilic Aromatic Substitution by Elimination—
Addition—The Benzyne Mechanism 402
9.4Benzyne—A Strained Cycloalkyne 405
9.5Nucleophilic Substitution of Pyridine—The Chichi-babin Reaction 408
9.6Nucleophilic Substitution to Arenechromium Carbonyl Complexes 408
Problems 410
Answers to the Problems 411
10. Photochemistry 413–455
10.1Absorption of Electromagnetic Radiation—Quantum Yield 413
10.2Excited States 414
10.3The Fate of the Molecule in S
1
and T
1
States (Jablonski Diagram) 415
10.4Energy Transfer 416
10.5Energy Transfer and Photosensitization 416

12. Contentsxi
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10.6Forbidden Transitions—Intersystem Crossing 417
10.7Photochemical Reactions 418
Problems 451
Answers to the Problems 453
11. Addition to Carbon-Carbon and Carbon-Hetero 456–473
Multiple Bonds
11.1Electrophilic Addition 456
11.2Nucleophilic Additions to Alkenes and Alkynes 467
11.3Addition of Carbenes 468
11.4Nucleophilic Additions to Carbonyl Compounds
(Aldehydes and Ketones) 469
11.5Nucleophilic Acyl Substitution Reactions 470
11.6Radical Additions to Alkenes 471
11.7Nucleophilic Attack on Carbon-Nitrogen Triple Bond 471
Problems 472
Answer to the Problems 472
12. Elimination Reactions 474–499
12.1The Bimolecular Mechanism for Elimination—E2 Process 475
12.2The Unimolecular Mechanism for Elimination—E1 Process 488
12.3Pyrolytic syn Elimination Reactions—(Ei—Elimination Internal) 490
Problems 496
Answers to the Problems 497
13. Oxidation Methods 500–545
13.1Oxidation of Alcohols to Aldehydes, Ketones or Carboxylic Acids 500
13.2Allylic Oxidation of Alkenes 507
13.3Oxidation of Saturated C—H Groups 509
13.4Addition of Oxygen at Carbon-Carbon Double Bonds 513
13.5Ozonolysis 529
13.6Cleavage of Glycols and Related Compounds 530
13.7Oxidation of Alkenes to Aldehydes and Ketones Catalysed with
Palladium and Oxidation of Alkylboranes 532
13.8Oxidation of Ketones 534
13.9Oxidation of  -Ketols 538
13.10Oxidative Decarboxylation of Acids 538
13.11Aromatic Rings of Phenols—Coupling 538
13.12Oxidation of Amines 539

13. xiiContents
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13.13Photooxidation of Alkenes 539
Problems 540
Answers to the Problems 542
14. Reduction Methods 546–590
14.1Catalytic Reduction-Reduction with Diimide and
Hydroboration 547
14.2Reduction by Dissolving Metals–Metal and Ammonia 556
14.3Addition of Hydrogen and Reductive Coupling of Carbonyl
Compounds—Dissolving Metal Reductions 562
14.4Reductive Removal of Functional Groups and
Reductive Fission—Hydrogenolysis 563
14.5Reductive Deoxygenation of Carbonyl Groups 564
14.6Reduction by Hydride Transfer Reagents—
Nucleophilic Reducing Agents 567
14.7Stereoselectivity of Reduction with Small Hydride Donors 575
14.8Stereoselectivity of Reduction with Hindered Hydride
Donors-Selectrides (Trialkyl Borohydrides) 576
14.9Reduction of Cyclic Ketones with Hydride Reagents –
A Summary 579
14.10Chiral Boranes—Enantioselective Reduction of
Carbonyl Compounds 581
14.11Meerwein-Ponndorf Reduction—The Hydride
Transfer Reaction 582
14.12Cannizzaro Reaction 582
14.13Reduction of Aldehydes and Ketones with an
Adjacent Stereocenter (Asymmetric Induction) 583
14.14Reduction of Epoxides 585
14.15Reductions with Enzymes—Bakers Yeast 586
14.16Less Reactive Modified Borohydrides—Sodium Cyanoborohydride and
Sodium Triacetoxy-Borohydride – Reductive Amination 587
Problems 588
Answers to the Problems 589
15. Molecular Rearrangements 591–616
15.1Rearrangements to Electron Deficient Carbon 593
15.2Rearrangements to Electron Deficient Nitrogen 602
15.3Rearrangements to Electron Deficient Oxygen 607
15.4Rearrangement to Electron Rich Carbon 608
15.5Aromatic Rearrangements 614
15.6Free Radical Rearrangements 614

14. Contentsxiii
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Problems 614
Answers to the Problems 615
16. Free Radical Reactions 617–652
16.1Structure, Stability and Geometry 617
16.2Preparation 618
16.3Properties of Free Radicals 622
16.4Aromatic Nucleophilic Substitution—S
RN
1 Substitution 636
16.5Homolytic Aromatic Substitution 636
16.6Some Name Reactions 637
16.7The Coupling of Alkynes 643
16.8Reactions involving Electron Transfer Steps 644
16.9Molecular Rearrangements 645
16.10Some Further Substitution and Other Reactions 648
Problems 650
Answers to the Problems 651
17. Chapterwise Review Problems with Solutions 653–690
Index 691–699

15. Basic Concepts1
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1.1 INTRODUCTION
One begins a study of reaction mechanisms by examining some of the basic principles. A basic
understanding of these concepts helps largely in understanding of reactions and their
mechanisms. Thiols undergo an oxidative coupling when treated with mild oxidizing agents
to give disulphides: 2RS—H + H
2
O
2
 RS—RS + 2H
2
O. The understanding of this reaction
requires a knowledge of bond dissociation energy. The bond dissociation energy of the S—H
bond of thiols (~ 80 kcal/mol) is much lower than the O—H bond of alcohols (~ 100 kcal/
mol). It is this weakness of the S—H bond which allows thiols to undergo an oxidative coupling,
and the alcohols do not display this reaction. On treatment with oxidizing agents, oxidation at
the weaker C—H bond (~ 85 kcal/mol) takes place rather than at the strong O—H bond. Thus
a knowledge of the nature and strength of bonds is essential for the chemical investigation of
organic molecules. Similarly the properties of molecules are influenced by their structure.
Both the length and strength of a carbon-hydrogen bond is dependent on the hybridization
of the carbon atom to which the hydrogen is attached. When there is more s character in the
orbital used by carbon to form the bond, the shorter and stronger bond results. An s orbital is
closer to the nucleus than a p orbital and thus the carbon-hydrogen bond formed by an
sp (50% s) hybridized carbon is shorter and stronger than carbon-hydrogen bond formed by
an sp
2
(33.3% s) hybridized carbon and this in turn is shorter and stronger than a carbon-
hydrogen bond formed by an sp
3
(25% s) hybridized carbon.
The following points may be noted:
In general shorter bonds are stronger bonds. Increasing s character shortens bonds, thus
bonds strength increase with increasing s character.
More the bonds holding two carbon atoms together, the shorter and stronger is the carbon-
carbon bond. Triple bonds are shorter (C
C, 1.20 Å) and stronger (CC, 200 kcal/mol)
than double bonds (CC, 1.33 Å, 152 kcal/mol), which are shorter and stronger than
single bonds (C—C, 1.54 Å, 88 kcal/mol).
Thus double bonds are both shorter and stronger than corresponding single bonds,
however, not twice as strong, since  overlap is less than  overlap. This means that a
 bond is stronger than a  bond. The difference in energy between a single bond, say
1
Basic Concepts
CHAPTER 1

16. 2Organic Reactions and their Mechanisms
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C—C, and the corresponding double bond is the amount of energy necessary to cause
rotation around the double bond.
1.2 ELECTRONEGATIVITY—DIPOLE MOMENT
Carbon is unique among the elements, since it is able to form a huge number of compounds
by bonding to itself and to the atoms of other elements e.g., hydrogen, oxygen, nitrogen,
sulphur and the halogens. This bonding is almost always covalent.
The sharing of electrons in a covalent bond is not exactly equal when the linked elements
are different. The relative attractive power exerted by an element on the electrons in a covalent
bond can be expressed by its electronegativity. According to one quantitative definition of
electronegativity, there is an increase in electronegativity along the series towards fluorine as
shown (Scheme 1.1).
In the first two periods, the nonmetallic elements are more electronegative than carbon
with the notable exceptions of hydrogen and boron. This electronegativty difference makes
carbon susceptible to reaction with ions and molecules that contain heteroatoms, most of which
are also more electronegative.
Hydrogen with electronegativity 2.1 is close in this respect with carbon (for further details
on electronegativity see, Scheme 3.9). When two atoms with different electronegativities form
a covalent bond, the atom with greater electronegativity draws the electron pair to it and a
polar covalent bond results as in hydrogen chloride and can be represented by the usual symbol
(I, Scheme 1.1) when necessary. In fact the hydrogen chloride molecule is a resonance hybrid
of two resonating structures.
SCHEME 1.1
As a consequence of a partially positive end (
+
) and a partially negative end (
–
) in HCl
molecule represents a dipole (II, Scheme 1.1) and therefore, has a dipole moment (a physical property). Thus the dipole moment is a property of the molecule which is due to charge
separations. It is defined as the product of the magnitude of the charge (e) in electrostatic
units (esu) and the distance (d) which separates them in centimeters (cm):
 = e × d
Dipole moments are typically of the order of
10
–18
esu cm, since charges are typically of the order
of 10
–10
esu and the distance is of the order of 10
–8
cm. For convenience this unit (1 × 10
–18
esu cm) is
defined as one Debye (abbreviated D). The direction
of polarity of a polar bond is usually symbolized by
a vector quantity (Scheme 1.2).
The arrow head points to be the negative part of the molecule, while the crossed end is the
positive end. A molecule with polar bonds, may, however, not possess a dipole moment i.e.,
SCHEME 1.2
Negative endPositive end
A vector quantity

17. Basic Concepts3
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the molecule itself may be non-polar. This is so when a particular molecule has a shape
(or symmetry) so that the dipoles of the individual bonds cancel each other. Thus one is
concerned with the total moment of the molecule which is the vectorial sum of the individual
bond moments as e.g., in the case of 1, 2-dichloroethene isomers (Scheme 1.3).
SCHEME 1.3
Similarly carbon in CO
2
is sp hybridized and the molecule is linear. The C—O bond moments
oppose each other and cancel, in SO
2
however, S is sp
2
hybridized with two  bonds to O and
one with unshared electron pair. The O—S—O bond angle is around 120° and S—O moments
do not cancel. Thus unlike CO
2
, SO
2
has  = 1.6 D.
The carbonyl group is polar. The carbon atom is bonded to the more electronegative oxygen
atom. The resulting imbalance in the electron density leads to a permanent dipole of
2–3 Debyes (D) in the case of simple carbonyl compounds (Scheme 1.4).
SCHEME 1.4 Resonance structures for the carbonyl group
2-chloroethanol is much more acidic than ethanol. This can be
explained due to electrostatic interaction of the C—Cl dipole with
the negative charge of the alkoxide ion (Scheme 1.5). The negative
charge on oxygen is nearer to the positive end of the dipole than it is
to the negative end. Consequently, electrostatic attraction exceeds
repulsion, leading to the stabilization of the anion. This stabilization of the anion increases its
ease of formation and the conjugate acid, 2-chloroethanol, is more acidic than ethanol itself.
In the equilibrium for 2-halocyclohexanones (Scheme 1.6) there is an increase in the per cent
of axial conformer on going from 1, 4-dioxane to heptane as solvent. The C—Cl and C
O
dipoles reinforce each other in the equatorial form, however, these cancel to some extent in
the axial form. Thus the equatorial form is more polar and should be favoured by the more
polar solvents.

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SCHEME 1.6 Conformers of 2-halocyclohexanone in different solvents
Bond polarity doesn’t entirely depend on electronegativity differences between atoms.
Another factor that contributes to the polarity of bonds is the polarizability of atoms. A
polarizable atom is one in which the distribution of its electrons is readily distorted or deformed
by outside influences. Polarizability of a large atom makes its bonds polar, even if the
electronegativity values indicate otherwise. For example, the electronegativity values of carbon
and iodine are identical (2.5). One might assume that the C–I bond is not polar. Significantly
CH
3
I reacts as if the carbon atom were partially positive and the iodine atom, partially negative.
Generally the bonds between carbon and any element in groups (VA)–(VIIA) are considered
as polar, with the carbon atom having the 
+
designation. For bonds between carbon and a
metal atom, including those of the transition metals, the carbon atom is 
–
(Scheme 1.6a).
SCHEME 1.6a
To cite one example of the involvement of carbon-halogen dipole is the electrophilic aromatic
substitution in a halobenzene. A halogen is o, p — directing substituent. Substitution at the
meta position of a halobenzene can lead to three resonance structures (Scheme 1.7). All the
three structures are strongly destabilized by electrostatic interaction of the positive charge in the ring with the carbon-halogen dipole. As a consequence the meta position in a halobenzene
is strongly deactivated. Though similar situation is also obtained during o and p attack,
however, in these cases additional stable halonium ion structures make the o , p attack for
more facile.
X
H
E
+
X
H
E
+
X
H
E
+
Reaction at the positionmeta
C—X
SCHEME 1.7

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1.3 INDUCTIVE AND FIELD EFFECTS
One may observe a change in the rate constant or equilibrium constant of a reaction by replacing
a hydrogen atom by another atom or group of atoms. These substituent effects may be the
result of the size of the substituent (steric effect) and/or its influence on the availability of
electrons (electronic effect) on the site of the reaction. The electronic effect which a substituent
can exert may be either electron releasing or electron withdrawing. These electronic effects
are further subdivided into an inductive and a resonance (mesomeric effect). The inductive
effect (I) is a result of a substituents’ intrinsic ability to supply electrons (electron donation, + I
effect) or withdraw electrons (electron withdrawing) – I effect, i.e., the inductive effect depends
on the electronegativity of the substituent. The inductive effect is transmitted through  bonds
and weakens as the distance between the substituent and the reactive center increases. Thus
the effect is greatest for the adjacent bond and may be felt weakly farther away (see,
Scheme 3.18). The effect may be represented for ethyl chloride (Scheme 1.8). In this case chlorine
atom has –I effect and thus C-1 atom loses some of its electron density and as a result C-1, Cl
bond is polarized and a slight positive charge is generated on C-2. In this way the replacement
of hydrogen atom by a more electronegative atom results in electron displacements throughout
the molecule.
CH
3
CH
2
Cl
dd+ d+ d– A – group will draw electronsI
A polarization of this bond
A slight positive charge on the C-2 atom
SCHEME 1.8 Operation of inductive effect
The acid catalysed halogenation of acetone stops after the first halogen has been introduced.
In the presence of base, however halogenation continues until the same -carbon is completely
halogenated. This is explained since under acid catalysis the rate determining step is enolisation. For halogenation to continue the halo-carbonyl compound must enolise again (Scheme 1.9).
Due to electron withdrawing nature (I effect) of
BrCH — C —
2
O
moiety, this is retarded.
Base mediated reaction, however, involves an enolate ion which instead increases the acidity
of remaining -hydrogens (Scheme 1.9, also see schemes 2.47-2.47c. Operation of inductive
effect is also seen in the acidity of halogen substituted carboxylic acids. An electronegative
SCHEME 1.9 Halogenation of a ketone under acidic catalysis 1 and basic conditions 2

20. 6Organic Reactions and their Mechanisms
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halogen atom pulls the bonding electrons toward itself through sigma () bonds (–I effect).
The conjugate base of the carboxylic acid (I, Scheme 1.10) will be stabilized by decreasing the
electron density about the oxygen atom. Stabilizing a base increases the acidity of its conjugate
acid. Put another way in the anion (hybrid structure, II) the negative charge is stabilised by
electrostatic interaction between the partial –ve charges on the oxygens and the +ve partial
charge on the carbon bearing the bromine atom.
O
C
HC
3
OH
Br
O
C
HC
2
OH
p = 4.76K
a
p = 2.86K
a
Acetic acid Bromoacetic acid
C
H
H
O
O
–
CBr
Inductive electron withdrawal
(I)
CH
2
Br
+
C
O
O
1
2
(II)
1
2
–
–
SCHEME 1.10 Inductive effect and electrostatic stabilisation of an anion
The other effect operates through space (and not through  bonds) or through solvent
molecules and is called the field effect. Normally the field effect depends on the geometry of
the molecule whereas the inductive effect depends only on the nature of the bonds. As an
example of the field effect (long range polar interactions) the two acids (I and II, Scheme 1.11)
have different pK
a
values. The inductive effect of the chlorine atoms on the position of the
electrons in the COOH group must be same since the same bonds intervene. Consequently,
the acidity must have been equal. However, this difference in pK
a
value shows the operation
of field effect, since the two chlorine atoms are placed closer in space to the COOH group in I
than in II.
Cl
ClH
H
CO H
2
(I)
Cl H
CO H
2
(II)
H
Cl
SCHEME 1.11 Operation of field effect
The inductive effect (+I) of alkyl group has been invoked to explain the carbocation stability.
This effect also helps in explaining the orientation and reactivity during electrophilic
substitution on benzene derivatives (Sec. 8.8).
The resonance effect (see, Schemes 2.14 and 2.15) involves delocalization of electrons through
resonance via the  system. Atoms and functional groups may be arranged according to their
ability to donate or withdraw electrons. The inductive and resonance effects of many groups
are in the same direction. Other groups display opposite effects in the two cases. Normally
atoms which are more electronegative than carbon and which also have non-bonding electrons
possess opposing characteristics. The halogens illustrate these opposing effects (Scheme 1.12).

21. Basic Concepts7
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A good example is found during electrophilic substitution when the inductive effect of a halogen
on the benzene ring slows the rate of further substitution (Scheme 1.12).
SCHEME 1.12 The C—Cl bond is strongly polarized. The 
+
on the carbon slows a substitution reaction
which places more positive charge on the ring. A halogen, however denotes electrons in
the ring via resonance. Thus a halogen in o, p director
EXERCISE 1.1
How one can explain that acetic acid (pK
a
= 4.7) is stronger acid than
2, 2, 2-trifluoroethanol (pK
a
= 12.8) and ethanol is the least acidic (pK
a
= 15.9)
from among these three compounds?
ANSWER. Consider the species after the loss of a proton from each of these
compounds. In the case of ethanol the negative charge resides on its single oxygen
i.e., the charge is localized (Scheme 1.13). In the carboxylate ion both inductive
withdrawal of electrons and the ability of two atoms to share the negative charge
via resonance renders the conjugate base of the carboxylic acid more stable than
the conjugate base from ethanol. 2, 2, 2-Trifluoroethanol is much stronger acid
than ethanol, since in the former the highly electronegative fluorines help in the
stabilization of its alkoxide ion.
CH CH O
32
—— :
::–
Charge is localized
on the alkoxide ion
CH C
3
—
O
–
– effect of carbonyl group in
acetate anion also stabilizes it
I
CH C
3
—
O
O::
:
–
CH C
3
—
O::
:–
O
Charge is shared by oxygen
atoms via resonance
F
CCH O
2
——
F
F
:
::–
d
–
d
+
Strong– effect of fluorines
stabilizes the alkoxide ion
I
O
SCHEME 1.13

22. 8Organic Reactions and their Mechanisms
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SCHEME 1.14
HC
3
CO—
–
d–d+
The electron donating inductive
effect destabilizes the
alkoxide ion
EXERCISE 1.2
Acidity order of alcohols in aqueous solution is :
CH
3
OH > CH
3
CH
2
OH > (CH
3
)
2
CHOH > (CH
3
)
3
COH. Can inductive effect explain
this order?
ANSWER. Electron donating inductive effect (+
I) of
alkyl groups will retard the formation of an alkoxide to
reduce the acidity of an alcohol. Thus t-Butanol is the
weakest acid (Scheme 1.14).
However, in the gas phase it is found that the acidity
order of alcohols is opposite to that found in solution.
Thus it is probably not the + I effect of alkyl groups that
is important but stabilizing effect of the solvent. A
smaller alkoxide ion is approached more easily by the
solvent to solvate it (see Scheme 3.18).
EXERCISE 1.3
Which of the alkenes (Scheme 1.15) is expected to react faster with HX?
ANSWER. Consider the protonation of the double bond in (I, Scheme 1.15) which
is according to Markovnikov rule.
ClCH CH
2
— CH
2
– effect of
chlorine
I
d
–
d
+
HX—
Cl CH CH CH H
22
————
d
–
d
+
+
Unfavourable interaction
between like positive charges
SCHEME 1.15
Due to the –I effect of chlorine a positive charge on the methylene group would be in opposition to the expected carbocation formed during the addition of HX. No such effect is operative in (II).
EXERCISE 1.4
Discuss in terms of resonance and inductive effect the addition of HX to methyl vinyl ether.
ANSWER. The –I effect of oxygen generates a partial positive charge on the
adjacent carbon, which will get enhanced on protonation (Markovnikov rule)
during the first step of addition of HX i.e., during the formation of intermediate

23. Basic Concepts9
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carbocation which is an unfavourable situation. However, resonance stabilization
dominates the inductive effect and the addition occurs smoothly (Scheme 1.16).
SCHEME 1.16
1.4 HYDROGEN BOND
The hydrogen atom which is bonded to an electronegative atom can form a hydrogen bond to
a second electronegative atom. The hydrogen bond, is thus a force of attraction between
opposite partial charges, e.g., 
+
charge on H in the OH group and 
–
charge on the O of
another group (Scheme 1.17).
No such partial charges exist in the molecules of alkanes since C and H have nearly same
electronegativities. Only three elements, F, O and N, have atoms that are electronegative enough
to participate significantly in hydrogen bonds. A hydrogen bond requires a hydrogen bond
donor and a hydrogen bond acceptor as in the alcohol molecule (Scheme 1.17).
SCHEME 1.17 Operation of hydrogen bonding
An ether has no O—H proton, therefore, the ether group cannot donate hydrogen bonds
and thus cannot form a hydrogen bond with another ether molecule. Since, ether molecules
are not held together by hydrogen bonds, they are more volatile than alcohols of the same
molecular weight. The oxygen of the ether group can however, form hydrogen bonds with
an alcohol or a other hydrogen bond donor e.g., water (Scheme 1.17). So ethers are more
soluble in water than in alkanes. The hydrogen bond is conventionally represented by a
dotted line.
The hydrogen bond (bond dissociation energy is about 1–9 kcal/mol) is weaker than an
ordinary covalent bond. When there are many such bonds as in carbohydrates, the total strength
is very great. The bond may be formed both between molecules of the same type as in alcohols

24. 10Organic Reactions and their Mechanisms
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(Scheme 1.17) and carboxylic acids
(Scheme 1.18) and molecules of different
type as in an ether and alcohol (Scheme 1.17)
or as in the interaction between the proton
of an alcohol and the oxygen of a carbonyl
group. Two types of hydrogen bonding
have been recognized: intramolecular
(within the same molecule) and
intermolecular (between two or more
molecules).
Due to hydrogen bonding, there is an increase in intermolecular ‘aggregation’ forces which
is reflected in the boiling point and solubility of the organic compound. There is an increase in
the boiling point since energy is required to separate the hydrogen bonded molecules in their
translation to the gaseous state. Hydrogen bonds exist in the liquid and solid phases and in
solution. Compounds which form strong hydrogen bonds may be associated even in the gas
phase. Thus acetic acid exists as a dimer in the gas phase.
Intramolecular hydrogen bonds may also be formed and these have particular significance.
When the resulting ring is five or six membered then the phenomenon is termed chelation. An
example of chelation is for the enolic form of acetylacetone (Scheme 2.50). Since on chelation,
intermolecular aggregation forces are not operative, chelated compounds have normal boiling
points (Scheme 1.19). Thus, o -nitrophenol is much more volatile than its p-isomer, since only
the latter can form intermolecular hydrogen bonds.
N
O O
+
O
H
–
o-Nitrophenol
(more volatile because of
intramolecular hydrogen bonding)
HO N
O
O
–
+
HO N
O
O
–
+
p-Nitrophenol
(less volatile because of intermolecular hydrogen bonding)
SCHEME 1.19
An important way to detect hydrogen bonding is via IR and NMR spectroscopy. A free OH
group of an alcohol or a phenol shows a sharp infrared absorption around 3600 cm
–1
(O—H stretching vibrations). On hydrogen bonding the band becomes broad and is shifted to lower frequencies (around 3400 cm
–1
). In several cases in dilute solutions, there may be partial
hydrogen bonding, i.e., some hydroxyl groups are free and others bonded. In these cases one therefore, observes two bands, one sharp band at high frequency (around 3600 cm
–1
) and
another broad band at lower frequency (around 3400 cm
–1
). A distinction can also be made
between inter- and intramolecular hydrogen bonding on the basis of infrared spectroscopy. In very dilute solution, formation of intermolecular hydrogen bonds does not take place as the molecules are widely separated. Increasing the concentration of the alcohol or phenol causes the sharp band around 3600 cm
–1
to be replaced by a broad and lower frequency band which
RC—
O
OH—
HO—
O
CR—
d– d+
d+ d–
O
H
OCH
3
—
CH C CH
33
——
SCHEME 1.18

25. Basic Concepts11
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is assigned to OH groups that are associated through intermolecular hydrogen bonding.
Intramolecular hydrogen bonds remain unaffected and as a result the absorption band also
remains unaffected. In the case of o-nitrophenol the OH band (intramolecular hydrogen
bonding) is at 3200 cm
–1
in KBr pellet as well as in CHCl
3
solution, whereas in the p -isomer,
the values are different in the two media KBr (pellet 3330 cm
–1
; CHCl
3
solution 3520 cm
–1
). In
the
1
H NMR spectrum a hydrogen bonded hydroxyl group shows a downfield shift of its
proton.
EXERCISE 1.5
(a)Why the O—H stretching frequency for t-butyl alcohol is a sharper band in
IR compared with methanol?
(b) Which of the following norbornane systems can be detected by IR spectroscopy:
CH
3HO
(I)
HC
3 OH
(II)
CH
3HO
(III)
RO
H
(IV)
SCHEME 1.20
ANSWER. (a) Due to steric effects it is far more difficult for t-butyl alcohol to
involve in intermolecular hydrogen bond formation.
(b) In (I, Scheme 1.20), intramolecular hydrogen bonding would be detected as
shown in (IV).
Hydrogen bonding affects structure (chemical properties) and molecular shape of molecules.
Thus e.g., the role of intramolecular hydrogen bonding is reflected in the large amount of enol
present in some tautomeric equilibria (see, Scheme 2.48). It also influences conformation of
molecules. The six membered heterocycles of oxygen closely resemble the chair conformation
of cyclohexane. In heterocyclic rings the steric repulsions for axial substituents are reduced
due to the replacement of a methylene groups of cyclohexane by oxygen or nitrogen. Since the
divalent oxygen has no substituents, therefore, the 1, 3-diaxial interactions which are the main
unfavourable interactions for axial substituents in cyclohexanes are absent (Scheme 1.20). With
the presence of a polar substituent, interactions between the substituent and the ring heteroatom
can become important. Thus, the preferred
conformation of 5-hydroxy-1, 3-dioxane
(Scheme 1.21) has the hydroxyl group in the
axial position. This conformation is favoured
due to hydrogen bonding of the hydroxyl
group with the ring oxygen which is possible
only with the axial hydroxyl group to serve as
a stabilizing force for this conformation.
OO
O
O
H
——NH OC
Peptide group
SCHEME 1.21

26. About the Book:
In the new fifth edition the text in almost all the chapters has been updated by adding new material and deleting the old
ones. The book is designed to provide a comprehensive coverage in the area of organic reaction mechanisms for
chemistry undergraduate and postgraduate students. Now-a-days the practice of medicine increasingly demands deep
knowledge of the behaviour of molecules. Therefore, the future biologists will have to be more of organic chemists,
among other things.
Key Features:
ŸIn writing this fifth edition the major goal has been to integrate the information about many fundamental organic
reactions.
ŸBased on the feedback given to me by hundreds of students and my learned colleagues, I have made changes by
completely rewriting the book at different places.
ŸNew reagents, new organic reactions and solved exercises have been added.
ŸThe study of organic reactions and there mechanisms is an enormously broad subject. A full analysis of reaction
mechanism requires a good knowledge about molecular structure, stereochemistry and thermodynamics. These
topics are, therefore, further developed by laying more emphasis in the fifth edition.
About the Author:
P S Kalsi obtained his PhD degree from Pune University, Pune under the guidance of Professor
S C Bhattacharya at National Chemical Laboratory, Pune in 1964. He has published over 150 research papers in national and
international journals of repute in the area of chemistry of natural products. Prof. Kalsi was honoured by the Punjab
Agricultural University in 1969 in recognition of his merit as a teacher. Prof. Kalsi was invited by the Swedish Royal Academy
of Sciences to submit proposals for the award of the Nobel Prize for Chemistry, 1985. Indian Chemical Society in 2003
conferred on him S C Ameta medal for his outstanding research contributions. In 2011, Indian Chemical Society conferred on
him Lifetime Achievement Award for his outstanding contributions to chemical education on the eve of International Year of
Chemistry. He was honoured as the best teacher of chemistry in India at 28th Gujarat Science Congress held at North Gujarat
University, Patan on 22nd–23rd February, 2014 deliberating on Excellence in Science Education in India–A challenge ahead.
He is actively involved in teaching in different universities/postgraduate colleges and serves as a UGC resource person to
deliver lectures in refresher courses. In an academic year he visits about 12 different universities and delivers about 200
lectures.
ORGANIC REACTIONS
AND THEIR MECHANISMS
ISBN : 978 93 89802 08 5
Price : 499.00
Pub Date : 2021
Format : Paperback
Extent : 724 Pages
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•
LONDON • NEW DELHI • NAIROBI
NEW AGE INTERNATIONAL (P) LIMITED, PUBLISHERS
Committed to Educate the Nation
IN INDIA
FIFTH EDITION
Contents:
Ÿ Basic Concepts
Ÿ Delocalized Chemical
Bonding
Ÿ Organic Acids and Bases
Ÿ Organic Reactions and the
Determination of their
Mechanisms
Ÿ Aliphatic Nucleophilic
Substitution and its
Synthetic Applications
Ÿ Common Organic Reactions
and their Mechanisms
Ÿ Reagents in Organic
Synthesis and Relevant
Name Reactions
Ÿ Electrophilic Aromatic
Substitution
Ÿ Aromatic Nucleophilic
Substitution
Ÿ Photochemistry
Ÿ Addition to Carbon-Carbon
and Carbon-Hetero
Multiple Bonds
Ÿ Elimination Reactions
Ÿ Oxidation Methods
Ÿ Reduction Methods
Ÿ Molecular Rearrangements
Ÿ Free Radical Reactions
Ÿ Chapterwise Review
Problems with Solutions
P S KALSI
Professor of Eminence, Shoolini University, Solan,
Visiting Professor, Gujarat Forensic Sciences University, Gandhinagar
Visiting Professor, Kanoria PG Mahila Mahavidyalaya, Jaipur
Former Visiting Professor of Chemistry, Indira Gandhi National Open University (IGNOU), New Delhi
Former Dean of Colleges, Punjab Technical University, Jalandhar
Former Professor and Head, Department of Chemistry, College of Basic Sciences & Humanities
Punjab Agricultural University, Ludhiana
Readership and Market Potential: Undergraduate/postgraduate students and faculties of Chemistry • University and college libraries • Biochemists, Medical and
Pharmacology students and other related professionals.