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ORGANIC CHEMISTRY A Modern Approach Volume-I ABOUT THE AUTHOR Nimai Tewari is a retired associate Professor, Department of Chemistry, Katwa College (affiliated to The University of Burdwan), West Bengal A PhD in Organic Chemistry from Calcutta University, he has taught the subject for a period of more than three decades He has published various research papers in national and international journals Apart from Organic Chemistry—A Modern Approach, Dr Tewari has authored three more books on Organic Chemistry for undergraduate and postgraduate students His research interest includes Organic Synthesis and Heterocyclic Chemistry ORGANIC CHEMISTRY A Modern Approach Volume-I Nimai Tewari Associate Professor (Retired) Department of Chemistry Katwa College (affiliated to The University of Burdwan) West Bengal McGraw Hill Education (India) Private Limited CHENNAI McGraw Hill Education Offices Chennai New York St Louis San Francisco Auckland Bogotá Caracas Kuala Lumpur Lisbon London Madrid Mexico City Milan Montreal San Juan Santiago Singapore Sydney Tokyo Toronto McGraw Hill Education (India) Private Limited Published by McGraw Hill Education (India) Private Limited 444/1, Sri Ekambara Naicker Industrial Estate, Alapakkam, Porur, Chennai 600 116 Organic Chemistry—A Modern Approach (Volume-I) Copyright © 2017, by McGraw Hill Education (India) Private Limited No part of this publication may be reproduced or distributed in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise or stored in a database or retrieval system without the prior written permission of the publishers The program listings (if any) may be entered, stored and executed in a computer system, but they may not be reproduced for publication This edition can be exported from India only by the publishers, McGraw Hill Education (India) Private Limited ISBN (13): 978-93-5260-565-1 ISBN (10): 93-5260-565-9 Managing Director: Kaushik Bellani Director—Science & Engineering Portfolio: Vibha Mahajan Lead—UG Portfolio: Suman Sen Content Development Lead: Shalini Jha Specialist—Product Development: Amit Chatterjee Production Head: Satinder S Baveja Sr Manager—Production: Piyaray Pandita General Manager—Production: Rajender P Ghansela Manager—Production: Reji Kumar Information contained in this work has been obtained by McGraw Hill Education (India), from sources believed to be reliable However, neither McGraw Hill Education (India) nor its authors guarantee the accuracy or completeness of any information published herein, and neither McGraw Hill Education (India) nor its authors shall be responsible for any errors, omissions, or damages arising out of use of this information This work is published with the understanding that McGraw Hill Education (India) and its authors are supplying information but are not attempting to render engineering or other professional services If such services are required, the assistance of an appropriate professional should be sought Typeset at Text-o-Graphics, B-1/56, Aravali Apartment, Sector-34, Noida 201 301, and printed at Cover Printer: Visit us at: www.mheducation.co.in Dedicated to my daughter Aindrila and my son-in-law Ritam Mukherjee whose forbearance, constant encouragement and inspiration made this work possible CONTENTS Preface xv Structure, Bonding and Properties of Organic Molecules 1.1 1.2 1.1–1.313 Hybridization, Bond Lengths, Bond Strengths or Bond Dissociation Enthalpies, Bond Angles and VSEPR Theory 1.2 1.1.1 Hybridization 1.2 1.1.2 Bond Length 1.9 1.1.3 Bond Dissociation Enthalpy or Bond Dissociation Energy 1.9 1.1.4 Bond Angle 1.10 1.1.5 VESPR Theory and Molecular Geometry 1.11 Solved Problems 1.13 Study Problems 1.21 Electronegativity and Bond Polarity 1.22 Solved Problems 1.33 Study Problems 1.42 1.3 Molecular Formula as a Clue to Structure: Double Bond Equivalent (DBE) or Index of Hydrogen Deficiency (IHD) 1.44 Solved Problems 1.44 Study Problems 1.46 1.4 Acids and Bases 1.46 1.4.1 Brönsted-Lowry Theory of Acids and Bases 1.46 1.4.2 Lewis Acid-Base Theory 1.52 Solved Problems 1.53 Study Problems 1.57 Inductive and Electrometric Effects 1.60 1.5.1 Inductive Effect 1.60 1.5.2 Field Effect 1.67 1.5.3 Electromeric Effect 1.67 Solved Problems 1.67 Study Problems 1.73 1.5 Contents viii 1.6 Resonance and Resonance Effect or Mesomeric Effect 1.75 1.6.1 Resonance Energy 1.77 1.6.2 Rules for Writing Meaningful Resonance Structures 1.78 1.6.3 Relative Contribution of Resonance Structures towards Resonance Hybrid 1.78 1.6.4 Resonance or Mesomeric Effect 1.80 1.6.5 Isovalent and Heterovalent Resonance 1.81 1.6.6 Effect of Resonance on the Properties of Molecules 1.81 Solved Problems 1.98 Study Problems 1.112 1.7 Hyperconjugation 1.117 1.7.1 Sacrificial and Isovalent Hyperconjugation 1.119 1.7.2 Effect of Hyperconjugation on the Physical and Chemical Properties of Molecules and on the Stabilities of Intermediates 1.119 Solved Problems 1.122 Study Problems 1.124 Steric Effect 1.125 1.8.1 Properties of Molecules Influenced by Steric Effect 1.126 1.8.2 Proton Sponges 1.132 1.8.3 Face Strain or F-Strain 1.133 1.8.4 Steric Acceleration and Steric Retardation 1.134 1.8.5 Bredt’s Rule 1.135 Solved Problems 1.138 Study Problems 1.142 Intermolecular Forces 1.147 1.9.1 Dipole–Dipole Interactions 1.147 1.9.2 van der Waals Forces 1.149 1.9.3 Effect of Intermolecular Forces on Different Properties of Compounds 1.150 Solved Problems 1.161 Study Problems 1.167 1.8 1.9 1.10 1.11 Reactive Intermediates 1.170 1.10.1 Nonclassical Carbocation 1.177 1.10.2 Carbonium Ion and Carbenium Ion or Carbocation 1.178 Solved Problems 1.193 Study Problems 1.212 Tautomerism 1.218 1.11.1 Mechanism of Keto-enol Tautomerism 1.221 1.11.2 Difference between Resonance and Tautomerism 1.222 1.11.3 Position of the Tautomeric Equilibrium 1.223 1.11.4 Ring-chain Tautomerism 1.228 1.11.5 Valence Tautomerism 1.228 Solved Problems 1.228 Study Problems 1.236 Contents 1.12 1.13 1.14 ix Aromaticity 1.240 1.12.1 Criteria for Aromaticity 1.240 1.12.2 Antiaromatic Compounds 1.242 1.12.3 Nonaromatic Compounds 1.242 1.12.4 Classification of Compounds as Aromatic, Antiaromatic and Nonaromatic by Comparing their Stabilities with that of the Corresponding Open Chain Compounds 1.243 1.12.5 Modern Definition of Aromaticity 1.243 1.12.6 Molecular Orbital Energy Diagram of Some Ions and Molecules 1.244 1.12.7 Use of Inscribed Polygon Method to Determine the Relative Energies of p Molecular Orbitals for Cyclic Planar and Completely Conjugated Compounds and to Classify Them as Aromatic and Antiaromatic 1.245 1.12.8 Classification of Some Molecules and Ions as Aromatic, Antiaromatic and Nonaromatic 1.247 1.12.9 Homoaromatic Compounds 1.251 1.12.10 Some Chemical and Physical Consequences of Aromaticity 1.251 Solved Problems 1.262 Study Problems 1.267 Thermodynamics, Energy Diagrams and Kinetics of Organic Reactions 1.276 1.13.1 Thermodynamics 1.276 1.13.2 Energy Diagram 1.278 1.13.3 Kinetics 1.281 1.13.4 Catalysis 1.283 1.13.5 Hammond Postulate 1.284 1.13.6 Kinetic Control versus Thermodynamic Control of a Chemical Reaction 1.286 Solved Problems 1.287 Study Problems 1.295 Methods of Determining Mechanisms of Reactions 1.298 1.14.1 Kinetic Isotope Effects 1.305 Solved Problems 1.307 Study Problems 1.312 Principles of Stereochemistry 2.1–2.230 Introduction 2.2 2.1 Projection Formulas of Stereoisomers 2.3 2.1.1 Flying-Wedge Projection Formula 2.3 2.1.2 Fischer Projection Formula 2.4 2.1.3 Sawhorse Projection Formula 2.10 2.1.4 Newman Projection Formula 2.11 2.1.5 Interconversion of Projection Formulas 2.12 Solved Problems 2.16 Study Problems 2.25 2.2 Symmetry Elements 2.29 2.2.1 Simple Axis of Symmetry or Rotational Axis of Symmetry (Cn) 2.2.2 Plane of Symmetry (s) 2.33 2.29 Structure, Bonding and Proper es of Organic Molecules 1.169 14 Between the anti- and syn-isomers of pyridine-2-carboxaldoxime the antiform predominates in the isomeric distribution of the compound Explain this observations [Hint: The anti-form gets extra stability due to intramolecular hydrogen bonding (chelation) No such chelating effect is observed in the syn-isomer For this reason, the anti-form predominates in the isomeric distribution of the compound 15 16 Resorcinol has higher boiling point than 2-nitroresorcinol — Why? 8-hydroxyquinoline can be separated from 4-hydroxyquinoline by steam distillation Account for this observation [Hint: Intramolecular H-bonding occurs in 8-hydroxyquinoline but not in 4-hydroxyquinoline 17 Draw the H-bonding arrangements in CH3OH — H2O and CH3NH2 — HCHO systems [ 18 19 20 ] Why is the mp of sulphanilic acid so high? [Hint: Sulphanilic acid exists as a salt (a dipolar ion or zwitter ion): – H3N— —SO3 Because of this, sulphanilic acid has a much higher melting point.] Explain why H2O has a higher boiling point than CH3OH (65°C), NH3 (–33°C) and HF (20°C) Arrange the following compounds in order of increasing boiling point Explain your answer 1.170 21 22 23 24 25 26 Organic Chemistry—A Modern Approach Explain why 1-pentanol has solubility of 2.7 g per 100 ml of water, where as ethanol is completely miscible in water Why does one expect the cis-isomer of an alkene to have a higher boiling point than the trans-isomer? Alcohols with fewer than four carbons are soluble in water, but alcohols with more than four carbons are insoluble in water — Why? The boiling point of propylamine (bp 49°C) is higher than that of ethylmethylamine (bp 37°C) which in turn is higher than that of triethylamine (bp 3.5 °C) Explain [Hint: Trimethylamine (MC3N) has no N — H bond and so it cannot form hydrogen bonds with each other Ethylmethylamine (CH3 CH2NH CH3) has one N — H bond and as it remains associated through intermolecular hydrogen bonding Propylamine (CH3CH2CH2NH2) with two N — H bonds is more extensively hydrogen bonded This explains their boiling points] There are four amides with the molecular formula C3H7 NO Write their structures One of these amides has melting and boiling point that is substantially lower than that of the other three Identify this amide and explain your answer [Hint: CH3CH2CONH2, CH3CONHCH3, HCONHCH2CH3 and HCON (CH3)2 The last one has a melting and boiling point that is substantially lower than that of the other three because it does not have a hydrogen that is covalently bonded to nitrogen and, therefore, its molecules cannot form hydrogen bonds to each other The other molecules all have a hydrogen covalently bonded to nitrogen, and therefore, hydrogen-bond formation is possible.] Which of the following compounds is expected to volatilize easily and why? OH NH CH N I NH CH N II [Hint: Due to intramolecular hydrogen bonding, I is expected to volatilize easily.] 1.10 REACTIVE INTERMEDIATES A chemical reaction involves conversion of a molecule into a new molecule The new molecule, i.e., the product, has different arrangement of atoms compared to the starting molecule, i.e., the reactant A redistribution of electrons also occurs during this change In most of the organic reactions, conversation of reactants into products takes place through some specific steps The mechanism of an organic reaction is a detailed description of these steps It not only acquient us the number of steps involved in the reaction, but also informs us regarding the sequence of breaking old bonds and making new bonds Study of reaction mechanism is of immense importance in organic chemistry as thousands of apparently different reactions occur through a limited number of common steps Structure, Bonding and Proper es of Organic Molecules 1.171 Organic reactions usually involve fission of weaker covalent bonds and formation of stronger ones, so that a relatively stable molecule is formed from a less stable molecule Breaking of bonds requires energy while formation of bonds involves release of energy A covalent bond is represented by a dash (—) and the transfer of electrons is shown by using arrow signs Curved arrow signs containing two barbs ( ) indicate the shifting of a pair of electrons while the transfer of a single electron is indicated by curved arrow signs containing one barb ( ) or fishhook arrow [it is to be noted that the symbol ( ) is incorrect] Fission or cleavage of covalent bonds can take place in two ways depending on the nature of the bond involved, the nature of the attacking agent and the conditions of the reaction (1) Homolytic fission or homolysis: It a covalent bond in a molecule undergoes fission in such a way that each of the two bonded atoms gets one electron of the shared pair, it is called homolytic fission or homolysis This type of bond cleavage results in formation of neutral species called free radicals, or often simply radicals, i.e., a radical is a reactive intermediate with a single unpaired electron Homolytic fission is usually favoured by conditions such as nonpolar nature of the bond, high temperature, presence of high energy (uv) radiations or presence of radical initiators such as peroxides The homolytic fission of a bond A—B leading to the formation of free radicals A and B (each containing odd electrons), may be shown as follows: e.g., (2) Homolytic bond cleavage requires less energy than heterolytic bond cleavage Heterolytic fission or heterolysis: When a covalent bond breaks in such a way that the pair of electrons stays with the more electronegative atom, such a fission is called heterolytic fission or heterolysis This type of fission results in formation of ionic (cationic and anionic) intermediates If in the molecule A–B, B is more electronegative than A, the heterolytic fission of the bond leading to the formation ≈ @ of the cation A and the anion B: may be shown as follows: e.g., 1.172 Organic Chemistry—A Modern Approach This type of bond cleavage resulting in the formation of charged species, i.e., ions, is favoured by conditions such as polar nature of the covalent bond and the presence of polar solvent If the fragments of a heterolytic fission are carbon species, then the cation is called carbocation and the anion is called carbanion Both of them are unstable reactive intermediates Reactive intermediates: Under the influence of attacking reagent, most of the organic compounds (substrates) undergo either hemolytic or heterolytic fission of a bond to form certain short lived and highly reactive (hence cannot be normally isolated) chemical species which are called reactive intermediates or reaction intermediates Some common examples of reactive intermediates are carbocations, carbanions, free radicals, carbenes, nitrenes, arynes, etc A one-step reaction is called a concerted reaction and in such a reaction, no matter how many bonds are broken or formed, a starting material is converted directly into a product, i.e., a concerted reaction involves no reactive intermediate A stepwise reaction involves more than one step The substrate is first converted into an unstable intermediate, which then goes on to form the product Concerted reaction: Substrate Ỉ Product Stepwise reaction: Substrate Ỉ Reactive intermediate Ỉ Product An understanding of the structure and properties of these intermediates (molecules, ions or radicals) is very much important in understanding organic reaction mechanism Matters related to various intermediates are discussed below: (1) Carbocations Carbocations are a group of reactive intermediates having positively charged carbon atom bearing only six electrons These are represented by the symbol R! For example, ≈ ≈ ≈ CH3 , CH3CH2 , (CH3 )2 CH, etc Because of having a strong tendency to complete the octet of the electron-deficient carbon, carbocations are highly reactive species Generation: Carbocations are generated by heterolytic fission of a bond to carbon in which the leaving group is removed along with its shared pair of electrons The principal ways of generating carbocations are as follows: (i) Heterolysis of C — L bond (L = halogen, OTs, OBs, etc.) of neutral substrates by the influence of polar solvents like H2O, EtOH, etc leads to the formation of carbocations For example: Structure, Bonding and Proper es of Organic Molecules 1.173 (ii) Deamination of aliphatic amines by nitrous acid leads to the formation of carbocation For example: (iii) Protonation of alcohols followed by loss of H2O leads to the formation of carbocations For example: (CH3)3C—OH (iv) H + H2SO4 + + (CH3)3C—OH2 (CH3)3C + H2O Protonation of alkenes leads to the formation of carbocations For example: (v) Carbocations can be generated by the action of Lewis acid such as AlCl3 on alkyl halides For example: (vi) Carbocations can be obtained by treating alkanes with FSO3H — SbF5 called super acid FSO3H — SbF5 is, in fact, an extremely strong acid which donates H! even to an alkane in order to form a carbocation by extracting a hydride ion (H①) H2 is liberated in this reaction For example: (vii) A more stable carbocation can be obtained when a relatively less stable carbocation abstracts H① For example: H + Ph3C + H + – H-shift Ph3CH + Cycloheptatrienyl cation (aromatic) Nomenclature: In naming a carbocation, the word ‘cation’ is added to the name of the ≈ ≈ ≈ alkyl or aralkyl group For example, CH ,(CH )2 CH, and C6 H5 CH are named as methyl cation, isopropyl cation and benzyl cation, respectively Classification: Carbocations are classified as primary (1°), secondary (2°) and tertiary (3°) on the basis of the number of carbon atoms (one, two or three) directly bonded to 1.174 Organic Chemistry—A Modern Approach ≈ the positively charged carbon atom Fro example, ethyl cation (CH CH ) is a primary, ≈ ≈˘ È ≈ isopropyl cation (CH 3CHCH ) is secondary and tert-butyl cation Í(CH )3 C˙ is a tertiary Ỵ ˚ ≈ carbocation Methyl cation (CH ) with one carbon atom is a special case Structure: The positively charged carbon atom of a carbocation is sp2-hybridized Therefore, the shape of a carbocation is trigonal planar and the bond angle is 120° The three sp2 orbitals are utilized in making bonds to three substituents The vacant p orbital is perpendicular to the plane of sp2 hybridized orbitals Stability: Any factor which tends to delocalize the positive charge must increase the stability of carbocations while any factor which tends to localize or intensify the positive charge must decrease the stability of carbocation The stability of alkyl carbocations decreases in the following order: Carbocations are stabilized mainly by +I, +R and hperconjugation effects The relative stability of carbocation can be easily assessed by determining the heterolytic R — H Ỉ R! + H① dissociation energies in the gas phase The lower the value of energy, the greater the stability of the carbocation The bond dissociation energies D (R+ — H–) where R = alkyl, aralkyl or aryl are given in the following table Structure, Bonding and Proper es of Organic Molecules 1.175 Heterolytic R — H Ỉ R+ + H– dissociation energies in gas phase D(R+ + H–) in kcal/mol Ion ≈ CH 314.6 C6 H5≈ 294 Stability increases ≈ CH2 == CH 287 C2 H5≈ 276.7 ≈ CH == CH — CH2 ≈ (CH )2 CH 256 249.2 246 ≈ C6 H5 CH ≈ (CH3 )3 C 238 231.9 Factors which determine the stability of carbocations: (a) Inductive effect: An ion is stabilized by a factor that disperses its charge The electron-donating inductive effect (+I) exerted by an alkyl group attached to the positive carbon of a carbocation neutralizes the charge partially As a result, the charge becomes dispersed over the alkyl groups and the system becomes stabilized For example, the methyl group in ethyl cation stabilizes the system through its +I effect (b) The stabilities of carbocations increase with increasing the number of electronreleasing alkyl groups attached to the positive carbon Hyperconjugation effect: An alkyl group may also delocalize the positive charge of a carbocation by the hyperconjugation effect The charge becomes dispersed over the a–H atoms and consequently, the system becomes stabilized Hyperconjugation in ethyl cation, for example, occurs as follows: 1.176 Organic Chemistry—A Modern Approach H≈ H H H È | ≈ | | Í ÍH — C — CH ´ H — C == CH ´ H≈ C == CH ´ H — C == CH | | | Í H≈ H H ÍỴ H Hyperconjugation in ethyl cation ˘ ˙ ˙ ˙ ˙˚ As the number of a-hydrogens, i.e., the number of hyperconjugation structures, increases, the stability of carbocations increases Therefore, the stability of methyl substituted carbocations increases in the following order: ≈ ≈ (least stable) C H CH C H (3 a - H) (c) ≈ ≈ (CH)2 C H (CH )3 C (most stable) (6 a - H) (9 a - H) Resonance effect: Resonance is the most important factor influencing the stability of carbocations When there is a double bond a to the positive carbon of a carbocation, effective charge delocalization with consequent stabilization occurs Allyl and benzyl cations, for instance, are found to be highly stabilized by resonance A nonbonding lone pair on a heteroatom which is directly attached to the electrondeficient C stabilizes a carbocation more than any other interaction This resonance satisfies the octet of the cationic carbon, even though there is developed a positive ≈ charge over the heteroatom For example, CH 3OCH is a very stable carbocation in spite of its primary nature In fact, the methoxymethyl cation is obtained as a ≈ stable solid, CH 3OCH SbF6@ (d) Steric effect: Stability of tertiary carbocation increases due to steric effect For ≈˘ È example, the alkyl groups of triisopropyl cation Í(Me2CH)3 C˙ having planar Ỵ ˚ arrangement with a bond angle of 120° are far apart from each other and so, there Structure, Bonding and Proper es of Organic Molecules 1.177 is less steric interaction among them However, when this carbocation undergoes nucleophilic attack, i.e., when a change of hybridization of the central carbon atom from sp2 (trigonal) to sp3 (tetrahedral) occurs, the bulky isopropyl groups are pushed together This results in a steric strain (B strain) in the product molecule Because of this, carbocation is not much willing to react with a nucleophile, that is, its stability is enhanced due to steric reason (e) Constituting an aromatic system: The vacant p orbital of a carbocation may be involved in constituting a cyclic and planar (4n + 2)p electron system, where n = 0, 1, … etc., i.e., a carbocation may be stabilized by constituting an aromatic system In fact, it acquires maximum stability by this process Cycloheptatrienyl cation, for example, is unusually stable because it is a planar 6p electron system and aromatic 1.10.1 Nonclassical Carbocation A carbocation in which the positive charge is delocalized by a double or triple bond that is not in the allylic position or by a single bond is called a nonclassical carbocation 7-Norbornenyl and norbornyl cations are two examples of nonclassical carbocation 1.178 Organic Chemistry—A Modern Approach In a carbocation, if there is one carbon atom between the positively charged carbon atom and the double bond, it is called a homoallylic carbocation 7-Norbornenyl cation is also an example of homoallylic carbocation 1.10.2 Carbonium Ion and Carbenium Ion or Carbocation There are intermediates in which a carbon bears a positive charge, but the formal covalency of the carbon atom is five rather than three The simplest example is the methanonium ion, ≈ CH5 Such pentacoordinated positive ions are called carbonium ions The intermediates in which there is positive charge at a carbon atom which is trivalent are called carbenium ≈ ≈ ions or carbocations For example, methyl cation (CH ) and ethyl cation (CH CH ), etc ≈ [N.B The methanonium ion CH5 has a three-centre two-electron bond It is not known whether this ion is a transition state or a true intermediate, but an ion CH5≈ has been detected by mass spectroscopy (2) Carbanions Carbanions are a group of reactive intermediates carrying a negative charge on carbon @ atom possessing eight electrons in its valence shell For example, CH (methyl carbanion) @ and CH CH (ethyl carbanion), etc They are represented by symbol R① Their reactivity is due to the presence of formal negative charge on the carbon atom Generation: Carbanions are generated by heterolytic fission of a bond to carbon in which the bonding electron pair remains with the carbon atom Structure, Bonding and Proper es of Organic Molecules 1.179 The principal methods of generating carbanions are as follows: (i) By an abstraction of an acidic hydrogen alpha to an electron-withdrawing group: Compounds containing acidic hydrogen attached to carbon alpha to —NO2, —CN or C == O groups produce carbanions when treated with a strong base For example: (ii) By abstraction of hydrogen from terminal alkynes using a strong base: Terminal alkynes being acidic produce carbanion when treated with strong bases For example: – – H2N: + H—C∫∫ C—CH3 (iii) C∫∫ C—CH3 + NH3 By metal halogen exchange: When organic halogen compounds are treated with strongly electropositive metals like Li, Na, etc in an inert solvent, carbanions are obtained in the form of organometallic compounds For example: ≈ @ ether CH — Br + 2Li ỉỉỉ Ỉ CH L i + LiBr @ ≈ ether Ph3C — Cl + 2Na ỉỉỉ Ỉ PH C Na + NaCl @ ≈ ether Ph — Br + 2Li ỉỉỉ Ỉ Ph Li + LiBr 1.180 Organic Chemistry—A Modern Approach (iv) By decomposition of carboxylate ions: When metal carboxylates are heated, they undergo decarboxylation to yield carbanions For example: (v) By addition of an anion to multiple bond containing electron-withdrawing groups: Carbanions are obtained when an alkene containing electron-attracting group undergoes nucleophilic attack by an anion For example: (vi) By addition of electrons to an unsaturated system: Unsaturated compounds may accept electrons from electropositive metals to generate carbanions For example, when cyclooctatraene is treated with metallic sodium, it is converted to cyclooctatetraenyl dianion which is a stable aromatic system containing (4n + 2)p electrons, where n = Nomenclature: In naming a carbanion, the word ‘anion’ is added to the name of the @ @ @ alkyl or aralkyl group For example, CH , (CH )2 CH and C6 H5 CH are named as methyl anion, isopropyl anion and benzyl anion, respectively Classification: Carbanions are classified as primary (1°), secondary (2°) and tertiary (3°) on the basis of the number of carbon atoms (one, two or three) directly bonded to @ the negatively charged carbon atoms For example, ethyl anion (CH CH ) is a primary, @ ≈ isopropyl anion (Me2 CH) is a secondary and tert-butyl anion Me3 C is a tertiary carbanion @ Methyl anion (CH ) with one carbon atom is a special case Structure, Bonding and Proper es of Organic Molecules 1.181 Structure: In simple carbanions, the negatively charged central carbon atom is sp3 hybridized; it is surrounded by three bonding electron pairs and one unshared pair of electrons occupying an sp3 orbital Therefore, a carbanion is expected to have the tetrahedral shape However, the shape is not exactly that of a tetrahedron and in fact, it is found to have the pyramidal shape just like ammonia Since the repulsion between the unshared pair and any bonding pair is greater than the repulsion between any two bonding pairs, therefore, the angle between two bonding pairs (i.e., between two sp3-s bonds) is slightly less than the normal tetrahedral value of 109.5° and for this reason, a carbanion appears to be shaped like a pyramid with the negative carbon at the apex and the three groups at the corners of a triangular base The central carbon atoms in resonance-stabilized carbanions are, however, sp2 hybridized and hence they are planar This is due to the fact that planarity is an essential criterion for resonance to occur Allyl anion, for example, is a planar carbanion The negative carbon atom of the following conjugated anion is, however, not resonancestabilized because according to the Bredt’s rule a double bond at the bridgehead position cannot be formed in bridged bicyclic compounds with small rings and for this reason, this carbon is sp3-hybridized Its shape is pyramidal 1.182 Organic Chemistry—A Modern Approach Stability: Any factor that tends to delocalize the negative charge must increase the stability of carbanions while any factor that tends to localize or intensify the negative charge must decrease the stability of carbanions [W = Electron-withdrawing group; it disperses the negative charge and thus, stabilizes the carbanion] [D = Electron-donating group; it intensifies the negative charge and hence destabilizes the carbanion] @ @ The stability of carbanions follows the order: Methyl anion (CH ) > R CH (primary or 1°) @ @ > R CH (secondary or 2°) > R C (tertiary or 3°) because an alkyl group destabilizes a carbanion Carbanions are stabilized mainly by –I and –R effects Functional groups in the a position stabilize carbanions in the following order: —NO2 > —COR > —COOR > —CN ~ —CONH2 > —X > —H The structural features responsible for the increased stability of carbanions are as follows: (a) s character of the anionic carbon atom: An s orbital being closer to the nucleus than the p orbital in a given main quantum level possesses lower energy An electron pair in an orbital having large s character is, therefore, more tightly held by the nucleus and hence of lower energy than an electron pair in an orbital having relatively small s character Thus, a carbanion in which the anionic carbon is sphybridized (50 percent s character) is more stable than a carbanion in which the anionic carbon is sp2 hybridized (33.33 percent s character), which in turn is more stable than a carbanion in which the negative carbon is sp3-hybridized (25 percent s character) Therefore, the order of decreasing stability of carbanions is (b) Inductive electron withdrawal: Substituents possessing electron-withdrawing inductive effects (–I) stabilize a carbanion by dispersing or delocalizing the negative charge Examples of some carbanions experiencing such stabilizing effect are as follows: Structure, Bonding and Proper es of Organic Molecules 1.183 (c) Resonance effect: If there is a double or triple bond conjugated with the anionic carbon, the carbanion is stabilized by delocalization of the negative charge with the p orbitals of the multiple bond Thus, allylic and benzylic carbanions and carbanions attached to the groups containing polarized multiple bond such as —NO2, —C ∫∫N, C ∫∫ O, etc are well stabilized by resonance For example: (d) Formation of an aromatic system: A carbanion becomes highly stabilized if it is involved in constituting an aromatic system [a planar (4l + 2) p electron system, where n = 0,1,2, …, etc.] Cyclopentadienyl anion, for example, is unusually stable because it is a close loop of 6p electron system and hence aromatic @ Optical inactivity of asymmetric carbanions like CH 3CH CHCH : Asymmetric carbanions are found to be optically inactive and cannot be separated into two enantiomers This can be explained in terms of umbrella effect There ... Ekambara Naicker Industrial Estate, Alapakkam, Porur, Chennai 600 116 Organic Chemistry? ? ?A Modern Approach (Volume- I) Copyright © 2017, by McGraw Hill Education (India) Private Limited No part... journals Apart from Organic Chemistry? ? ?A Modern Approach, Dr Tewari has authored three more books on Organic Chemistry for undergraduate and postgraduate students His research interest includes Organic. . .ORGANIC CHEMISTRY A Modern Approach Volume-I ABOUT THE AUTHOR Nimai Tewari is a retired associate Professor, Department of Chemistry, Katwa College (affiliated to The University of Burdwan),