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First Edition : 1995Seventh Revised Edition : 2004Eighth Revised Edition : 2005Reprint : 2006, 2007, 2009Ninth Revised Edition : 2010Tenth Revised Edition : 2012Eleventh Revised Edition : 2013Reprint : 2014Twelfth Revised Edition : 2015(As per New Syllabus)Thirteenth Edition : 2016Fourteenth Revised Edition : 2017(As per New Syllabus)Fifteenth Edition : 2018Sixteenth Edition : 2019

COLLEGEORGANIC

CHEMISTRYS.Y.B.Sc.

(Written according to Revised Syllabus of University of Mumbaiwith effect from the academic year 2017-18)

R.S. RaoVice-Principal & Associate Professor,

Dept. of Chemistry,G.N. Khalsa College of Arts, Science & Commerce,

Matunga, Mumbai.

Dr. (Mrs.) Sushil PuniyaniM.Sc., M.Phil., Ph.D.,

Rtd. Head, Dept. of Chemistry,K.C. College of Arts, Science and Commerce,

Churchgate, Mumbai.

Prof. Dr. A.K. UpadhyayM.Sc., M.Phil., Ph.D.

Rtd. Head, Dept. of Chemistry,Smt. CHM College,

Ulhasnagar.

Dr. Tanuja ParulekarAssociate Professor,

Department of Chemistry,S.I.W.S. College,

Wadala, Mumbai.

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SYLLABUS

COURSE CODE – USCH301PAPER I

THEORY: 45 LECTURES

SEMESTER - III

3.1 Reactions and Reactivity of Halogenated Hydrocarbon (4L)

3.1.1 Alkyl Halides

Nucleophilic substitution reactions: SN1, SN2 and SNimechanisms with stereochemical aspects and factorsaffecting nucleophilic substitution reactions-nature ofsubstrate, solvent, nucleophilic reagent and leaving group.

3.1.2 Aryl Halides

Reactivity of aryl halides towards nucleophilic substitutionreactions. Nucleophilic aromatic substitution (SNAr)addition-elimination mechanism and benzyne mechanism.

3.1.2. Organomagnesium and Organolithium Compounds(3L)

Nomenclature, nature, type and reactivity of carbon-metalbond. Preparation using alkyl/aryl halide. Structure,stability and reactions with compounds containing acidichydrogen, carbonyl compounds, CO2, cyanides andepoxides.

3.2 Alcohols, Phenols and Epoxides (8L)

3.2.1. Alcohols

Nomenclature, preparation: Hydration of alkenes,hydrolysis of alkyl halides, reduction of aldehydes andketones, using Grignard reagent. Properties: Hydrogenbonding, types and effect of hydrogen bonding on differentproperties. Acidity of alcohols, Reactions of alcohols.

PREFACE

This book is written according to the revised syllabus prescribed bythe University of Mumbai for S.Y.B.Sc. class, as per the UGC guidelines.This syllabus will come into effect from the academic year 2017-18.

The book gives a good foundation of the theoretical aspects such asacidity, basicity, tautomerism, resonance, H-bonding, etc. In the earlierclass, the aliphatic compounds are covered. But in this book, the emphasisis on study of aromatic compounds. The chapter on organometalliccompounds has been introduced in the book.

Several reaction mechanisms have been included in which electronshifts have been clearly shown by curved arrows. An introduction hasbeen made about simple heterocyclic compounds. Throughout the bookIUPAC names have been given along with the trivial names. Further,reactions involving interconversions of compounds are included.

In the book several charts, tables, P.E. diagrams and illustrations aregiven to make the concepts clear. Many questions and excercises havebeen given for the students to practice. Any suggestions for improvementof this edition from teachers as well as students will be highly appreciated.We thank the publisher for bringing out fine edition of this book.

AUTHORS

3.2.2. Phenols

Preparation, physical properties and acidic character.Comparative acidic strengths of alcohols and phenols,resonance stabilization of phenoxide ion. Reactions ofphenols.

3.2.3. Epoxides

Nomenclature, methods of preparation and reactions ofepoxides: reactivity, ring opening reactions by nucleophiles(a) In acidic conditions: hydrolysis, reaction with halogenhalide, alcohol, hydrogen cyanide. (b) In neutral or basicconditions: ammonia, amines, Grignard reagents,alkoxides.

PAPER II

Carbonyl Compounds (15L)

3.1 Nomenclature of aliphatic, alicyclic and aromatic carbonylcompounds. Structure, reactivity of aldehydes and ketones andmethods of preparation; oxidation of primary and secondaryalcohols using PCC, hydration of alkynes, action of Grignardreagent on esters, Rosenmund reduction, Gattermann - Kochformylation and Friedel Craft acylation of arenes.

3.2 General mechanism of nucleophilic addition, and acid catalyzednucleophilic addition reactions.

3.3 Reactions of aldehydes and ketones with NaHSO3, HCN, RMgX,alcohol, amine, phenyl hydrazine, 2,4-Dinitrophenyl hydrazine,LiAlH4 and NaBH4.

3.4 Mechanisms of following reactions: Benzoin condensation,Knoevenagel condensation, Claisen-Schmidt and Cannizzaroreaction.

3.5 Keto-enol tautomerism: Mechanism of acid and base catalysedenolization

3.6 Active methylene compounds: Acetylacetone, ethyl acetoacetatediethyl malonate, stabilised enols. Reactions of acetylacetone andethyl acetoacetate (alkylation, conversion to ketone, mono- anddicarboxylic acid).

COURSE CODE – USCH401PAPER I

SEMESTER - IV

3.1 Carboxylic Acids and their Derivatives (11L)

3.1.1. Nomenclature, structure and physical properties, acidityof carboxylic acids, effects of substituents on acid strengthof aliphatic and aromatic carboxylic acids.

3.1.2. Preparation of carboxylic acids: oxidation of alcohols andalkyl benzene, carbonation of Grignard and hydrolysis ofnitriles.

3.1.3. Reactions: acidity, salt formation, decarboxylation,reduction of carboxylic acids with LiAlH4, diborane, Hell-Volhard-Zelinsky reaction, conversion of carboxylic acidto acid chlorides, esters, amides and acid anhydrides andtheir relative reactivity.

3.1.4. Mechanism of nucleophilic acyl substitution and acid-catalysed nucleophilic acyl substitution. Interconversionof acid derivatives by nucleophilic acyl substitution.

3.1.5. Mechanism of Claisen condensation and Dieckmanncondensation.

3.2 Sulphonic acids (4L)

Nomenclature, preparation of aromatic sulphonic acids bysulphonation of benzene (with mechanism), toluene andnaphthalene; Reactions: Acidity of arene sulfonic acid,comparative acidity of carboxylic acid and sulfonic acids. Saltformation, desulphonation. Reaction with alcohol, phosphorouspentachloride, IPSO substitution.

PAPER IINitrogen containing compounds and heterocyclic compounds

3.1 Amines: (4L)

Nomenclature, effect of substituent on basicity of aliphatic andaromatic amines; Preparation: Reduction of aromatic nitrocompounds using catalytic hydrogenation, chemical reductionusing Fe-HCl, Sn-HCl, Zn-acetic acid, reduction of nitriles,

(v) (vi)

(vii) (viii)

ammonolysis of halides, reductive amination, Hofmannbromamide reaction.

Reactions: Salt Formation, N-acylation, N-alkylation, Hofmann’sexhaustive methylation (HEM), Hofmann-elimination reaction,reaction with nitrous acid, carbylamine reaction, Electrophilicsubstitution in aromatic amines: bromination, nitration andsulphonation.

3.2 Diazonium Salts: (3L)

Preparation and their reactions/synthetic application: Sandmeyerreaction, Gattermann reaction, Gomberg reaction, replacement ofdiazo group by –H, –OH. Azo coupling with phenols, naphtholsand aromatic amines, reduction of diazonium salt to aryl hydrazineand hydroazobenzene.

3.3 Heterocyclic Compounds: (8L)

3.3.1. Classification, nomenclature, electronic structure,aromaticity in 5-numbered and 6-membered ringscontaining one heteroatom.

3.3.2. Synthesis of Furan, Pyrrole (Paal-Knorr synthesis, Knorrpyrrole synthesis, and Hantzsch synthesis), Thiophene,Pyridine (Hantzsch synthesis).

3.3.3. Reactivity of furan, pyrrole and thiophene towardselectrophilic substitution reactions on the basis of stabilityof intermediate and of pyridine on the basis of electrondistribution. Reactivity of pyridine towards nucleophilicsubstitution on the basis of electron distribution.

3.3.4. Reactions of furan, pyrrole and thiophene: halogenation,nitration, sulphonation, Vilsmeier-Haack reaction, Friedel-Crafts reaction. Furan: Diels-Alder reaction, ring opening.Pyrrole: Acidity and basicity of pyrrole. Comparison ofbasicity of pyrrole and pyrrolidine.

3.3.5. Pyridine: Basicity. Comparison of basicity of pyridine,pyrrole and piperidine. Sulphonation of pyridine (with andwithout catalyst), reduction and action of sodamide(Chichibabin reaction).

CONTENTS

SEMESTER III

Paper Unit Chapter Name of Topic No. of Page No.Lectures

I 3.1 – Reactions and Reactivity of 7Halogenated Hydrocarbons

3.1.1 1 Alkyl Halides 3 – 14

3.1.2 2 Aryl Halides 15 – 23

3.1.3 3 Organomagnesium and 24 – 41Organolithium Compounds

3.2 – Alcohols, Phenols, Epoxides 83.2.1 4 Alcohols 42 – 64

3.2.2 5 Phenols 65 – 86

3.2.3 6 Epoxides 87 – 95

II 3.1 7 Carbonyl Compounds 15 96 – 137

SEMESTER IVI 3.1 8 Carboxylic Acids and their 11 141 – 172

Derivatives

3.2 9 Sulphonic Acids 4 173 – 182

II 3.1 10 Amines 5 183 – 206

3.2 11 Diazonium Salts 2 207 – 218

3.3 12 Heterocyclic Compounds 8 219 – 240

SEMESTER - III

College Organic Chemistry – S.Y.B.Sc.Alkyl Halides

CHAPTER 1

UNIT 3.1.1

Alkyl Halides

Nucleophilic substitutions in Alkyl halides:When a substitution is brought about by a nucleophile it is called

nucleophilic substitution or SN reaction.The number of molecules taking part in a chemical reaction as represented

by a simple chemical equation is called molecularity. Thus the reactions canbe classified as uni or mono molecular, bimolecular, ter-molecular etc. However,the number of molecules whose concentration actually affects the rate of thereaction is called order of the reaction. The SN reactions are further classifiedas SN1 and SN2 in which the order of the reaction is 1 and 2 respectively.

(i) Mechanism of SN2 reaction (Substitution NucleophilicBimoleculer)

Example: Mechanism of alkaline hydrolysis of primary alkyl halide:Consider the action of aqueous sodium or potassium hydroxide on a

primary alkyl halide such as methyl bromide.CH3Br + NaOH ? ? ? CH3OH + NaBr

In the ionic form the above equation can be written as:CH3Br + OH

– ? ? ? CH3OH + Br–

During the hydrolysis a stronger (more basic) nucleophile namely :OH–ions displaces or substitutes the weaker nucleophile, Br– ions. Therefore, thereaction is a nucleophilic substitution reaction (SN reaction).

Kinetics of the reaction: Experimentally it is observed that the rate ofreaction depends on the concentrations of both CH3Br and :OH

– ions.Rate = k [CH3Br] [OH

–]Therefore, it is a second order nucleophilic substitution reaction or SN2

reaction.Mechanism of Hydrolysis: This is a one step reaction whose mechanism

is explained as follows.

HO + C Br??+ ?+? ?

HH H

SlowH

Nucleophile

??HO ---- C ---- Br

H

H

RapidHO C + Br?

HH

H

Inversion ofconfiguration

Transition state

The C? Br bond is polar. The nucleophile OH– is repelled by the halogenatom having fractional negative charge. Therefore, the nucleophile, i.e., theOH– group approaches the carbon atom from the backside, i.e., the side oppositeto the group to be substituted (Br). The formation of C? OH bond and breakageof C? Br bond takes place simultaneously. Therefore, a transition state isformed in which both OH and Br are loosely attached and negative charge isevenly distributed between them. In this state the molecule has highest energycontent as five groups are attached to the central carbon atom. Therefore, thetransition state represents a highly unstable arrangement and very rapidlyeliminates :Br– ion to give the product.

The other three H-atoms attached to the central carbon atom move througha coplanar configuration in the transition state. Due to the backside attack ofnucleophile the product is formed with inversion of configuration, i.e., theproduct has exactly the opposite configuration as compared to the substrate.

Energy profile diagram: It is a graph of potential energy changes takingplace during the course of the reaction plotted against the reaction co-ordinates.

3.1 Reactions and Reactivity of HalogenatedHydrocarbons

4

— 3 —

College Organic Chemistry – S.Y.B.Sc.Alkyl Halides

E = Energy of activationH = Heat of reactiona

?Ea

?HCH Br + :OH3

CH OH + :Br3

Reactants

ProductReaction co-ordinates

H

?? ??H

H

(Transition state)HO ----- C -----Br??Po

tent

ial

ener

gy

Fig. 1.1 Energy profile diagram of alkaline hydrolysis of methyl bromide bySN2 mechanism.

StereochemistryFor example, chlorosuccinic acid and malic acid can be interconverted

by nucleophilic substitutions.

COOHCHO

HCH COOH2

PCl5

KOH

COOHC Cl

HCH COOH2

AgOH

HC OH

COOH

CH COOH2

(–) Malic acid (+) Chlorosuccinic acid (+) Malic acid

By these reactions, one enantiomer of malic acid is converted into anotherby nucleophilic substitution at the chiral carbon atom. These reactions wereearlier known as Walden inversions.

Hydrolysis of (–) 2-bromooctane gives (+) octan-2-ol.

CH

BrAq.NaOH

–NaBr

SN2 reaction

HO CH

(–) 2-Bromooctane (+) Octan-2-ol

C H6 13

CH3 CH3

C H6 13

Mechanism: SN2 reaction occurs in one step. The nucleophile ( OHion) attacks the carbon from the side opposite to the leaving group (bromine).In the transition state, the carbon atom is partially bonded to both –OH and–Br. The other three groups and carbon atom become coplanar. When C–Obond is completely formed, at the same time C–Br bond is completely broken.The overall reaction is a concerted process occuring in one step without anyintermediate. Hence, in their final product, the entering group occupies aposition different from the leaving group and thus causing inversion ofconfiguration in the product.

C6 C6H13 H13

HH3C

BrOH

HOH

C H6 13CC

(–) 2-Bromooctane (+) Octan-2-ol

Br–:Br

HO C

CH3CH3 H

T.S

?– ?– –

Therefore, SN2 reactions proceed with stereochemical inversion calledWalden inversions.

Thus, every SN2 reaction at a chiral carbon atom results in 100% inversionof configuration. The reaction with the reagents NaOH, PCl5, etc. follow SN

2

mechanism and hence cause inversion of configuration. However, reactionwith AgOH does not cause inversion.

Hughes confirmed these facts by following experimentWhen optically active, (+) 2-iodooctane was treated with radioactive

sodium iodide in dry acetone solution, it not only exchanged ordinary iodidewith radioactive iodide, but also lost its optical activity. It was observed thatthe rate of loss of optical activity was twice the rate of isotopic exchange.

To account for the observed facts, it was concluded that the nucleophileI

–* attacks from the backside and passes through the T.S. and invertsconfiguration.

CH

I

I — C H

C H136 C H136

CH3 CH3

C H136I I I

–* * *?– ?–CCH3H

65

College Organic Chemistry – S.Y.B.Sc.Alkyl Halides

Since the rotation of the inverted molecule exactly cancels the rotationof unreacted molecule. Thus racemisation takes place. For every iodine atomsubstituted by radioactive iodine atom (I*) two molecules get racemised.Therefore, rate of loss of optical activity must be twice the rate of isotopicexchange. Experimentally it is confirmed that the ratio of rate of racemizationand the rate of iodine exchange is nearly equal to 2.

Further, the rate of reaction depended upon the concentration of alkyliodide as well as radioactive iodide ions, conforming its SN2 character. Thesefacts prove that every SN2 reaction of chiral carbon atom results in the inversionof configuration.

(ii) Mechanism of SN1 reaction (Substitution NucleophilicUnimolecular)

Example: Mechanism of Alkaline hydrolysis of tert-alkyl halide.Consider the action of aqueous sodium hydroxide or potassium hydroxide

on a tertiary alkyl halide such as t-butyl bromide.(CH3)3CBr + KOH ? ? ? (CH3)3COH + KBr

The ionic form of the reaction is:

(CH3)3 CBr + OH ? ? ? (CH3)3 COH + :Br–

During this hydrolysis the stronger nucleophile, :OH– has displaced theweaker nucleophile, Br– and therefore, it is nucleophilic substitution (SN)reaction.

Kineties of reaction: Experimentally is observed that the rate of thereaction depends only on the concentration of (CH3)3CBr and is independentof the concentration of the OH– ions, i.e.,

Rate = k [(CH3)3CBr]Therefore, this reaction is first order nucleophilic substitution reaction

SN1 reaction.Mechanism: This nuclephilic substitution takes place in two steps which

can be represented as follows.Step (i): Ionization of t-butyl bromide:

C Br??+Slow

CH3

(T.S.)1

CH3CH3

C ----- Br

CH3

CH3CH3

??

Carbocation

C + Br

CH3

H3C CH3

The three methyl groups of tert-butyl bromide sterically hinder theapproach of the nucleophile and thus prevent the backside attack. Therefore inthe first step C–Br bond ionizes to give t-butyl carbocation and bromide ion.Due to gradual breaking of the bond a transition state (T.S)1 is formed. Theelectron repelling inductive effect of the methyl groups facilitates the ionization,by stabilising, carbocation.

Step (ii): Attack of nucleophile ( OH ):

?+?+

(T.S.)2(T.S.)2

C ---- OHHO ---- C

CH3CH3Backsideattack

t-Butyl aclohol

CH3CH3CH3CH3

????CH3

H3C

(Planer)

CH3

HO: + C + :OH Frontsideattack

HO C?

CH3

CH3CH3

Retention

C OH?

CH3

CH3CH3

Inversion

+

The nucleophile ( OH ) attacks the carbocation forming t-butyl alcohol.But due to gradual formation of the C–OH bond a transition state (T.S)2 is firstformed. The carbocation has planar configuration, hence it can be attacked bythe nucleophile from either side. The frontside attack results in the productwith retention of configuration. However, the backside attack results in theproduct with inversion of configuration. Since the attack from either side isequally probable, there will be retention in 50% of the molecules and inversionin the 50% of the molecules.

Energy profile diagram is obtained by plotting the potential energiesof all the species against the reaction co-ordinates. The two-humps in the graphindicates two steps in the reaction.

Activation energy is the energy which must be supplied to reactants inorder to form the transition state. It is equal to difference in potential energiesof reactants and the transition state. The step-(i) has a higher activation energy,hence it is slow. The step-(ii) has lower activation energy, hence it is fast.

87

College Organic Chemistry – S.Y.B.Sc.Alkyl Halides

Step I(T.S)

(CH ) C ----- Br1

3 3

?H(CH ) Br + :OH3 3

(CH ) C OH + :Br3 3

Reactants

Product

Intermediate

Reaction co-ordinates

Pote

ntia

l en

ergy

Step II

(CH ) C ----- OH

(T.S)2

3 3

C

CH3 CH3

Ea2Ea1

CH3+ :Br

Fig. 1.2 Energy profile diagram of alkaline hydrolysis of t-butyl bromide by SN1mechanism.

Ea1 = Energy of activation for step 1Ea2 = Energy of activation for step 2?H = Heat of reaction.In multi-step reactions the slowest step determines the overall rate of

reaction. This is called the rate controlling step. In the slow step only t-butylbormide takes part and not the nucleophile. Hence it is a first order reaction.

StereochemistrySN1 reaction results in racemisation, e.g., when optically active 3-bromo-

3-methylhexane is heated with aqueous solution of acetone, the racemic mixtureof 3-methylhexan-3-ol is obtained.

C H2 5C H2 5 C H2 5

C BrCH3

(+) 3-Bromo-3-methylhexane (+) Isomer (–) Isomer

C OHCH3 CH3

C

Aqueous

+ HO

3-methyl-3-hexanol(Racemic mixture)

CH COCH33

C H3 7 C H3 7 C H3 7

Mechanism: It is a two step reaction:(i) In first step, the alkyl halide ionizes to form a carbocation and bromide

ion. The carbocation has trigonal planar structure and is achiral.(ii) In the second step, the nucleophile attacks from either side with equal

rate. The attack from the opposite side results in inversion of configurationand attack from the same side results in retention of configuration. Thus, theSN1 reaction results in almost complete racemisation. Thus, the faces ofcarbocation are enantiotropic.

BrC?+

C Br?–

C + Br

(+) Isomer (T. S)

C H2 5

C H3 7C H3 7

CH3CH3

C H2 5 C H2 5

C H3 7

+

H3C

C+ :OH2

2

CH3–H+

Backsideattack

2

1

1

HO

OH

C

C

CH C H3 73

(–) Isomer

(Inversion)

(Retention)

(+) Isomer

Racemicmixture

Frontsideattack

C H2 5

C H3 7

C H3 7CH3

C H2 5

C H2 5H O:2

However, the mixture is found to be slightly optically active. The bromideion which is detached may take some more time to move away from thecarbocation. Hence, the attack of nucleophile from the same side will be slightlydelayed. Hence, the product with inverted configuration will be slightly more.Hence, the mixture will be slightly optically active.

Factors Affecting SN1 and SN2 Reactions:(i) Nature of substrate: As we go from primary to tertiary alkyl halides

the stability of the corresponding carbocation increases (primary < secondary< tertiary). Hence tertiary halides would prefer SN1 mechanism. Further, thecrowding of the alkyl groups increases from primary to tertiary halides. Hencethe attack of the nucleophile is hindered sterically by the alkyl groups. Hence,

109

College Organic Chemistry – S.Y.B.Sc.Alkyl Halides

SN2 type of mechanism can take place in primary halides where steric repulsionis minimum.

SN1 mechanism? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?

Primary Secondary Tertiary? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?

SN2 mechanism(ii) Strength of nucleophile: In SN2 reaction the attack of nucleophile

on the carbon atom takes place in the rate controlling step. Hence strongnucleophiles are required to promote SN2 reaction. In SN1 reaction thenucleophile reacts with carbocation only in the fast step. Hence weaknucleophiles like water can also result in SN1 reactions.

(iii) Concentration of the base: With the increase in concentration ofthe base the probability of attack of the base ( OH ) on the carbon atomincreases, hence SN2 reaction is favoured. In SN1 reaction the base ( OH ) hasto react with the cation. Hence low concentration of the base will be sufficientto cause SN1 reactions.

(iv) Nature of Solvent: The polarity of the solvent has a marked affecton the mechanism of the reaction. The SN1 reaction involves formation of acarbocation which can be stabilised by solvents through solvation. Hence,more polar solvents favour SN1 reaction. Whereas, in SN2 reaction there is adispersal of charge in the transition state. Hence, less polar solvent are sufficientfor SN2 reactions.

(v) Nature of leaving group: A group attached to the substrate whichdeparts along with the electron pair from the molecule is called the leavinggroup. The nature of the leaving group also decides the mechanism. The basicityis one of the factors, i.e., a weaker base is the better leaving group. For example,halides are good leaving groups as they are weak bases. The other factor is thesize of the group, i.e., a larger group will be a better leaving group. Amonghalogens, iodine being largest, acts as a better leaving group.

The –OH group is not a good leaving group but the aryl sulphonate(–OSO2Ar) is excellant leaving group. A good leaving group favours SN

2

reaction.

SN1 Reactions(i) Rate of the reaction depends only

on concentration of alkyl halide

(ii) Rate depends on the structure ofhalides in the order of 3o > 2o > 1o> CH3X

(iii) Reaction is favoured by more polarsolvents

(iv) Nature of the leaving groups doesnot affect position of reactionequilibrium.

(v) The reaction takes place even withweak nucleophile

SN2 ReactionsRate of the reaction depends onconcentration of alkyl halide andnucleophileRate depends on the structure ofhalides in the order of CH3X > 1

o

> 2o > 3o.Reaction is favoured by less polarsolvents.Nature of the leaving groupaffects position of reactionequilibrium.The reaction is favored by morepowerful nucleophile (strongbase).

(iii) SNi ReactionWhen an alcohol is treated with thionyl chloride, the hydroxyl group is

replaced by chlorine atom. This is a nucleophilic substitution which follows asecond order kinetics. When a chiral alcohol is used, there is no change in theconfiguration, i.e., there is retention of configuration. For example,

CH3

C OH + SOCl2

C H6 5

H HC H6 5

C

CH3

Cl + SO + HCl2

Mechanism: The reaction does not follow SN2 mechanism, as there isno inversion of configuration. The alcohol first reacts with thionyl chlorideforming an intermediate alkyl chlorosulphite (R-OSOCl) which can be isolatedunder mild conditions. In this step, no inversion is possible as the bond betweenchiral carbon atom and oxygen atom is not broken. Then the chlorosulphiteloses SO2 and forms an intimate ion pair. The intimate ion pair is short-livedintermediate in which the carbocation carbon and chloride are in closeproximity. Before the carbocation changes its configuration, it combines withchloride ion to form alkyl chloride having similar configuration as the chiralalcohol.

1211

College Organic Chemistry – S.Y.B.Sc.Alkyl Halides

CH3

HC

C H6 5

OH + Cl

OS

Cl

CH3

HC O

C H6 5

OS

Cl–SO2

CH3

C+ Cl–

H C H6 5

C

CH3Cl

H

C H6 5

Alkyl chlorosulphite

Intimate ion-pair

Retention of configuration

From

same side

Due to the intimate ion-pair, the halide ion joins C-atom from the sameside and does not allow any change in the configuration. Hence, there is noinversion of configuration even though the bond to the asymmetric carbonatom is broken. As the reaction is intramolecular, it is called substitution,nucleophilic internal, i.e., SNi reaction.

QUESTIONS1. Explain the mechanism of alkaline hydrolysis of methyl bromide giving energy

profile diagram.2. Explain Walden inversion with example.3. Explain with mechanism: Hydrolysis of (–) 2 bromooctane gives (+) octan-2-ol.4. Explain the mechanism of alkaline hydrolysis of tert-butyl bromide giving energy

profile diagram.5. Explain with mechanism: SN1 reaction results in racemisation.6. Distinguish between SN1 and SN2 reactions.7. What are the factors which effect SN1 and SN2 reactions?8. Explain with mechanism SNi reaction.9. Explain the stereochemistry of the following reactions.

(i) 2-Iodooctane + NaI (acetone)(ii) 3-Bromo-3-methyl hexane + Aqueous acetone

(iii) 1-Phenyl ethanol + Thionyl chloride

(iv) (–) Malic acid PCl5? ? ?? (+) Chlorosuccinic acid AgOH? ? ? ? (+) Malic acid.

State True or False(i) In SN2 reaction rate of the reaction depends only on concentration of alkyl halide.

(ii) SN2 reactions proceed with stereochemical inversion called Walden inversion.(iii) In SN1 reactions the rate of the reaction depend only on the concentration of

alkyl halide.(iv) SN1 reactions are favoured by less polar solvent.(v) SN1 reactions result in racemization.

(vi) In SNi reactions there is no inversion of configuration.

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Front cover.pdfTitle page 2019.pdfUnit 3.1.1_Alkyl halides_Tanuja Parulekar.pdf