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FUNDAMENTALS OF VOLUME II ORGANIC CHEMISTRY for the JEE (Main and Advanced) ANANYA GANGULY Fundamentals of Organic Chemistry for the J E E (Main and Advanced) Volume II Ananya Ganguly 00 Prelims Volume II.indd 4/7/2015 2:17:55 PM To my son Ayushman Editor—Acquisitions: Jitendra Gaur Editor—Production: G Sharmilee Copyright © © 2016 2015 Pearson Pearson India India Education Education Services Services Pvt Pvt Ltd Ltd Copyright Copyright by © 2012, 2013 Pearson India Education Services Pvt Ltd Published Pearson India Education Services Pvt Ltd, CIN: U72200TN2005PTC057128, formerly known as Tutor This book is sold subject to the condition that it shall not, by way of trade or otherwise, be lent, resold, hired out, or Vista Global Pvt Ltd, licensee of Pearson Education in South Asia otherwise circulated without the publisher’s prior written consent in any form of binding or cover other than that in which is this published similar condition including conditionwithout being imposed on the subsequent No partitof eBookand maywithout be useda or reproduced in any mannerthis whatsoever the publisher’s prior writtenpurchaser consent and without limiting the rights under copyright reserved above, no part of this publication may be reproduced, stored in or introduced intoor a retrieval any form or the by any means (electronic, mechanical, photocopying, This eBook may may notsystem, includeoralltransmitted assets thatinwere part of print version The publisher reserves the right to recording or otherwise), without the prior written permission of both the copyright owner and the publisher of this book remove any material in this eBook at any time ISBN 978-93-325978-93-325-4695-0 ISBN 4696-7 eISBN 978-93-325First Impression Head Office: A-8 (A), 7th Floor, Knowledge Boulevard, Sector 62, Noida 201 309, Uttar Pradesh, India Published by Pearson Pvt Ltd, CIN: City, U72200TN2005PTC057128, formerly known Registered Office: 4th India Floor,Education Software Services Block, Elnet Software TS-140, Block & 9, Rajiv Gandhi Salai,asTaramani, TutorVista Global Pvt Ltd, licensee of Pearson Education in South Asia Chennai 600 113, Tamil Nadu, India Head 080-30461003, Office: A-8(A),Phone: 7th Floor, Knowledge Boulevard, Sector 62, Noida 201 309, Uttar Pradesh, India Fax: 080-30461060 Registered Office: Module G4, Ground Floor, Elnet Software City, TS-140, Blocks & 9, Rajiv Gandhi Salai, www.pearson.co.in, Email: companysecretary.india@pearson.com Taramani, Chennai 600 113, Tamil Nadu, India Fax: 080-30461003, Phone: 080-30461060 www.pearson.co.in, Email: companysecretary.india@pearson.com Compositor: Mukesh Technologies Pvt Ltd Printed in India at Contents Preface Acknowledgments vii viii Chapter ALKYL HALIDES AND ARYL HALIDES Classification Nomenclature Preparation of Alkyl Halide Physical Properties Aliphatic Nucleophilic Substitution Reaction The Variables in Nucleophilic Substitution The Nucleophile The Site of Substitution Solvent Effects Neighbouring Group Participation Elimination Reactions Substrate Structure for E1 Substitution and Elimination Organometallic Compounds Grignard Reagents Dihalogen Derivatives Trihalogen Derivatives Unsaturated Halides Solved Conceptual Examples Conceptual Assignments Solutions 1.1–1.183 1.1 1.3 1.6 1.18 1.20 1.25 1.28 1.38 1.45 1.64 1.69 1.75 1.85 1.96 1.97 1.106 1.108 1.111 1.113 1.121 1.164 Chapter aLCOhOL, pheNOL aND ether Alcohol Ether Oxirane or Expoxides Phenols Some Important Reactions of Phenol Distinction Between Glycol: Ethylene Glycol Glycerol or Glycerine Solved Conceptual Examples Conceptual Assignments Solutions 2.1–2.151 2.2 2.41 2.55 2.65 2.77 2.92 2.93 2.95 2.100 2.108 2.138 00 Prelims Volume II.indd 4/7/2015 2:17:55 PM iv Contents Chapter ALDEHYDE AND KETONE3.1–3.183 Nomenclature 3.1 Structure of a Carbonyl Group 3.5 Methods of Preparation 3.5 Physical Properties 3.20 Reactions of Aldehyde and Ketones 3.23 Carbon as Nucleophiles 3.27 Oxygen as Nucleophile 3.30 Sulphur as Nucleophile 3.35 Nitrogen as Nucleophile 3.36 Beckmann Rearrangement 3.39 Reactions with Phosphorus and Sulphur Halide 3.47 Aldol Condensations 3.53 Cannizzaro Reaction 3.61 Reaction of a, b-Unsaturated Aldehyde or Ketones 3.66 Claisen Condensation 3.82 Reformatsky Reaction 3.86 Knoevenagel Reaction 3.88 Benzilic Acid Rearrangement 3.92 Darzen Condensation–Aromatic Aldehyde and Ketone 3.95 Oxidation Reaction 3.96 Baeyer-Villiger Oxidation 3.105 Oppenauer Oxidation 3.112 Reduction Reaction 3.113 Clemmensen Reduction 3.113 Wolff-Kishner Reduction 3.117 Meerwein-Ponndorf-Verley Reduction 3.119 Wittig Reaction 3.121 Perkin Reaction 3.126 Benzoin Condensation 3.128 Solved Conceptual Examples 3.134 Conceptual Assignments 3.142 Solutions 3.168 Chapter CARBOXYLIC ACIDS AND DERIVATIVES4.1–4.121 Nomenclature 4.1 General Methods of Preparation 4.8 Physical Properties 4.19 Chemical Properties of Carboxylic Acid 4.20 Individual Member 4.32 Conversion into Acid Derivatives 4.35 Acid Derivatives 4.36 Acid Chloride 4.39 Acid Anhydride 4.43 Ester 4.47 Amides 4.60 Solved Conceptual Examples 4.68 Conceptual Assignments 4.71 Solutions 4.108 00 Prelims Volume II.indd 4/7/2015 2:17:56 PM Contents v Chapter NITROGEN CONTAINING COMPOUNDS5.1–5.94 Amines 5.1 Diazonium Salts 5.33 Nitro Compounds 5.45 Cyanide or Nitriles (–C ≡ N) 5.49 Isocyanides 5.53 Urea (Carbamide) 5.55 Aryl Nitro Compounds 5.58 Solved Conceptual Examples 5.68 Conceptual Assignments 5.71 Solution 5.85 Chapter CARBOHYDRATES, AMINO ACIDS AND POLYMERS6.1–6.113 Classification 6.2 D(+) Glucose, Dextrose (or) Grape Sugar 6.5 Cyclic Structure of Glucose 6.6 Reactions of Monosaccharides with Phenylhydrazine: Osazones 6.11 Reducing and Nonreducing Sugars 6.12 Glycoside Formation 6.13 Periodate Oxidations: Oxidative Cleavage of Polyhydroxy Compounds 6.16 Preparation of Sugar 6.19 Action of Alkali 6.23 Comparison of Glucose and Fructose 6.24 The Anomeric Effect 6.25 Disaccharides 6.25 Polysaccharides 6.27 Amino Acids 6.30 Proteins 6.53 Polymers 6.59 Some Important Polymers 6.66 Solved Conceptual Examples 6.77 Conceptual Assignments 6.85 Solutions 6.102 00 Prelims Volume II.indd 4/7/2015 2:17:56 PM This page is intentionally left blank Preface I am indeed very delighted to present the book Fundamentals of Organic Chemistry for the JEE (Main and Advanced) Volume II to the readers with elaborated concepts along with more solved and unsolved problems In all competitive examinations, there are several vital areas that a candidate needs to master—organic chemistry being one of them There are various books in the market on this topic having different approaches; however this book provides simple, shortcut methods and time-saving tactics which are helpful during the examination to the students It will also help you to gain confidence through the right approach to a particular questions rather than attempting number of questions This book would be extremely useful for the students who enroll for examinations like JEE (Main and Advanced) and for other engineering entrance examinations In order to bridge the gap between theory and practical, each concept is explained in detail, in an easy-to-understand manner supported with numerous worked-out examples and practical These will stimulate thought and facilitate advanced learning One of the important factors that contribute to the success of a book is the way it has been developed This book reflects my experience and understanding of the requirements of the students The methods and approaches discussed in this book are tried and tested modes of instruction in the class Fundamentals of Organic Chemistry, Volume II cover topics like, Alkyl Halides and Aryl Halides, Alcohol, Phenol and Ether, Aldehyde and Ketone, Carboxylic Acids and Derivatives, Nitrogen Containing Compounds, Carbohydrates, Amino Acids and Polymers Volume I covers, IUPAC Nomenclature, General Organic Chemistry, Hydrocarbon, Electrophilic Aromatic Substitution, and Analysis of Organic Compounds I am sure that readers will appreciate this book and will find this book very useful to prepare for various examinations Your comments and suggestions would be very useful in improving the subsequent editions of this book Although we have taken utmost care to prepare the manuscript and checking subsequent proofs, there may be a possibility of some errors creeping inside the book We will welcome suggestions for further improvement of the book Please mail us your suggestions on: chemistrycoach1@gmail.com Ananya Ganguly 00 Prelims Volume II.indd 4/7/2015 2:17:56 PM Acknowledgments Fundamentals of Organic Chemistry, Volume II is the result of encouragement that I received from my students, who insisted that, my knowledge and experience should benefit a wider audience I would like to thank my friends who have, over the years, been my support and strength Writing this book has been a long but fulfilling journey and I am fortunate to be assisted by a talented team of editors I am indebted to my family for keeping me motivated during all stages of the project I always feel a divine power supporting my efforts when my family is around They actually made me work harder on the project I extend my sincere thanks to Pearson editorial team for their constant encouragement and support during the publication of this book 00 Prelims Volume II.indd 4/7/2015 2:17:56 PM Alkyl Halides and Aryl Halides Introduction The replacement of hydrogen atom(s) in a hydrocarbon, aliphatic or aromatic, by halogen atom(s) results in the formation of an alkyl halide (haloalkane) and an aryl halide (haloarene), respectively Haloalkanes contain halogen atom(s) attached to the sp3 hybridized carbon atom of an alkyl group, whereas haloarenes contain halogen atom(s) attached to sp2 hybridized carbon atom(s) of an aryl group The general formula of saturated mono substituted alkyl halide is CnH2n+1X, where X is a halogen atom Alkyl halides are usually represented by R – X where R is an alkyl group CLASSIFICATION Halides can be classified depending on the nature of carbon to which halogen is attached like alkyl or aryl or can be classified depending on the number of halogen, as di, tri, tetra halides According to the nature of halogen, they can be called as fluorides, chlorides, bromides or iodides Alkyl halides can be classified as methyl halide, primary alkyl halide, secondary alkyl halide (2°) and tertiary alkyl halide (3°), according to the number of other carbon atoms attached to the carbon bearing the halogen atom + + + ; + + 0HWK\OKDOLGH 3ULPDU\DON\O KDOLGH 1.28 Alkyl Halides and Aryl Halides Problem Arrange each of the following sets of ions in decreasing order as leaving groups in nucleophilic substitution a +2&+ 62± 2± 21 2± b H–, Cl–, Br–, CH3CO2– , HO– Problem Suggest an explanation for the following results: H 2O (CH )3 COH + NaC1 → no reaction H 2O/HCl (CH )3 COH + NaC1 →(CH3 )3 CCl THE NUCLEOPHILE A nucleophile uses its lone pair electrons to attack an electron-deficient atom other than a proton Nucleophilicity is a measure of how readily the nucleophile is able to attack such an atom It is measured by a rate constant (k) Addition of the nucleophile takes place after formation of the carbocation in reactions that proceed by the SN1 mechanism Therefore, the nucleophile does not influence the reaction rate of an SN1 reaction By contrast, the SN2 mechanism requires participation of the nucleophile The nucleophile is often said to displace, or push off, the leaving group Nucleophilicity of the reagent is extremely important in the SN2 reaction We would expect good nucleophiles to be good electron donors, i.e., good Lewis bases Nucleophilicity and basicity correlate in many cases Their relation is most useful for comparison of a series of compounds in which the same atom is the nucleophile For instance, the oxygen nucleophile may be encountered as the reactive portion of an alcohol or a phenol The alcohol will be the better nucleophile because resonance delocalization of the oxygen electrons in the phenol reduces their availability This was the same rationale we used to account for acid-base characteristics of alcohols and phenols A similar type of relation is found for nucleophiles along a row of the periodic table The nitrogen atom is more nucleophilic than an oxygen atom in a series of similar compounds Again, this is a result of the greater availability of an electron pair for bonding from the less electronegative nitrogen atom (Table) Table 1.3 Relation of Nucleophilicity to Basicity for Atoms in the Same Row of the Periodic Table O-atom Nucleophiles N vs O Nucleophiles + 1 &+2 +1 &+1+ +2 +2 + 1 &+2 +1 &+1+ &+&2 S12&+1+ +2 ± 'HFUHDVLQJEDVLFLW\ 'HFUHDVLQJ QXFOHRSKLOLFLW\ N-atom Nucleophlles +2 Problem – Many nucleophiles are anions, but some anions are not nucleophiles Explain why BF4 is not a nucleophile Correlation of nucleophilicity with basicity is useful but not exact Two different kinds of reactions, normally in different solvents, are being compared Basicity is an equilibrium phenomenon which measures the position of equilibrium of a reagent with a proton, usually in water Nucleophilicity involves the rate of reaction (kinetics) with a carbon atom, usually in non-aqueous solvents Alkyl Halides and Aryl Halides 1.29 The basicity-nucleophilicity correlation is not useful for comparison of atoms down a family of the periodic table As atomic number increases, nucleophilicity increases and basicity decreases (Table) Table 1.4 Relation of Nucleophilicity to Basicity Commonly Observed for Atoms in Same Family of the Periodic Table Group VI Nucleophiles Group VII Nucleophiles 53 56 , 51 52 %U &O ) 'HFUHDVLQJEDVLFLW\ 'HFUHDVLQJ QXFOHRSKLOLFLW\ Group V Nucleophiles The outer electrons of the larger atoms are diffused over greater volumes than are those of smaller atoms The lesslocalized electrons form weaker bonds to a proton, and the atoms are less basic However, the outer electrons of the larger atoms are also less tightly held by the nucleus They are more polarizable and are more available for forming bonds to a carbon atom; they are more nucleophilic Table summarizes the reactivities of a series of nucleophiles from different families of the periodic table Bisulfite ion shows the rather interesting dependence of basicity and nucleophilicity on the atoms within a single ion Charge is delocalized over the oxygen and sulfur atoms The more basic oxygen atom typically reacts with a proton, and the more nucleophilic sulfur atom reacts at a carbon atom The bisulfite addition product from an aldehyde illustrates that difference Ions in which charge is delocalized so that reaction can take place at two different atoms are known as ambident ions +2²6²2 + +2²6²2+ +2²6 2 2 5& +2²6 +2²6²&+5 + 2 2 Table 1.5 Relative Rates of Nucleophilic Substitution (SN2) Nucleophile k2 (rel.) Nucleophile k2 (rel.) CH3OH F– CH CO -2 × 102 × 104 C6H5SH C6H5O– N 3- × 105 5.6 × 105 × 105 Cl– (CH3O)3P 2.3 × 104 1.6 × 104 Br– CH3O– × 105 × 106 † 1.30 Alkyl Halides and Aryl Halides Nucleophile k2 (rel.) Nucleophile k2 (rel.) 1.7 × 105 CN– × 106 NH3 3.2 × 105 (C2H5)2NH × 107 (CH3)2S 3.5 × 105 (C6H5)3P × 107 C6H5NH2 × 105 I– × 107 C6H5S– × 109 MeOH For the reaction Nu: + CH3I → Nu–CH3 Method arbitrarily set as standard Problem Explain the difference in positions of reaction by sulfite anion in the examples below + 62 &+ , 2²6²2+ 2²6²2 ,± &+ The size and shape of a reagent is another variable associated with nucleophilicity When a Lewis base interacts with a proton, steric considerations are not normally important because the proton is small However, approach of the same reagent to a tetracoordinate carbon atom can involve severe steric interactions The important oxygen anions methoxide and tert-butoxide are illustrative They are of similar basicity, being the conjugate bases of aliphatic alcohols Methoxide, however, is a small species which can easily approach a carbon atom during nucleophilic substitution In contrast, tert-butoxide is very bulky, so that steric restraints reduce its ability to function as a nucleophile ++ + + +²&²2 + 0HWKR[LGH & + +²&²&²2 + & + ++ WHUW%XWR[LGH In comparing molecules with the same attacking atom, there is generally a direct relationship between basicity and nucleophilicity Both describe a process involving the formation of a new bond to an electrophile by donation of an electron pair Stronger bases are better nucleophiles The hydroxide ion is a stronger base than a cyanide ion, but cyanide ion is a stronger nucleophile than hydroxide ion For example, a compound with negatively charged oxygen is a stronger base and a better nucleophile than a compound with neutral oxygen (Table) Alkyl Halides and Aryl Halides 1.31 Table 1.6 Relation of Nucleophilicity to Basicity Stronger Base, Better Nucleophile HO– Weaker Base Poorer Nucleophiles > H2O CH3O– > CH3OH > H3N > CH3CH2NH2 H2N– CH3CH2NH – In comparing molecules with attacking atoms of approximately the same size, the stronger bases are again the better nucleophiles Within a group of nucleophiles that attack at the electrophile with the same atom, the nucleophilicity decreases with decreasing basicity of the nucleophile Decreasing basicity is equivalent to decreasing affinity of an electron pair for a proton RO– > HO– > C6H5O– > RCOO– >> ROH, H2O >>> RSO3– This parallel relationship between nucleophilicity and basicity can be reversed by steric effects Therefore, less basic but sterically unhindered nucleophiles have a higher nucleophilicity than strongly basic but sterically hindered nucleophiles Thus, although t-butoxide ion is a stronger base than ethoxide ion, the bulky t-butoxide is a weaker nucleophile ± 1 /L (W ± 1 /L !! !! !! 5SULP2±!5VHF2±!5WHUW2± (W 5SULP2+!5VHF2+!5WHUW2+ The atoms across the second row of the periodic table have approximately the same size If hydrogens are attached to the second-row elements, the resulting compounds have the following relative acid strengths CH4 < NH3 < H2O < HF Nucleophilicity decreases with increasing electronegativity of the attacking atom Consequently, the conjugate bases have the following relative base strengths and nucleophilicities For example, the methyl anion is the strongest base as well as the best nucleophile CH3 > –NH2 > HO– > F– ; – RS– >> Cl–; Et3N >> Et2O In comparing molecules with attacking atoms that are very different in size, the direct relationship between nucleophilicity and basicity is maintained if the reaction occurs in the gas phase If, however, the reaction occurs in a solvent this relationship between nucleophilicity and basicity depends on the solvent In a comparison of the atomic centres from the same group of the periodic table, the trend is as follows: Basic strength: Nucleophilic ability: RO– > RS–; RS– > RO–; ROH > RSH; RSH > ROH; F– > Cl– > Br– > I– I– > Br– > Cl– >> F– The divergence of nucleophilic ability from basic strength stems from the fact that as the atom-donating electron pair increases in size, the electrons in its outer shell will be further away from, and hence, held less tightly by the atomic nucleus These outer electrons are thus more polarizable They are more readily available to form a bond with the atom being attacked Polarizability appears to be much more important in nucleophilic ability than in the equilibrium situation involved in basicity; thus, species in which the relevant atom is large are commonly found to be better nucleophiles than their strength as bases might suggest If the solvent is aprotic (it is not a hydrogen bond donor), the direct relationship between nucleophilicity and basicity holds For example, both the nucleophilicities and basicities of the halogens decrease with increasing size in an aprotic solvent such as dimethyl formamide 1.32 Alkyl Halides and Aryl Halides If the solvent is protic (it is a hydrogen bond donor such as water, alcohols, etc.), the relationship between basicity and nucleophilicity becomes inverted: as basicity decreases, nucleophilicity increases Thus, iodide ion, which is the weakest base of the halogen family, is the poorest nucleophile of the family in an aprotic solvent and the best nucleophile in a protic solvent ,QFUHDVLQJQXFOHRSKLOLFLW\ LQDSURWLFVROYHQW ,QFUHDVLQJ )± VL]H &O± %U± O± ,QFUHDVLQJ ,QFUHDVLQJQXFOHRSKLOLFLW\ EDVLFLW\ LQDQDSURWLFVROYHQW How does the solvent’s ability to be a hydrogen bond donor affect the relationship between nucleophilicity and basicity? When a negatively charged species is placed in a protic solvent, the solvent molecules arrange themselves so that their partially positively charged hydrogens point toward the negatively charged species An aprotic solvent does not have a partially positively charged hydrogen G + G+ + G+ + G– G+ G+ G+ G– + 2 + &O + G + G G+ G– LRQGLSROHLQWHUDFWLRQVEHWZHHQ DQXFOHRSKLOHDQGZDWHU The interaction between the ion and the dipole of the protic solvent is called an ion-dipole interaction The change from a direct relationship between basicity and nucleophilicity in an a protic solvent to an inverse relationship in a prom solvent results from the ion-dipole interactions between the nucleophile and the protic solvent This occurs because at least one of the ion-dipole interaction; must be broken before the nucleophile can participate in an SN2 reaction Weal bases interact weakly with protic solvents; strong bases interact more strong because they are better at sharing their electrons It is, therefore, easier to break the ion-dipole interactions between an iodide ion and the solvent than between the more basic fluoride ion and the solvent, because the latter is a stronger base As a result, iodide ion is a better nucleophile in a protic solvent RS− >R P>I− >CN− >R N>HO− >RO− >Br− > NH >Cl− >CH CO− 2>F− >CH OH 3 3 → decreasing Nucleophilicty The nucleophilicity of a given nucleophilic centre is increased by attached heteroatoms that possess free electron-pairs (a-effect) The reason for this is the unavoidable overlap of the orbitals that accommodate the free electron-pairs at the nucleophilic centre and its neighbouring atom HO–O– > H–O; H2N–NH2 > H-NH2 1.33 Alkyl Halides and Aryl Halides It is worth summarizing the characteristics of the two types of nucleophiles Hard Nucleophile X small charged basic like to attack C=O such as RO–, NH–, MeLi Soft Nucleophile Y large neutral not basic like to attack saturated carbon such as RS–, I–, R3P The sp3 carbon is a soft electrophile, whereas the proton is a hard electrophile Thus, according to the HSAB theory, a soft anion should act primarily as a nucleophile, giving the substitution product, whereas a hard anion is more prone to abstract a proton, giving the elimination product The property of softness correlates with high polarizability and low electronegativity Hardness reflects a high charge density and is associated with small highly electronegative species Because there are many different kinds of nucleophiles, a wide variety of organic compounds can be synthesized by means of SN2 reactions Table shows just a few of the many kinds of organic compounds that can be synthesized in this way Table 1.7 Some Organic Compounds That Can Be Synthesized By SN2 Reactions HO– + CH3CH2Cl → CH3CH2OH + Cl– HS– + CH3CH2Br → CH3CH2SH + Br– RO– + CH3CH2l → CH3CH2OR + l– RS– + CH3CH2Br → CH3CH2SR + Br– H2N– + CH3CH2F → CH3CH2NH2 + F– RC≡C– + CH3CH2Br → CH3CH2C≡CR + Br– N≡C– + CH3CH2l → CH3CH2C≡N + l– If the difference between the basicities of the nucleophile and the leaving group is not very large, the reaction will be reversible For example, in the reaction of ethyl bromide with iodide ion, Br– is the leaving group in one direction and I– is the leaving group in the other direction Because the pKa values of the conjugate acids of the two leaving groups are not very different (pKa of HBr = –9; pKa of HI = –10), the reaction is reversible CH 3CH Br + I - CH 3CH I + Br - One can drive a reversible reaction toward the desired products by removing one of the products as it forms Le Chatelier’s principle states that fl equilibrium is disturbed, the components of the equilibrium will adjust to offset the disturbance In other words, if the concentration of product C is decreased, A and B will react to form more C and D so that the equilibrium constant maintains its value A+B C+D [C] [D] K eq = [A] [B] For example, the reaction of ethyl chloride with methanol is reversible because the difference between the basicities of the nucleophile and the leaving group is not very large If the reaction is carried out in a neutral solution, the protonated product will lose a proton This disturbs the equilibrium and drives the reaction toward the products &+&+&O&+2+ ±+ IDVW &+&+2&+&+&+2&+ +&O± 1.34 Alkyl Halides and Aryl Halides It is found in practice that in (highly polar) SNl reactions attack takes place on the carbocationic intermediate, R⊕, through the atom in the nucleophile on which electron density is the higher With, for example, halides that not readily undergo SN1 attack this can be promoted by use of the silver salt of the anion, e.g AgCN, as Ag⊕ promotes R⊕ formation by precipitation of AgHal: l & 5²%U$J>&1@ & $J%UĻ5>&1@ VORZ IDVW 5²1& In the absence of such promotion by Ag⊕, e.g with Na⊕ [CN] Θ, the resulting SN2 reaction is found to proceed with preferential attack on the atom in the nucieophile which is the more polarisable: G± G± 1&5²%Uĺ 1&5%U Á ĺ1&²5%U 76 This is understandable as, unlike SNl, bond formation is now taking place in the T.S for the rate-limiting step, for which ready polarisability of the bonding atom of the nucleophile is clearly important—the beginning of bonding at as great an internuclear separation as possible Illustrations 18 Why is PhO– a weaker nucleophile than RO– Ans The ucleophilic pairs of electrons on oxygen are involved in resonance in the case of PhO– This resonance interaction decreases their availability to participate in nucleophilic processes ± ± HWF 2 No such resonance is possible in the case of the alkoxide ion It is also this resonance interaction that makes phenoxideless basic than alkoxide, as well as making phenols more acidic than alcohols 19 RBr when treated with AgCN in a highly polar solvent gives RNC whereas when it is treated with NaCN it gives RCN Explain Ans As[CN]– is an ambident nuicleophile which have two nucleophile which have two nucleophilic sites and can attack from either side In a highly polar solvent, AgCN promotes the formation of carbocation R+, by precipitation of AgBr VORZ &1ĺ&1 5²%U$J>&1±@ 5&1±$J%UĻ IDVW 5±1&± In the absence of such promotion by Ag+, with Na+[CN]–, the resulting SN2 reaction is found to proceed with preferential attack on the atom in the nucleophile which is more polarisable i.e C 1&±5²%U G± 1&ÂÂÂ5ÂÂÂ%U G± 7UDQVLWLRQ6WDWH 1&²5%U± Alkyl Halides and Aryl Halides 1.35 ## Practice Exercise Explain the fact that a small amount of NaI catalyzes the general reaction RCl + RO–Na+ → ROR′ + NaCl SOLUTION With I– the overall reaction occurs in two steps, each of which is faster than the uncatalysed reaction Step 1: RCl + I– → RI + Cl– This step is faster because I–, a soft base has more nucleophilicity than OR–, a hard base Step 2: RI + R′O:– → ROR′ + I′ This step is faster because I– is a better leaving group than Cl– Illustrations 20 Give the organic products of the following reactions: 2 (b) L3U%U> 6& (c) (W%U> 662@± WKLRVXOIDWH ĺ (d) ClCH2CH2CH2I + CN– (one mole each) → (a) Q3U%U 1²2 ±ĺ 1 @± LVRF\DQDWH ĺ + -H (e) H2NCH2CH2CH2CH2 Br → Ans The nucleophiles in (a), (b), and (c) are ambident since they each have more than one reactive site In each case, the more nucleophilic atom reacts even though the other atom may bear a more negative charge (a) n-PrNO2 (b) i-PrSCN (c) [EtSSO3]– (with its cation it is called a Bunte salt) (d) CICH2CH2CH2CN I– is a better leaving group than Cl– (e) + When the nucleophilic and leaving groups are part of the same molecule, an intramolecular displacement occurs if a three-, a five-, or a six-membered ring can form 21 Generalize about the relationship of basicity and nucleophilicity from the following relative rates of nucleophilic displacements: (a) +2 ±!!+2 DQG+1 ±!!+1 (b) +& ±!+1 ±!+2 ±! ) ± (d) 0H2 ±!+2 ±!0H&22 ± (e) +22 ±!+2 ±DQG+11+!+1 (c) , ±! %U ±! &O ±! ) ±DQG+6 ±!+2 ± 1.36 Alkyl Halides and Aryl Halides Ans (a) Bases are better nucleophiles than their conjugate acids (b) In going from left to right across the periodic table, basicity and nucleophilicity are directly related—they both decrease (c) In going down a group in the periodic table, they are inversely related—nucleophilicity increases while basicity decreases (d) When the nucleophilic and basic sites are the same atom (here an O), nucleophilicity parallels basicity (e) These relative rates are counter to relative basicities When the atom bonded to the nucleophilic site also has an unshared pair of e-‘s, e.g., G—Nu:, nucleophilicity of the species increases 22 Define soft and hard bases (nucleophiles) (b) Which have the greater nucleophilicity? (c) Discuss the relationship of polarizability and nucleophilicity Ans (a) Soft bases have larger, more polarizable basic site atoms (e.g., I, Br, S, and P) Hard bases have smaller, more weakly polarizable sites (e.g., N, O, and F) (b) Soft bases have enhanced nucleophilicities; hard bases have diminished nucleophilicities (c) Distortion of the electron cloud of the active site atom [polarizability] concentrates electron density at its head as it approaches the C The approach is more facile even though these are larger atoms 23 (a) Distinguish between soft and hard acids (electrophiles) (b) Which combinations of soft and hard acids and bases give the best reactions? (c) Is the attacked C of RX a soft or hard electrophilic site? Ans (a) The more polarizable electrophilic sites are soft, while the less polarizable sites are hard (b) Soft bases (nucleophiles) bind best with soft acids (electrophiles) The same is true for hard bases and acids This is known as the SHAB (Soft and Hard Acid-Base) principle (c) Since the softer bases react best in displacement reactions, the attacked C must also be soft-like 24 Compare the following displacement reactions and account for any difference (a) ROH + NaBr → (b) ROH + HBr→ Ans (a) No reaction The extremely weak base Br– cannot displace the very strong base OH– (b) This reaction occurs in two steps: ROH + HBr → ROH+2 + Br– → RBr + H2O Displacement is on the onium ion of ROH whose good leaving group is the very weakly basic H2O 25 Explain the fact that a small amount of Nal catalyzes the general reaction RCI + R’O:– Na+ → ROR' + NaCI Ans With I- the overall reaction occurs in two steps, each of which is faster than the uncatalyzed reaction Step RCl + I– → RI + Cl– This step is faster because I– a soft base, has more nucleophilicity than OR–, a hard base Step RI + R’O:– → ROR + I– This step is faster because I- is a better leaving group than Cl– 26 (a) Rationalize the orders of relative rates for reactions carried out by the following nucleophiles in a weakly polar aprotic solvent, such as acetone, (i) LiI > LiBr > LiCl > LiF, (ii) CsF > RbF > KF > NaF > LiF, and (iii) Bu4N+ Cr > Bu4N+ Br– > Bu4N+ I– (b) Show how the results in (i) are a good application of the SHAB principle (question 24) Ans (a) In weakly polar aprotic solvents, ion-pairing influences the reactivity of Nu– The more pervasive is ion-pairing, the less nucleophilic is Nu– The counteraction plays an important role in ion-pairing (i) The-charge on the largest anion, I–, is the most diffused; I– least readily forms ion pairs The — charge on the smallest anion, F–, is the least diffused; F– most readily forms ion pairs (ii) As the alkali metal cation gets smaller, ion-pairing to a given X– gets stronger and the nucleophilicity of X- diminishes The order given for diminishing nucleophilicity Alkyl Halides and Aryl Halides 1.37 of F– is the same as the order of reducing the size of M+ (iii) Bu4N+ has the positively charged N at its center, surrounded by four Bu’s, and consequently has practically no tendency to ion-pair with X– With no encumberance from ion-pairing or H-bonding, the reactivity of X– is directly related to the order of basicity (b) The smallest M+ is the “hardest” acid and the smallest X– is the “hardest” base They combine to give the strongest bond—the most effective ion-pair 27 (a) What experimental conditions should be used to study the intrinsic nucleophilicity of halide ions? (b) Predict the relative decreasing order of nucleophilicities of X in the following reaction: (C5Hn)4N+X–(s) (where X– is Cl, Br, I) →(C5H11)3N + C5H11X Ans (a) Run without solvent; use the gaseous or solid state, in which there is no ion-pairing, (b) This reaction fits the conditions suggested in part (a) The order of nucleophilicities should fit the order of basicities: CI– > Br- > I– ## Practice Exercise Provide an explanation for the order of reactivity when each of the following compounds reacts with iodomethane (methyl iodide) to give a methylpyridinium iodide product &+, ,± &+ &+ &+ 1 1 + & 5HODWLYH UDWH &+ & &+ SOLUTION SN2 reaction is slowed down by steric bulk on the nucleophile 1.38 THE Alkyl Halides and Aryl Halides SITE OF SUBSTITUTION Structure around the site of substitution is the major factor in determining reaction mechanism SN1 reactions are favored at a tertiary carbon atom and SN2 reactions are favored at a primary carbon atom Comparison of rate constants for solvolytic reactions (first-order) shows an increase of 106 in passing from primary to tertiary substrates (Table) Rate constants for second-order substitution reactions decrease by >106 as the number of alkyl groups surrounding the reaction center increases (Table) Let us see how the mechanistic descriptions of the two pathways can account for the observed structurereactivity effects Table 1.8 Effect of Structure on the Relative Rates of First-Order Substitution SN1 R k1 (rel.) CH3— CH3CH2— (CH3)2CH— (CH3)3C— 1 12 1,20,000 * For the reaction R—Br + H2O → ROH Table 1.9 Effect of Sructure on the Relative Rates of Second-Order Substitution SN2 R k2 (rel.) CH3— CH3CH2— R'CH2CH2— (CH3)2CH— (CH3)3C— (CH3)3CCH2— 30 0.4 0.002 0.001 0.00001 * For the reaction R—X + Nu: → R—Nu: Consider the SN1 reaction We proposed that substitution by the SNl pathway requires formation of a carbocation as the initial rate-controlling step Recall that the rate of a reaction is dependent on the free energy of activation (∆G‡) of the rate-controlling step Since only a small change in configuration of the atoms takes place between the transition state and the cationic intermediate, it seems reasonable that the transition state leading to the most stable intermediate would be of lower energy than that leading to a less stable intermediate A tertiary carbocation is relatively more stable than a secondary carbocation, which is more stable than a primary carbocation This result was attributed to stabilization of the positive charge because of the electron-donating effect of alkyl groups on the cationic carbon atom For the SN1 reaction, this corresponds to a tertiary carbocation being formed more rapidly than a secondary or primary carbocation (CH3)3 CBr → (CH3)3 C+ + Br CH3Br → CH3+ + Br– Favourable Unfavourable The carbocation intermediates of SN1 reactions also benefit from delocalization of their positive charge into adjacent unsaturated groups Thus the 2-propenyl (allyl) group undergoes SN1 reactions more rapidly than does an ethyl or a propyl group Aryl groups are even better at stabilizing an adjacent carbocation and the transition state leading to it The influence of various hydrocarbon groups on the relative rates of ethanolysis for a series of substrates is illustrated in the table overleaf Carbocations connected to heteroatoms possessing nonbonded electrons are particularly well stabilized Thus the reaction of l-chloro-2-oxabutane with ethanol proceeds at a rate over 109 faster than the similar reaction of the tertiary haloalkane 2-chloro-2-methylpropane Alkyl Halides and Aryl Halides 1.39 Table 1.10 Effect of Carbocation Stability on the Relative Rates of First-Order Substitution SN1 R k1 (rel.) CH3CH2— 1.2 × 10–4 CH2=CH—CH2— 0.04 C6H5CH2— 0.08 C6H5CH— CH3 (CH3)3C— 1‡ (C6H5)2CH— 300 (C6H5)3C— × 106 EtOH * For the reaction R - Cl + C2 H 5OH → ROC2 H + HCl ‡ tert-Butyl arbitrarily taken as standard &+&+2&+&O&+2+ (W2+ &+&+2&+2&+&++&O &KORURR[DEXWDQH &KORURPHWK\OHWK\OHWKHU .. .Fundamentals of Organic Chemistry for the J E E (Main and Advanced) Volume II Ananya Ganguly 00 Prelims Volume II. indd 4/7/2015 2:17:55 PM To my son Ayushman... 2-chloro-2-methylbutane; and (v) (CH3)3CCH2Br, l-bromo-2,2-dimethyl-pro-pane (b) The “form” method is used for the HCX3 type of compounds: HCF3, fluoroform; HCCl3, chloroform; HCBr3, bromoform; HCl3, iodoform;... extremely useful for the students who enroll for examinations like JEE (Main and Advanced) and for other engineering entrance examinations In order to bridge the gap between theory and practical,