Wileys problems in organic chemistry 1

PROBLEMS IN ORGANIC CHEMISTRY PROBLEMS IN ORGANIC CHEMISTRY Dr K Singh K Singh Vision Classes, Patna Problems in Organic Chemistry Copyright © 2015 by Wiley India Pvt Ltd., 4435-36/7, Ansari Road, Daryaganj, New Delhi-110002 Cover Image: stuartbur/iStockphoto All rights reserved No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or scanning without the written permission of the Publisher Limits of Liability: While the publisher and the author have used their best efforts in preparing this book, Wiley and the author make no representation or warranties with respect to the accuracy or completeness of the contents of this book, and specifically disclaim any implied warranties of merchantability or fitness for any particular purpose There are no warranties which extend beyond the descriptions contained in this paragraph No warranty may be created or extended by sales representatives or written sales materials Disclaimer: The contents of this book have been checked for accuracy Since deviations cannot be precluded entirely, Wiley or its author cannot guarantee full agreement As the book is intended for educational purpose, Wiley or its author shall not be responsible for any errors, omissions or damages arising out of the use of the information contained in the book This publication is designed to provide accurate and authoritative information with regard to the subject matter covered It is sold on the understanding that the Publisher is not engaged in rendering professional services Trademarks: All brand names and product names used in this book are trademarks, registered trademarks, or trade names of their respective holders Wiley is not associated with any product or vendor mentioned in this book Other Wiley Editorial Offices: John Wiley & Sons, Inc 111 River Street, Hoboken, NJ 07030, USA Wiley-VCH Verlag GmbH, Pappellaee 3, D-69469 Weinheim, Germany John Wiley & Sons Australia Ltd, 42 McDougall Street, Milton, Queensland 4064, Australia John Wiley & Sons (Asia) Pte Ltd, Fusionpolis Walk #07-01 Solaris, South Tower Singapore 138628 John Wiley & Sons Canada Ltd, 22 Worcester Road, Etobicoke, Ontario, Canada, M9W 1L1 Edition: 2015 ISBN: 978-81-265-5582-6 ISBN: 978-81-265-8216-7 (ebk) www.wileyindia.com Printed at: PREFACE Organic Chemistry is understood by reading the textbook, listening to lectures and memorizing name reactions with reagents But perhaps most importantly, it is learnt by doing, that is, solving problems It is not uncommon for students who have performed below expectation in JEE to explain that they honestly thought they understood the text and lectures The difficulty, however, lies in applying, generalizing, and extending the specific reactions and mechanisms they have “memorized” to the solution of a very broad array of related problems In doing so, students will begin to “internalize” Organic Chemistry to develop an intuitive feel for, and appreciation of, the underlying logic of the subject Acquiring that level of skill requires but goes far beyond rote learning It is the ultimate process by which one learns to manipulate the myriad of reactions and, in time, gains a predictive power that will facilitate solving new problems Mastering Organic Chemistry is challenging It demands memorization, but then requires application of those facts to solve real problems It features a highly logical structural hierarchy and builds upon a cumulative learning process The requisite investment in time and effort, however, can lead to the development of a sense of self-confidence in the subject, an intellectually satisfying experience, indeed Many excellent textbooks are available to explain the theory of Organic Chemistry; all provide extensive exercises Better performing students, however, consistently ask for additional exercises It is the purpose of this book, then, to provide supplementary problems and their solutions that reinforce and extend those textbook exercises This book has reached its destination in five years; three years used in collection of problems chapter-wise and two years for correction/eliminating errors I have designed this book in such a manner that it will be useful for JEE aspiring students If you go through previous years’ JEE problems, you will find that objective type questions have subjective nature, that is why it was decided to write an objective pattern book in a subjective way The beauty of this book is in the solutions, which are at par with international level treatment Each and every question has detailed solution with reaction sequence, bond cleavage and formation of products The book has separate chapters for Substitution and Elimination reaction and Carbonyl alpha-Substitution and Carbonyl Condensation reactions, because JEE has always been framing problems from these chapters Arrangement is according to classical functional group organization, with each group typically divided into Reactions, Conversions and Mechanisms To emphasize the vertical integration of the subject, problems in later chapters heavily draw upon and integrate reactions learned in earlier chapters It is desirable, but impossible, to write a problem book that is completely text-independent Most problems will follow a similar developmental sequence, progressing from alkane/alkene/alkyne to aromatic to aldehyde/ketone to carboxylic acid to enol/enolate to amine chemistry But within the earlier domains, placement of the basics, stereochemistry, SN/E mechanisms, and other functional groups varies considerably The sequence is important because it establishes the concepts and reactions that can be utilized in subsequent problems It is the intent of this problem book to follow a consensus sequence that complements a broad array of Organic Chemistry textbooks Consequently, instructors utilizing a specific textbook may on some occasion offer their students guidance on the corresponding problem book chapter and select problems for practice FM - Copy.indd v 8/17/2015 5:42:36 PM vi PREFACE I acknowledge the blessings and support of my mother Smt Tileshwari Devi, father Shri Triveni Singh, wife Mrs Sarika Singh and sons, Aayushman and Shauryaman I would like to thank Dr Sarika Mehta Jain for her unconditional support to make this an error-free book She spent a lot of time on this book, and solved each and every problem to ensure authentic and error free solutions I can say without any hesitation that she is the mother of this book, without her support this book would have never reached to my students, readers and teachers I also sincerely thank all the members of Wiley India team and especially Paras Bansal, Anjali Chadha and Seema Sajwan in bringing out this book in such a nice form It is all made possible by the grace of God Almighty I devote this book to the feet of God At the end, constructive criticism and valuable suggestions from the readers are most welcome to make the book more useful Dr K Singh ksingh.visiondirector@gmail.com FM - Copy.indd vi 8/17/2015 5:42:37 PM ABOUT THE AUTHOR Dr K Singh has done his post-graduation in Chemistry with special paper in Organic Chemistry and his PhD in Water Pollution from Veer Kunwar Singh University, Ara, Bihar He is an accomplished faculty in Chemistry, imparting quality education to JEE aspirants across north India He has a vast teaching experience of more than 19 years and has taught more than 60,000 students He has churned out thousands of successful students in JEE, many of whom attained top hundred ranks and studied in institutes of their choice Many of these students are doing exceedingly well in the field of research and in corporate sector He is one of the very few faculties from Bihar who had the privilege of teaching at Kota in early days He then went on to establish the first Coaching Institute at Patna, Bihar, which is based on Kota teaching pattern As a leading educationist of Patna, he very successfully stopped the migration of the students from Bihar to Kota Dr K Singh is a member of American Chemical Society and has been conferred with many awards Some of which are: “Bharat Gaurav Award” by World Economic Progress Society; Rashtriya Siksha Ratna Award” by I.E.D.R.A and “Bhojpuri Academy Samman” by Govt of Bihar He has also participated in the “National Conference on Excellence in Higher Education” His name is counted among the most successful and respected faculty in the world of JEE coaching FM - Copy.indd vii 8/17/2015 5:42:38 PM FM - Copy.indd viii 8/17/2015 5:42:38 PM 70 CHAPTER STEREOCHEMISTRY shift (j) (k) S2.51 (a) Addition of hydrogen into alkynes is syn and is followed by the anti-addition of bromine to lead to racemic product Chapter Part II.indd 70 8/17/2015 11:10:31 AM SOLUTIONS 71 (b) Reduction of alkyne to a trans-olefin, followed by Br2 addition, yields the meso-dibromide (c) Chapter Part II.indd 71 8/17/2015 11:10:32 AM Chapter Part II.indd 72 8/17/2015 11:10:33 AM ALKANES AND CYCLOALKANES PROBLEMS ALKANES (c) C2C12 3.1 General P3.1 Identify the compound with the highest melting point (a) n-Octane (b) 2,5-Dimethylhexane (c) 2,3,4-Trimethylpentane (d) Bicyclo[2.2.2]octane (d) cis-l,3-Dichlorocyclobutane 3.2 Nomenclature P3.7 Write the IUPAC names for each of the following structures and specify the stereochemistry where required P3.2 Identify which compound has the highest boiling point (a) n-Pentane (b) Neopentane (2,2-Dimethylpropane) (a) (b) (c) (d) (c) Isopentane P3.3 Dodecahedrane is a regular polyhedron consisting of 12 cyclopentane rings (like a football) while eicosane is a straight-chain compound Explain the difference in their melting points of 420° (dodecahedrane) and 37° (eicosane), even though both are C20 hydrocarbon alkanes P3.4 How many constitutional (structural) isomers exist for the following molecular formulae? (a) C6H14 (b) C7H16 (c) Dichlorocyclohexane Cl (d) C6H12 that have a cyclopropyl ring in their structure P3.5 How many different kinds (constitutional) hydrogens are present in the following compounds? (e) (a) 2,3-Dimethylpentane (f) F (b) 2,2,4-Trimethylpentane (c) 4-Ethyl-3,3,5-trimethylheptane (d) 2,5,5-Trimethylheptane (g) (e) 2,4-Dimethylpentane (f) 3-Ethylpentane P3.6 The compound with the highest molecular dipole moment (µ) among the following is: Cl (a) Cl Chapter Part I.indd 73 (h) (b) Anti conformer of 2,3-dichlorobutane 8/17/2015 11:12:53 AM 74 CHAPTER ALKANES AND CYCLOALKANES 3.3 Conformational Analysis (i) (j) Isohexyl iodide P3.10 Represent the following as their Newmann projection: (a) Most stable conformer of 2-cyclopentyl-6-methylheptane, looking down the C2 C3 bond (k) (b) Gauche conformer of 1-phenylbutane, looking down the C1 C2 bond (l) P3.8 Provide the correct IUPAC names for the following compounds with their bond line structures (a) 2-Isopropyl-4-methylheptane (b) 3-(l-Methylbutyl) octane P3.11 Draw the Newman projections of isopentane and plot the graph for potential energy vs rotation about the C2 C3 bond Identify the conformer that corresponds to the highest minimum in the plot P3.12 Why the molecular dipole moment (µ) for FCH2CH2OH is much larger than that for FCH2CH2F Use conformational analysis to explain P3.13 The least stable conformation of 3-bromo-2-methylpentane, viewed through the C-2 C-3 bond (i.e., C-2 in the front, C-3 in the back) is: (c) 3-S-Butyl-7-t-butylnonane (d) Tetraethylmethane P3.9 Using bond line notation, represent structural formulae for the following: (a) (b) (c) (d) (a) Neopentyl alcohol (b) Isobutyl n-pentyl ether (c) Allyl bromide CYCLOALKANES 3.4 General P3.14 How many cis/trans stereoisomers exist for the following? (e) Vinylcyclopentane P3.16 Identify the bicyclic compound with the least strain? (a) (b) P3.15 How many different kinds [i.e., constitutional and geometric (cis/trans)] of hydrogens are there in the following? (c) (d) (a) 1-Ethyl-1-methylcyclopropane P3.17 Three structural isomers are possible for methylbicyclo[2.2.1]heptane One of them has two stereoisomeric forms Draw structures for all four isomers (a) Dichlorocyclopentane (b) Diphenylcyclohexane (c) 2-Chloro-4-ethyl-l-methylcyclohexane (b) Allylcyclobutane (c) Methylcyclobutane (d) Chlorocyclopentane Chapter Part I.indd 74 P3.18 Identify the number of structural isomers that exist for methylbicyclo[2.2.2]octane? 8/17/2015 11:12:55 AM 3.5 NOMENCLATURE 75 P3.19 Identify the more stable stereoisomer in each of the following pairs, and justify your choice (a) cis-or trans-1-Isopropyl-2-methylcyclohexane (c) (b) cis-or trans-1-Isopropyl-4-methylcyclohexane (c) or H3C CH3 CH3 (d) or H3C CH3 CH3 (e) H3C or CH3 H3C H3C CH3 P3.22 CH3 (a) Which of the following has the least negative heat of combustion ( Hcomb)? CH3 (f) cis-or trans-1-Isopropyl-3-methylcyclohexane P3.20 In each of the following groups of compounds, identify the one with the largest heat of combustion and the one with the smallest In which cases can a comparison of heats of combustion be used to assess relative stability? (a) Cyclopropane, cyclobutane, cyclopentane (b) cis-1,2-Dimethylcyclopentane, methylcyclohexane,1,1,2,2tetramethylcyclopropane (b) Identify the structures in (a) that are the same compound 3.5 Nomenclature (c) H H H H H H P3.23 Give the IUPAC name for each of the following and specify the stereochemistry where required (d) (a) (b) (c) (d) P3.21 State which has the most negative heat of combustion ( Hcomb) in each of (a), (b) or (c)? (a) t -Bu t -Bu (b) (e) Chapter Part I.indd 75 8/17/2015 11:12:56 AM 76 CHAPTER ALKANES AND CYCLOALKANES P3.25 In each of the following, predict whether Keq is approximately equal to, greater than or less than one: Ph (g) (f) Cl (a) Ph Ph (h) (i) Ph (b) F (j) (k) (c) (l) (m) P3.26 Many alkyl halides undergo loss of HX (dehydrohalogenation) in the presence of base to form alkenes The reaction is known as trans-diaxial elimination because mechanism generally requires both the leaving proton and halide to occupy axial positions Based on this which of the two geometric isomers would react faster, cis-l-chloro-2-t-butylcyclohexane or trans-1-chloro-2-t-butylcyclohexane? P3.27 Explain why the trans-4-fluorocyclohexanol exists largely in a chair conformation, while its cis-isomer favours a twist-boat conformation (n) (o) P3.28 Two possible configurations of glucose are shown as follows: 3.6 Conformational Analysis P3.24 Draw the most stable conformer of (a) In the structures below, complete the chair conformations to represent the most stable conformer of (1) and (2) Chapter Part I.indd 76 8/17/2015 11:12:57 AM 3.6 (b) Which configuration would have a more negative heat of combustion? (e) CH3CH2 P3.30 (a) How many cis/trans stereoisomers exist for 1,2,3,4,5,6-hexamethylcyclohexane? and CH3 CH3CH2 (b) Draw the conformations for three stereoisomers for which Keq = for conformational chair-chair flipping (f) P3.31 Determine whether the two structures in each of the following pairs represent constitutional isomers, different conformations of the same compound, or stereoisomers that cannot be inter-converted by rotation about single bonds H (a) H and H3C CH3 CH3 H3C (b) H H H and P3.32 Write the structural formula of the most stable conformation for each of the following compounds (a) 2,2,5,5-Tetramethylhexane (Newman projection of conformation about C3 C4 bond) (b) cis-1-Isopropyl-3-methylcyclohexane (c) trans-1-Isopropyl-3-methylcyclohexane and H3C CH3 CH3 (c) Which is the least stable isomer of those three? CH3 77 and P3.29 One of the chair conformations of cis-l,3-dimethylcyclohexane is 5.4 kcal/mol more stable than the other If the steric strain of 1,3-diaxial interactions between hydrogen and methyl is 0.9 kcal/mol, what is the amount of strain of a 1,3-diaxial interaction between the two methyl groups? (d) Which stereoisomer would be least likely to undergo conformational flipping? CONFORMATIONAL ANALYSIS CH3 H (c) cis-1,2-Dimethylcyclopentane and trans-1,3-dimethylcyclopentane (d) cis-1-tert-Butyl-4-ethylcyclohexane (e) cis-1,1,3,4-Tetramethylcyclohexane H CH3 (f) (d) Chapter Part I.indd 77 H3C H H CH3 8/17/2015 11:12:58 AM 78 CHAPTER ALKANES AND CYCLOALKANES SOLUTIONS ALKANES 3.1 General S3.1 The compound with highest melting point is bicyclo[2.2.2]octane It is most spherical in shape S3.2 The compound with highest boiling point is n-pentane It is straight chain compound S3.3 Dodecahedrane has higher melting point than eicosane because spherical molecules tend to pack more closely in the solid state than linear ones, thus requiring more energy to separate (melt) them Eicosane, mp 37º Dodecahedrane, mp 420º S3.4 (a) The number of constitutional isomers for the molecular formula C6H14 is five (b) The number of constitutional isomers for the molecular formula C7H16 is nine Chapter Part II.indd 78 8/17/2015 11:13:19 AM SOLUTIONS 79 (c) The number of constitutional isomers for dichlorocyclohexane is four (d) The number of constitutional isomers for C6H12 that contain a cyclopropyl ring is six S3.5 Different kinds (constitutional) of hydrogens in the given compounds are as follows: (a) There are six different kinds of hydrogens present in 2,3-dimethylpentane (b) There are four different kinds of hydrogens present in 2,2,4-trimethylpentane (c) There are ten different kinds of hydrogens present in 4-ethyl-3,3,5-trimethylheptane (d) There are seven different kinds of hydrogens present in 2,5,5-trimethylheptane Chapter Part II.indd 79 8/17/2015 11:13:22 AM 80 CHAPTER ALKANES AND CYCLOALKANES (e) There are three different kinds of hydrogens present in 2,4-dimethylpentane (f) There are three different kinds of hydrogens present in 3-ethylpentane S3.6 Highest molecular dipole moment is observed in cis-1,3-dichlorocyclobutane, as the dipole moment of the two C bond is not cancelled out Cl The dipole moment is zero in case of (a), (b) and (c) because of symmetrical structures of molecules, in which dipole moment of two C Cl bonds are cancelled out 3.2 Nomenclature S3.7 (a) 4-Ethyl-2,2,5-trimethyl-3-nitroheptane (b) 2,3,7-Trimethyl-4-n-propyloctane (Numbering is started from carbon atom with more branching) Chapter Part II.indd 80 8/17/2015 11:13:23 AM SOLUTIONS 81 (c) 5-Ethyl-3,5-dimethyloctane (d) 5-(2-Chloro-2-methylpropyl)-4-methyldecane (e) The two possible names for the compound are 4-t-butyl-2, 2, 6-trimethyl-4-n-propylheptane and 4-t-butyl-2,2-dimethyl4-(2-methylpropyl)heptane Since, in case of chains of equal length, the chain with the greater number of substituents are preferred over lesser number of substituents, hence, the second possible name of the compound is incorrect 4-Butyl-2,2,6-trimethyl-4-(1,1-dimethylpropyl) heptane (f) 4-Fluoro-2-methyl-2-phenylheptane (g) 5-(1,2-Dimethylpropyl)nonane Chapter Part II.indd 81 8/17/2015 11:13:23 AM 82 CHAPTER ALKANES AND CYCLOALKANES (h) 7-Bromo-3-ethyl-2-iodo-5,6-dimethylnonane (i) 2,3-Dimethyl-4-n-propylnonane (j) 1-Iodo-4-methylpentane (k) 4-Ethyl-3,3,5-trimethylheptane (l) 5-Ethyl-3,4-dimethyloctane S3.8 The correct IUPAC names for the following compounds are: (a) 2,3,5-Trimethyloctane Chapter Part II.indd 82 8/17/2015 11:13:23 AM SOLUTIONS 83 (b) 5-Ethyl-4-methyldecane (c) 3,7-Diethyl-2,2,8-trimethyldecane (d) Diethylpentane S3.9 The following are the structural formulae of the given compounds, with bond-line notation: (a) (b) (c) 3.3 Conformational Analysis S3.10 (a) The compound 2-cyclopentyl-6-methylheptane is most stable when the largest R groups are positioned anti to each other Hence, the most stable conformer is Chapter Part II.indd 83 8/17/2015 11:13:23 AM 84 CHAPTER ALKANES AND CYCLOALKANES (b) Gauche conformer of 1-phenylbutane implies that the largest R groups are at a dihedral angle of 60° to each other Hence, the Newman projection is S3.11 The Newman projections of isopentane are The plot for potential energy vs rotation about C2—C3 bond for these structures is as follows: Conformer E in which the dihedral angle between both the methyl groups is 60° corresponds to highest minimum in the graph S3.12 The dipole moment (μ) of F CH2CH2OH is much larger than FCH2CH2F because an intramolecular hydrogen bonding between highly electronegative F and H of ( OH) leads to stabilization of a gauche conformer of FCH2CH2OH thereby adding to the net dipole moment in the same direction On the other hand, the net dipole moment in FCH2CH2F is zero because of oppositively oriented F atoms S3.13 Eclipsed conformation is least stable Steric repulsion between the molecules raises the energy of the conformation which makes the molecule less stable Thus, the correct option is (c) CYCLOALKANES 3.4 General S3.14 (a) The number of cis/trans stereoisomers for dichlorocyclopentane is two pairs Chapter Part II.indd 84 8/17/2015 11:13:23 AM ... PROBLEMS 99 4 .1 General 99 4.2 Reactions 10 0 4.3 Conversions 10 5 4.4 Mechanisms 10 7 SOLUTIONS 10 9 14 1 ALKYNES PROBLEMS 14 1 5 .1 Reactions 14 1 5.2 Conversions 14 2 5.3 Mechanisms 14 3 SOLUTIONS 14 5... PROBLEMS IN ORGANIC CHEMISTRY PROBLEMS IN ORGANIC CHEMISTRY Dr K Singh K Singh Vision Classes, Patna Problems in Organic Chemistry Copyright © 2 015 by Wiley India Pvt Ltd., 4435-36/7,... COMPOUNDS PROBLEMS 245 10 .1 General 245 10 .2 Reactions 247 10 .3 Conversions 252 10 .4 Mechanisms 254 SOLUTIONS 257 11 299 ALDEHYDES AND KETONES PROBLEMS 299 11 .1 General 299 11 .2 Reactions 299 11 .3