Preview Problems In Organic Chemistry by K. Singh (2015)

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.
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ISBN: 978-81-265-5582-6
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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.

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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


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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.

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CONTENTS

1

THE BASICS

PROBLEMS

1

1

1.1

Hybridization, Molecular Formula and Physical Properties

1

1.2

Acids and Bases

4

1.3

Resonance

7

1.4

Reaction Basics

8

SOLUTIONS

2

STEREOCHEMISTRY

9

31

PROBLEMS

31

General

31

2.1

Elements of Symmetry

31

2.2

Chiral Molecules

36

2.3

Number and Types of Stereoisomers

37

2.4

Nomenclature

41

2.5

R- and S- Configuration

42

2.6

Optical Activity

43

2.7

Miscellaneous

44

Reactions

2.8

Stereochemistry of Reactions

44
44

SOLUTIONS

46

General

46

Reactions

67

3

73

ALKANES AND CYCLOALKANES

PROBLEMS

73

Alkanes

73

3.1

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General

73

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x CONTENTS

3.2

Nomenclature

73

3.3

Conformational Analysis

74

Cycloalkanes

74

3.4

General

74

3.5

Nomenclature

75

3.6

Conformational Analysis

76

SOLUTIONS

78

Alkanes

78

Cycloalkanes

84

4

99

ALKENES AND CARBOCATIONS

PROBLEMS

99

4.1

General

99

4.2

Reactions

100

4.3

Conversions

105

4.4

Mechanisms

107

SOLUTIONS

109

5

141

ALKYNES

PROBLEMS

141

5.1

Reactions

141

5.2

Conversions

142

5.3

Mechanisms

143

SOLUTIONS

145

6

159

SUBSTITUTION AND ELIMINATION REACTIONS

PROBLEMS

159

6.1

General

159

6.2

Reactions

161

6.3

Conversions

163

6.4

Mechanisms

164

SOLUTIONS

167

7

195

ALKYL HALIDES AND RADICALS

PROBLEMS

195

7.1

Reactions

195

7.2

Conversions

195

7.3

Mechanisms

196

SOLUTIONS

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197

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CONTENTS xi

8

ALCOHOLS AND ETHERS

205

PROBLEMS

205

Alcohols

205

8.1

Reactions

205

8.2

Conversions

207

8.3

Mechanisms

208
209

Ethers

8.4

Reactions

209

8.5

Conversions

210

8.6

Mechanisms

210

SOLUTIONS

212

Alcohols

212

Ethers

225

9

CONJUGATED SYSTEMS

PROBLEMS

233
233

9.1

Reactions

233

9.2

Conversions

234

9.3

Mechanisms

235

SOLUTIONS

237

10

245

AROMATIC 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

Conversions

306

11.4

Mechanisms

309

SOLUTIONS

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316

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xii CONTENTS

12

CARBOXYLIC ACIDS AND THEIR DERIVATIVES

371

PROBLEMS

371

Carboxylic Acids

371

12.1

General

371

12.2

Reactions

371

12.3

Conversions

373

12.4

Mechanisms

374

Carboxylic Acid Derivatives

375

12.5

Reactions

375

12.6

Conversions

378

12.7

Mechanisms

380

SOLUTIONS

383

Carboxylic Acids

383

Carboxylic Acid Derivatives

399

13

423

CARBONYL SUBSTITUTION AND CONDENSATION REACTIONS

PROBLEMS

423

Carbonyl α-Substitution and Enolates

423

13.1

Reactions

423

13.2

Conversions

424

13.3

Mechanisms

425

Carbonyl Condensation

426

13.4

Reactions

426

13.5

Conversions

427

13.6

Mechanisms

427

SOLUTIONS

430

Carbonyl α-Substitution and Enolates

430

Carbonyl Condensation

439

14

453

AMINES

PROBLEMS

453

14.1

Reactions

453

14.2

Conversions

454

14.3

Mechanisms

456

SOLUTIONS

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CONTENTS xiii

15

BIOMOLECULES AND POLYMERS

475

PROBLEMS

475

Carbohydrates

475

15.1

General

475

15.2

Reactions

475

15.3

Mechanisms

476

Amino Acids

477

15.4

General

477

15.5

Mechanism

478

Polymers

478

15.6

General

478

15.7

Mechanisms

480

SOLUTIONS

481

Carbohydrates

481

Amino Acids

493

Polymers

497

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1

THE BASICS

PROBLEMS

1.1

Hybridization, Molecular Formula
and Physical Properties

P1.1 State the formal charge over any atom that possesses it
in the following structures.
(a) C

C

(b)

(c)

(d) The conjugate base of NH2CH3
(e)
(B)
(a) The molecular formula for each compound.
(b) All the electron pairs in both the structures.
(c) The orbitals that overlap to form the covalent bonds indicated by arrows (1), (2), (3), (4).
(d) The hybridization state of both oxygen atoms in Compound
(A) and of double bonded nitrogen in Compound (B)?

(f)

P1.3 In the following pairs of compounds, identify
P1.2 For the Compounds (A) and (B) given below, identify

(a) Compound with higher boiling point and why?

(b) Compound with lower melting point and why?

(A)

Catechol

Hydroquinone

P1.4 Carbene [:CH2], a highly reactive species containing
divalent carbon having no charge and surrounded by
sextet of electrons. It can exist in two forms:

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2 CHAPTER 1 THE BASICS
Singlet carbene with two unshared electrons in the same
orbital and triplet carbene with unpaired electrons in different
orbitals and linear HCH bond angle.
(a) Identify the orbital housing the unshared electrons in singlet carbene and predict the HCH bond angle.
(b) Identify the orbitals housing the two unshared electrons
in triplet carbene.
P1.5 Would the carbon atoms indicated by arrows in each of
the following structures lie in the same plane?
(a)

(B)
(a) Write the molecular formulas for each compound.

(b) Identify the type of orbital (s, p, sp, sp2, sp3) in which the
lone pairs of electrons on the atoms indicated by arrows
(1), (2) and (3) are present.

(b)

(c) In Compound (A), the bond between the carbonyl carbon
and nitrogen lies between a single and a double bond, suggesting resonance structure. Identify the type of orbital in
which the lone pair of electrons on that nitrogen resides.
(c)

(d) How many lone pairs of electrons are present in each of
the given structures?

(d)

P1.7 Among the following pairs, identify the species with
higher molecular dipole moment.
(a) CHCl3 or CFCl3
(e) (CH3)3C+ all four carbons

(c) CO2 or SO2
(g)

(f)

(b) CH3NH2 or CH3NO2

P1.8 Identify the functional groups present in each of the
following compounds?

(a)

(h)

(b)
P1.6 For the structures given below for Compound (A) and
Compound (B):

(c)

(A)

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1.1

HYBRIDIZATION, MOLECULAR FORMULA AND PHYSICAL PROPERTIES 3

P1.9 Identify the number of lone pairs of electrons that are
present in the following structures.
(b)
(a)

P1.10 In the structures of Compounds (A), (B) and (C) given
below, identify

(B)

(a) The molecular formulae.
(b) The number of sp2 and sp3 carbon atoms present.
(c) The number of lone pairs of electrons present.
P1.12 The following compound shows presence of two
nitrogen atoms. In this structure, identify

(A)

(B)
(a) The nature of orbital in which the lone pair on each N
atom reside.
(b) The hybridization of each N atom in the compound.
P1.13 In the following compounds:

(a) State the hybridization of the N atom?
(C)
(a) The molecular formula for each compound.

(b) Which of these compounds is the most basic?
P1.14 In the structure of acetonitrile given below

(b) Number of lone pairs of electrons present in each compound.
P1.11 Compounds (A) and (B) have the following structures. In both the compounds, identify

(a) What is the hybridization of both the C atoms and N atom?
(b) In what type of orbital does the lone pair on N reside?
(c) Identify the type of orbitals used to form each bond.
P1.15 In the structure of two naturally occurring compounds
given, identify the orbitals used to form the bonds
indicated with the arrows.

(A)

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4 CHAPTER 1 THE BASICS

(c)

(a)

(d)
(b)

P1.19 In the following compounds, identify the number of
hydrogen atoms present around the carbon atom indicated with arrows.

P1.16 In the structure of antibiotic given below, identify:

(4)

(a)

(1)

(2)

(3)

(b)
(a) The hybridization of the atoms marked with an arrow.
(b) The number of π bonds in the antibiotic.
P1.17 In the structure of compound given below

(a) Considering all the bonds, which is the shortest C
bond?
(b) Identify the shortest and longest C

C

C single bond.

(c) Explain why the bond lengths of the two C
bonds (1) and bond (2) are different.

P1.18 In the following compounds, indicate the orbitals used
to form the bonds marked with arrows. Also indicate
the individual orbitals used to form the multiple bonds.

(a)

(b)

1.2 Acids and Bases

C single
P1.20 Label each of the following compounds as a Lewis

acid or a base.

(a)

(b) Cl3C+

(c) BCl3

(d) CH3 — Cl

(e)

(f) H3O+

P1.21 Arrange the following ions in order of increasing
basicity.
(a)
(b)

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1.2
−
−
−
−
(c) CH3COO , CH3CH2O , ClCH2COO , Cl2CHCOO ,

−
Cl3CCOO

ACIDS AND BASES 5

P1.25 For the structures of the compounds given below,
write a Brønsted-Lowry equation for the reaction of
the conjugate base of the compound with
(a) Ammonia (pKa = 35)

(d)

P1.22 Complete the following reactions showing the movement of electrons using curved arrows. Identify the
reactants as Lewis acids or Lewis bases and place the
formal charge on atoms as appropriate.
(a)

(b)

(b) Ammonia (pKa = 35)

(c)
(d)

(c) Sodium bicarbonate (pKa = 10.2)

(e)
P1.23 Complete the following reactions showing the movement of electrons using curved arrows. Identify the
reactants as Lewis acids or Lewis bases.
(a)

P1.26 Consider the structure of ibuprofen (Compound A)
given below.

(b)

(c)
(d)
(e)

(A)
(a) Identify the most acidic proton in the structure.
(b) Write the reaction for the conjugate base of Compound
(A) with acetaminophen (Compound B) and identify the
weak and strong acids and bases.

P1.24 (a) Which among the two is the stronger base:
(CH3)2NH or CH3 O CH3?
(b) Which is the strongest base that can exist in ammonia?
Write the Lewis acid base reaction when NaH is added to
NH3. Comment on the strength of NaH as base.

Chapter 1 Part I.indd 5

(B)
P1. 27 Which compound among the following has the lowest
pKa value and why?

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6 CHAPTER 1 THE BASICS
(a) C2H5OH

(b) CH3COOH

(c) H2O

(d) C6H5OH

(e) H2

(f) NH3

P1.28 Arrange the following compounds in order of increasing acidity

P1.30 In the following three compounds, account for the
observed values pKa of three different C H bonds.
Write all possible structures of the conjugate base of
the compounds.
(a)

(a) NH3, H2O, HF

(b)

(b) HBr, HCl, HF
(c) H2O, H3O+, HO−
(d) NH3, H2O, H2S
(e) CH3OH, CH3NH2, CH3CH3
(f) HCI, H2O, H2S

(g) CH3CH2CH3, CICH2CH2OH, CH3CH2OH
(h) HC

CCH2CH3, CH3CH2CH2CH3, CH3CH

(c)
CHCH3

P1.29 Identify the products in each of the following acidbase reactions. Label the acid and base in the reactants and the conjugate acid and base in the products
formed.
(a)

P1.31
(a) Write the structures of conjugate acids of the compounds
given below.

(b)

(A)
(c)
(d)
(e)

What would be the structure of the conjugate base of
Compound (A)?
(b) Identify the most basic electron pair in the structure of
Compound (B).

(f)
(g)

(h)

(i)

P1.32 Explain the following:
(a) The C–H bond of nitromethane, pKa = 1.0, is more acidic
than most C–H bonds.

(j)

(b) The C–H bond of 1,4-pentadiene (marked) is more
acidic than the C–H bond of pentane.

(k)

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1.3

(c) The pKa values of isomers dimethyl ether (pKa = 40) and
ethanol (pKa = 16) are different.

RESONANCE 7

(f)

P1.33 In the following compounds, indicate the most acidic
proton and explain why.
(g)
(a)

(h)
(b)
P1.36 For each of the following ions, draw a resonance
structure that is more stable than the one given.
(c)

1.3 Resonance
P1.34 Draw the conjugate acid forms of the compounds
given below and on the basis of their stability predict
protonation of which (a) oxygen and (b) nitrogen atom
is favoured.
(a)

(a)

(b)

(c)

(b)
(d)
P1.35 Identify the number of resonance structures possible
for the following ions.
(a)

P1.37 On the basis of resonance explain the following:
(a) The pKa value of Compound (B) is lower than that of
Compound (A).
H

(c)

C
(A)

N:

H

O

C
(B)

N:

(b) The pKa value of H1 in Compound (C) is only about 10.

(d)

(C)
(e)

Chapter 1 Part I.indd 7

(c) The molecular dipole moment of Compound (E) is larger
than that for Compound (D).

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8 CHAPTER 1 THE BASICS
(e)

(D)

(E)

(f)

P1.38 Identify the type of orbital in which the electrons
specified by the arrows are present.

1.4 Reaction Basics
P1.40 Illustrate the movement of electrons in the following reactions using curved and or half-headed curved
arrows.
(a)

(b)
P1.39 Identify the number of resonance structures possible
for the following.
(c)
(a)
(d)
(b)

(c)

(e)

(f)

(d)

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SOLUTIONS 9

SOLUTIONS
1.1

Hybridization, Molecular Formula and Physical Properties

S1.1 Formal charge is the charge that each atom in the molecule would carry if the electrons in the bonds were divided equally
between the two atoms. It considers all bonds as if they were non-polar or the difference in the electronegativity of the
atoms is not taken into consideration. The formal charges in the given compounds can be represented as follows:
(a)

(b)

(c)

(e)

(d)
(f)

S1.2
(a) The molecular formulae of the given compounds are:
Compound (A): C32H41NO2
Compound (B): C12H20N4O7
(b) The presence of electron pairs in the structures is indicated as follows

(c) The bonds indicated by the arrows are:
(1): C

C bond in which both the carbon atoms are sp3 hybridized, so orbitals that overlap are sp3

sp3;

(2): C C bond in which one carbon is sp3 hybridized and the second carbon (of aromatic ring) is sp2 hybridized,so orbitals
that overlap are sp3 sp2;
(3): C

O bond in which both atoms are sp2 hybridized, so orbitals that overlap are: sp2

sp2

(4): C

N bond in which carbon and nitrogen both are sp2 hybridized, so orbitals that overlap are: sp2

sp2

(d) Hybridization of both the oxygen atoms in Compound (A) is sp3 and of nitrogen indicated in Compound (B) is sp2.
S1.3
(a) The amine with the following structure has higher boiling point because this isomer can form intermolecular hydrogen bond. This
increase in the intermolecular attractive forces leads to an increase in its boiling point as compared to the other isomer.

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