www.pdfgrip.com

Conceptual Problems
in Organic Chemistry
(Volume 1)
[For Engineering and Medical Entrance Examinations]

D. K. SINGH

ChandigarhyDelhiyChennai


www.pdfgrip.com

The aim of this publication is to supply information taken from sources belived to be valid and
reliable. This is not an attempt to render any type of professional advice or analysis, nor is it to
be treated as such. While much care has been taken to ensure the veracity and accuracy of the
information presented within, neither the publisher nor its authors bear any responsibility for any
damage arising from inadvertent omissions, negligence or inaccuracies (typographical or factual)
that may have found their way into this book.
Copyright © 2009 Dorling Kindersley (India) Pvt. Ltd
This book is sold subject to the condition that it shall not, by way of trade or otherwise, be lent,
resold, hired out, or otherwise circulated without the publisher's prior written consent in any form
of binding or cover other than that in which it is published and without a similar condition including this condition being imposed on the subsequent purchaser and without limiting the rights under
copyright reserved above, no part of this publication may be reproduced, stored in or introduced
into a retrieval system, or transmitted in any form or by any means (electronic, mechanical, photocopying, recording or otherwise), without the prior written permission of both the copyright owner
and the above-mentioned publisher of this book.
ISBN 978-81-317-2463-7
First Impression
Published by Dorling Kindersley (India) Pvt. Ltd., licencees of Pearson Education in South Asia.

Head Office: 482, F.I.E., Patparganj, Delhi 110 092, India.
Registered Office: 14 Local Shopping Centre, Panchsheel Park, New Delhi 110 017, India.
Laser typeset by Rachna Graphics, Chandigarh
Printed in India


www.pdfgrip.com

To my mother for her immense
love, patience and support


www.pdfgrip.com

This page is intentionally left blank


www.pdfgrip.com

About the Author
Devendra Kaumar Singh did his post-graduation at Muzaffarpur, Bihar and received his doctorate in 1991 from Indian Institute of Science, Bangalore under the suprtvision of Professor S. N.
Balasubrahmanyam. Following this he worked as a post-doctoral fellow first with Professor Uday
Maitra in the same institute for two years (1991–1993) and then with Professor R. C. Corcoran in
the University of Wyoming, USA for one year (1993–1994).
His research interest was in the area of Physical Organic Chemistry. He has got three research
papers—One in Bull. Chem. Soc. Jpn, one in Indian J. Chem. and one in J. Org. Chem.
He received state merit scholarship in M.Sc., research fellowship for Ph.D. degree and postdoctoral fellowship from Indian Institute of Science, Bangalore followed by post-doctoral fellowship from the University of Wyoming, USA.
When his academic training was over, the profession of private teaching in Patna attracted
him. Therefore, he left the idea of obtaining a suitable job and self employed himself by way of
teaching Organic Chemistry to students intended to appear in the medical and engineering entrance examinations. This he started in 1996 and today he has a large number of students and four

non-teaching staff working under him.


www.pdfgrip.com

This page is intentionally left blank


www.pdfgrip.com

Contents
Preface

ix

CHAPTER 1 HYBRIDIZATION, RESONANCE AND AROMATICITY

3 – 28

Hybridization
Resonance
Aromaticity

CHAPTER 2 ISOMERISM

31 – 99

Structural Isomerism
Geometrical Isomerism
Optical Isomerism

Conformational Isomerism

CHAPTER 3 SUBSTITUENT EFFECTS, REACTIVE INTERMEDIATES,

103 – 167

ACID-BASE AND ELECTROPHILES/NUCLEOPHILES
Substituent Effects
Reactive Intermediates
Acid-base
Electrophiles/Nucleophiles

CHAPTER 4 REACTION MECHANISM
Nucleophilic Substitutions
Elimination Reactions
Electrophilic Addition Reactions
Nucleophilic Addition Reactions
Electrophilic Aromatic Substitutions
Nucleophilic Aromatic Substitutions

171 – 233


www.pdfgrip.com

This page is intentionally left blank


www.pdfgrip.com


Preface

Organic Chemistry is not a fomulative science. It requires to generate the feeling about
the natures of molecules and their reactions. In other words, realization is the best way
to learn Organic Chemistry. Without realization, one has to memorize which cannot help
much specially when problems are based on application of concepts as in engineering and
medical entrance examinations.
Therefore, two-fold training is required. Students should gain clear concepts of the
subject first and then they should learn to apply these concepts in solving problems.
Now, when it comes to making and designing questions for this purpose, one has to
imagine from all angles of each topic included in syllabus. The present book is an attempt
in this direction.
Though Conceptual Problems in Organic Chemistry (Volume 1) is only a practice workbook, it will effectively catalyse the process of learning the subject. In fact, the questions
are designed in a way that will encourage the students to learn more and more of Organic
Chemistry.
It is proposed to write the book in two volumes. First volume is in your hand. It includes
topics in structural organic chemistry and general reaction mechanism. The second volume,
yet to come, will contain questions on organic reactions and their specific features.
The contents of this volume is the basis of the contents in second volume. Without
understanding the first volume one can never be able to answer the questions in second
volume. Therefore, having the first volume alone in hand, can turn out to be a nice starter.
I will appreciate valuable suggestion from readers. In fact, I will be happy to know your
opinion.

D. K. SINGH


www.pdfgrip.com

This page is intentionally left blank



www.pdfgrip.com

1

CH

2

O

N
a

Chapter

H

H
C=C=C
H

H

H

N
C
H


O

Hybridization,
Resonance and
Aromaticity


www.pdfgrip.com

Question Bank
1. For exhibiting tetravalency, carbon atoms
have to be excited. Now, which of the following statements is true?
a Excitation occurs before bonding
b Bonding occurs before excitation
c Both bonding and excitation occur
simultaneously
d Two bonds are formed first, then excitation occurs followed by formation of
another two bonds
2. Bond formation is
a always exothermic
b always endothermic
c neither exothermic nor endothermic
d sometimes exothermic and sometimes
endothermic
3. Which of the following statements is true
about bonding and excitation?
a Energy required for excitation of carbon atoms (96 Kcal/mol) is less than
energy released in bonding
b Energy required for excitation is more

than energy released in bonding
c Energy required for excitation is equal
to the energy released in bonding
d None of these
4. Which of the following statements is true
about hybridization?
a Only those atomic orbitals can be hybridized which do not differ much in
shape.
b Only those atomic orbitals can be hybridized which do not differ much in
energy.
c Only those atomic orbitals can be hybridized which do not differ much in
size.
d Only those atomic orbitals can be hybridized which do not differ much in
overlap integrals.

5. Which of the following statements is true
about hybridization?
a Hybridization generates new set of
atomic orbitals identical in shape but
not in size and energy.
b Hybridization generates new set of
atomic obitals identical in size but not
in shape and energy.
c Hybridization generates new set of
atomic obitals indentical in energy but
not in shape and size.
d Hybridization generates new set of
atomic orbitals indentical in shape, size
and energy.
6. Which of the following statements is true

about hybridization?
a Hybrid orbitals frequently undergo
linear overlaps making sigma bonds.
b Hybrid orbitals frequently undergo
lateral overlaps making π-bonds. In
other words, there are several compounds in which π-bonds are formed
using hybrid orbitals.
c Hybrid orbitals are molecular orbitals.
d A hybrid orbital bigger in size makes
shorter bond.
7. In 2sp hybridization, 2s-orbital can be
mixed with
a Only 2p
x
b Only 2py
c Only 2p
z
d Any one of 2px, 2py and 2pz
8. In 2sp2 orbital, character of 2pz orbital will
be
a always 33.33%
b always 0%
c always 66.66%
d either 33.33 % or 0 %


www.pdfgrip.com
4

Conceptual Problems in Organic Chemistry


9. In 2sp3 orbital
a characters of 2p , 2p and 2p orbitals
x
y
z
are equal to each other but less than
that of 2s-orbital
b characters of 2px, 2py and 2pz orbitals
are equal to each other but more than
that of 2s
c characters of 2s, 2p , 2p and 2p orbitx
y
z
als are equal to each other
d characters of 2px, 2py and 2pz are not
equal to each other
10. Which of the following statements about
the energy of hybrid orbitals resulting
from mixing of 2s and 2p orbitals is
true?
a E2p > Esp3 > Esp2 > Esp > E2s
b Esp3 > Esp2 > Esp > E2p > E2s
c E2p > Esp > Esp2 > Esp3 >E2s
d E2p > Esp > E2s > Esp2 > Esp3
11. Which of the following statements about
the size of hybrid orbitals resulting from
mixing of 2s and 2p orbitals is true?
a 2s > 2p > sp3 > sp2 > sp
b 2p > 2s > sp3 > sp2 > sp

c 2p > sp3 > sp2 > sp >2s
d sp3 > sp2 > sp > 2p > 2s
12. Shapes of sp3, sp2 and sp orbitals are like
a

b

c

d

13. Which of following statements about
overlap integrals of the various orbitals is
true?
a 2p > 2s > 2sp3 > 2sp2 > 2sp
b 2p > 2sp3 > 2sp2 > 2sp > 2s
c 2p > 2sp > 2sp2 > 2sp3 > 2s
d 2sp3 > 2sp2 > 2sp > 2p > 2s
14. Energy content of a molecule will be less
if

a bond energy is more
b bond energy is less
c magnitude of ovelap is very less
d none of these

15. Which of the following orders is correct
for electronegativity?
a sp3C > sp2C > spC
b spC > sp2C > sp3C

c sp2C > spC > sp3C
d sp3C > spC >sp2C
16. In which of the following compounds,
C has used sp2 orbital in making C–O
bond?
OH
OH
a

b

OH
c

OH
d

17. The orbitals used by C in forming C-H and
C-Cl bonds of CH2Cl2 are
a four sp orbitals
b four sp2 orbitals
c four sp3 orbitals
d none of these
18. CH2 = CH - CN
3
2
1
Cl-C2 bond of this molecule is formed
by
a sp3-sp2 overlap

c sp2-sp overlap

b sp2-sp3 overlap
d sp2-sp2 overlap

19.
O

In this molecule, there are two C-O bonds.
Which of the following statements is true
about these bonds?
a Both C-O bonds are formed by sp3-sp3
overlaps
b Both C-O bonds are formed by sp2-sp2
overlaps


www.pdfgrip.com
Hybridization, Resonance and Aromaticity
c One C-O bond is formed by sp2-sp3

overlap while the other C-O bond is
formed by sp3-sp3 overlap
d Both C-O bonds are formed by sp2-sp3
overlaps
20.

NH

21. CH2 = C = CH2

In this molecule (allene)
a all three C-atoms are sp2 hybridized
b both terminal C-atoms are sp2 hybridized while central C-atom is sp-hybridized
c both terminal C-atoms are sp-hybridized while central C-atom is sp2 hybridized
d none of these
22. CH2 = C = CH2
Which of the following structures is most
appropriate for this molecule?
H
H
a
C=C=C
H
H
H
H
C
=
C
=
C
b
H
H
H

H
C=C=C

H

H

H
H
C=C=C
H

H
a

H

H
C=C=C=C

H
H
b

O
In this molecule,
a both C-N bonds are formed by sp2-sp2
overlaps
b one C-N bond is formed by sp2-sp2
overlap while the other by sp2-sp 3
overlap
c both C-N bonds are formed by sp2-sp3
overlaps
d both C-N bonds are formed by sp2-sp2
overlaps


d

23. CH2 = C = C = CH2
Which of the following structures is most
appropriate for this molecule?

H
C

c

5

H
H
C=C=C=C

H

H

H
c

H
C=C=C=C

H


H

H

H

d

C=C=C=C
H

H

24. Which of the following compounds has
maximum number of sp-hybridized Catoms?
a (CN)
2
b CH ≡ C–CN
c HC ≡ C–CH –CH = C = C = CH
2
2
d CH2 = C = CH–CN
25. CH3–O–CH = CH2
Oxygen atom of this molecule is
a sp3 hybridized
b sp2 hybridized
c sp-hybridized
d unhybridized
26. CH3
C=O

CH3
Oxygen atom of this molecule is
a sp3-hybridized
b sp2-hybridized
c sp-hybridized
d unhybridized

27. CH3 – C ≡ N
Nitrogen atom of this molecule is
a sp3-hybridized
b sp2-hybridized
c sp-hybridized
d unhybridized


www.pdfgrip.com
6

Conceptual Problems in Organic Chemistry

28. In which of the following molecules, all
atoms are coplanar?
a
a

c

b

CN


CH3

CH3

CH3

CH3

CH3

b
d

C=C
CN

CH3

29. In which of the following molecules, –NO2
group is not coplanar with phenyl ring?

CH3

CH3

c

a


N
O
CH3

O

CH3

CH3

CH3

b

N
O

c

d

O

I

CH3
CH3

I


31. In which of the following molecules, all
atoms are not coplanar?

N
O

CH3
CH3

O

a

O

d

O

N=O
O

30. In which of the following molecules both
phenyl rings are not coplanar?

O
b


www.pdfgrip.com

Hybridization, Resonance and Aromaticity

d

c

O
d

O

32. In which of the following molecules, all
bond angles are not similar?
CHO
a

b

c

d

33. Which of the following is the correct order
of bond angles?
a NH > H O > CH
3
2
4
b H2O > NH3 > CH4
c CH > NH > H O

4
3
2
d CH4 > H2O > NH3
34. Which of the following is the correct order
of bond angles?
NH4 > H3O + > H2O
+
b H3O + > NH4 > H2O
+
c H O > H O + > NH4
2
3
+
d NH4 > H2O > H3O +
a

+

35. Which of the following order of bond
angles is correct?
a

7

>

b HC ≡ CH >

> HC ≡ CH


>

HC ≡ CH >

36. Which of the following orders of bond
energy is correct?
a C–C > N–N > O–O
b O–O > N–N > C–C
c N–N > C–C > O–O
d C–C > O–O > N–N
37.

1
2
3
4
I CH2 = CH–CH = CH2
1
2
3
4
II CH3–CH2–CH2–CH3
1
2
3
4
III CH2 = CH–CH2–CH3
1
2 3

4
IV CH2 = CH–C = CH
Which of the following is the correct order
of C2-C3 bond length?
a II > I > III > IV
b II > III > I > IV
c III > I > IV > II
d IV > I > III > II

38. CH3CN
CH3CHO
CH3-CH3
I
II
III
Which of the following is the correct order
of C–C bond length among these compounds?
a I > II > III
b III > II > I
c III > I > II
d II > I > III
39. H

H
l1
C=C=C
H
H
l2
Which of the following is the correct order

of spatial distances l1 and l 2?
a l =l
b l1 > l2
1
2
c l d
l2 = 2l1
1
2

40. H
H

c

>

> HC ≡ CH

>

l1
C=C=C=C
l2

H
H

Which of the following is the correct order

of spatial distances l1 and l2?


www.pdfgrip.com
8

Conceptual Problems in Organic Chemistry
a l=l
1
2
c l1
2

b l1 >l2
d l2 = 2l1

41. Resonance involves
a change in the positions of atoms
b change in the positions of both σ and
π electrons
c change in the positions of only π electrons
d change in the positions of only σ electrons
42. CH3 – CH = O
I
CH2 = CH – OH
II
–
+
CH3 – CH – O

III
Among these, which are canonical structures?
a I and II
b I and III
c II and III
d all
43. CH 2 = CH – CH = CH2
I
–
+
CH2 – CH = CH – CH2
II
–
+
CH2 – CH = CH – CH2
III
Among these which are canonical structures?
a I and II
b I and III
c II and III
d all of these
44. Which of the following statements is
true?
a Resonance is an intermolecular process
b Canonical structures are imaginary
c Canonical structures are real
d A canonical structure explains all features of a molecule
45. For a resonating molecule, real structure
is

a the resonance hybrid
b the canonical structures
c sometimes resonance hybrid and some-

times canonical structures

d none of these

46. A resonance hybrid is the one which is
contributed by
a all canonical structures
b only the least stable canonical structure
c only the most stable canonical structure
d none of these
47. In the resonance hybrid
a all canonical structures contribute
equally
b more stable canonical structure contributes more than less stable canonical
structure
c more stable canonical structure contributes less than less stable canonical
structure
d there is no contribution of any canonical structure
48. Delocalization of electrons increases molecular stability because
a electrons-nuclei attraction increases
b electrons-electrons repulsion increases
c potential energy of the molecule increases
d potential energy of the molecule remains unaffected
49. Resonance energy is
a equal to the energy of resonance hybrid
b equal to the energy of most stable canonical structure

c equal to the energy of least stable canonical structure
d equal to the difference in energies of
the most stable canonical structure and
resonance hybrid


www.pdfgrip.com
Hybridization, Resonance and Aromaticity

50. Resonance energy is
a stored in the molecule
b released by the molecule
c neither stored nor released by the molecule
d sometimes stored and sometimes released by the molecule
51. All canonical structures of a molecule
a must have different number of unpaired electrons
b must have different number of paired
electrons
c must be always equivalent
d need not be always equivalent but they
should not differ much in stability
–
–
+
+
CH2 – O
CH2 – O
52. CH2 = O
I
II

III
Which of these structures is practically
not a valid canonical structure of formaldehyde?
a I
b II
c III
d none of these
+
53. CH2 = CH – CH = CH – NH3
I
–
+
+
CH2 – CH = CH – CH – NH3
II
+
CH2 – CH = CH – CH = NH3
III
Which of these structures is not a valid
canonical structure?
a I
b II
c III
d none of these

54.

–
+ O
CH3 – N

O
+ O
CH3 – N
–
O
O
CH3 – N
O

9
a I
c III

b II
d none of these

55. A canonical structure will be more stable
if
a it has more number of π-bonds than if
it has less number of π-bonds
b it has charge separation than if it has
no charge separation
c it has greater length of charge separation than if it has lower length of
charge-separation
d it has positive charge on more electronegative atom than if it has positive
charge on less electronegative atom
56. Which of the following statements is true
about the contributions of canonical structures in the resonance hybrid?
a A charge-separated canonical structure
contributes more than a canonical

structure without charge-separation
b A canonical structure with less number
of π bonds contributes more than a
canonical structure with more number
of π bonds
c A canonical structure with negative
charge on less electronegative atom
contributes more than a canonical
structure with negative charge on more
electronegative atom
d contribution of a charge-separated
canonical structure will be more in a
polar solvent than in a non-polar solvent
57.

O
I

C
H

I

O
II

II

H

C
+

–

OH

O–

III

Which of these structures is not a valid
canonical structure of nitromethane?

OH

C

III
H

+
OH


www.pdfgrip.com
10

Conceptual Problems in Organic Chemistry

Among these canonical structures, the
correct order of stability is
a I > II > III
b III > II > I
c I > III > II
d II > I > III
58.

O
I

C
H

The correct order of stability among these
canonical structures is
a I > III > II
b III > I > II
c II > III > I
d II > I > III
61.

–
O

O
I

–

NH
H

H

O–
II
H

C
+

O

–
O

NH

C
II

H

III

59. I

II

–
NH

O

C

H
O
Among these canonical structures, the
correct order of stability is
a I > II > III
b II > I > III
c I = III > II
d II > I ≡ III

C
III

H

H

The correct order of stability among these
canonical structures is
a I > II > III
b I > III > II
c II > I > III
d III > I > II

CH2 = CH – CH = O
+
–
CH2 = CH – CH – O

62.
I
N

+
–
CH2 – CH = CH – O

III
The correct order of stability among these
canonical structures is
a III > II > I
b II > III > I
c I > III > II
d I > II > III

60.

II

+

C
H

N
–

+

O
I

–
CH2

III
N–

O–
II
H

C
+

–
CH2

IV
N
–

O–
C

III
H

CH2

H

–

O–

V
N

+


www.pdfgrip.com
Hybridization, Resonance and Aromaticity

Among these canonical structures of pyridine, the correct order of stability is
a (I = V) > (II = IV) > III
b (II = IV) > (I = V) > III
c (I = V) > III > (II = IV)
d III > (II = IV) > (I = V)
63.

11

65.

3

6

2

3

5

N

2
1

H
In pyrrole-electron density is maximum
on
a 2 and 3
b 3 and 4
c 2 and 4
d 2 and 5

4
5

4

66.

N
1
In pyridine electron density is maximum
on
a 2 and 6
b 3 and 5
c 4
d 3 and 4

O
I
N
H
O+

64.

II

I

N

N

H

H

–

–
O

N
+
H

II

IV

N+

–

67.

N
+
H

–
V

H
The least stable canonical structure among
these is

a I
b II
c III
d all are equally stable

–

N
+
H

III

–

III

N
+
H
Among these canonical structures of pyrrole, the correct order of stability is
a (III = IV) > (II = V) > I
b I > (II = V) > (III = IV)
c I > (III = IV) > (II = V)
d (II = V) > (III = V) > I

O

+
I

O

+

II
+
O

III

The most stable canonical structure among
these is
a I
b II
c III
d all are equally stable


www.pdfgrip.com
12

Conceptual Problems in Organic Chemistry

68. 3

72.

2

m

o

m

o

N

p
+1
In this cation, π electron density is more
on
a C1
b C2
c C3
d π-electron-density is same on each Catom.

69.

O

O
In nitrobenzene, π-electron density is
maximum on
a ortho positions
b meta positions
c para position
d none of these

73. Which of the following compounds has
maximum electron density in ring?
NO2
OH
a

2

b

3

–
1

O

4

5
In this anion, π-electron-density is maximum on
a C1
b C2 and C5
c C3 and C4
d same on each C-atom

O

O

70. H - C - H
I

H - C - OH
II

71.

o

m

o

d

74.
Which of following statements is true
about this compound (biphenyl)?
a Both rings act as π donors
b Both rings act as π acceptors
c One ring acts as π donor while the other
as π-acceptor
d Resonance does not occur between
both rings

O
H - C - NH2
III

Which of these compounds has maximum
π-electron-density on the carbonylic oxygen?
a I
b II
c III
d Same in all
m

c

–
COO

–

75.

I

CH2 = CH – CH = CH – OCH3

II

–
+
CH2 – CH = CH – CH = OCH3

III

–
+
CH2 = CH – CH – CH – OCH3

OH

p

In phenol, π-electron-density is maximum
on
a ortho and meta positions
b ortho and para positions
c meta and para positions
d none of these

–
+
IV CH2 = CH – CH – CH – OCH3
Among these canonical structures which
one is least stable?
a I
b II
c III
d IV


www.pdfgrip.com
Hybridization, Resonance and Aromaticity

I

CH2 = CH – CH = O

II

–
+
CH2 – CH = CH – O

III

–
+
CH2 – CH – CH = O

76.

13
a - CH = O

b - CO NH2

77.

4
3

1
2
In this molecule, π-electron-density is more

on
a C1 and C3
b C2 and C4
c C2 and C3
d C1 and C4
4

3

2

1

78. CH2 = CH—C = CH2
|
OCH3
In this molecule, π-electron density is
maximum on
a C1
b C2
c C3
d C4
79. Resonance energy will be more if
a Canonical structures have charge separation than if canonical structures have
no charge separation
b Molecule is nonaromatic than if molecule is aromatic
c Canonical structures are equivalent
than if canonical structures are non
equivalent
d π-bonds are nonconjugted than if πbonds are conjugated

80. Which of the following groups acts as πdonor in resonance?

c - NH - C - H

d - NO2

=

=

81. Which of the following groups act as πacceptor in resonance?
O
O
a -O-C-R

b - C - OR

=

O

c - NH - C - H

d - NH2

82. Which of the following groups can act
either as a π-donor or a π-acceptor depending upon situation?
a - Ph
b -OH
c - CH=O

d - NO2
83. Which of the following groups can act
either as a π-donor or as a π-acceptor depending upon situation?
a - NO
b -NO2
c -O-N=O
d - NH2
84. Which of the following group cannot participate in resonance with any group?
+
+
a - CH2
b - NH3
O

=

–
+
IV CH2 – CH – CH = O
Among these canonical structures, which
one is least stable?
a I
b II
c III
d IV

=

O

c -N+ -O–

85. CH3COOH
I

d - NO

CH3COONa CH3CONH2
II
III

Among these compounds, the correct
order of resonance energy is
a I > II > III
b III > II > I
c II > III > I
d II > I > III
86.

I
II

–
HN = CH – NH
–
O = CH – NH

III
O = CH – OH
Among these species, the correct order of

resonance energy is
a I > II > III
b II > I > III
c III > I > II
d III > II > I


www.pdfgrip.com
14

Conceptual Problems in Organic Chemistry

87.

O

91.

O

I

O

III
IV
Among these compounds, which one has
maximum resonance energy?
a I
b II

c III
d IV

88.

I CH3COOH
II CH3CONH2
III CH3COF
Among these compounds, the correct
order of resonance energy is
a I > II > III
b III > II > I
c II > III > I
d II > I > III

92.

89.

II

II

I

CH2 = CH – CH = CH – CH =CH2

II

–

+
CH2 = CH – CH – CH = CH – CH2

III

–
+
CH2 – CH = CH – CH = CH – CH2

Among these three canonical structures,
(though more are possible) what would be
their relative contribution in the hybrid?
a I > II > III
b III > II > I
c I > III > II
d III > I > II

OCH3

I

I

Which of the following statements is true
about this resonating system?
a Contribution of II is more than that of
I in resonance hybrid
b Contribution of I is increased by a polar
solvent
c Contribution of II is increased by a

polar solvent
d Relative contributions of I and II are
independent of solvent

II
O

CH2 = CH – OH

–
+
CH2 – CH = OH

OCH3

93.

CH2 = CH – OCH3

OCH3
III

Among these compounds the correct order
of resonance energy is
a III > I > II
b II > I > III
c I > II > III
d III > II > I
90.

I

CH2 = CH – N

II

III

Among these compounds, the correct
order of resonance energy is
a I > II > III
b II > I > III
c III > I > II
d I > III > II

CH3
CH3

II
I

–
+
CH2 – CH = OCH3

–
+ CH3
CH2 – CH = N
CH3

Which of the following statements is true
about the contribution of I and II in respective resonance hybrids?
a I contributes more than II
b II contributes more than I
c Both I and II contribute equally
d none of these


www.pdfgrip.com
Hybridization, Resonance and Aromaticity

94. Heat of hydrogenation of cyclohexene
is –28.6 kcal/mol and that of benzene is
–49.8 kcal/mol. Then resonance energy
of benzene will be
a 46 kcal/mol
b 39 kcal/mol
c 36 kcal/mol
d 30 kcal/mol

15

99.

97.

CH3

I
CH3

CH3

95. Diene
Heat of Hydrogenation
CH2 = CH-CH=CH-CH3 –54.1 kcal/mol
I
CH3-CH=C=CH-CH3
–70.5 kcal/mol
II
CH2=CH-CH2-CH=CH2 –60.2 kcal/mol
III
Examine these dienes and their heat of
hydrogenations. Now, what will be the
stability order among these dienes?
a I > II > III
b II > III > I
c I > III > II
d II > I > III
96. Heat of hydrogenation of CH3-CH=CH2 is
–30 kcal/mol and resonance energy of 1,3butadiene is 3 kcal/mol. What will be the
heat of hydrogenation of 1,3-butadiene?
a –63 kcal/mol
b –57 kcal/mol
d
–54 kcal/mol
(c) –27 kcal/mol

CH3

CH2

II
CH3
CH2

CH2

III
CH2
The heats of hydrogenation of these compounds will be in the order as
a I > II > III
b III > II > I
c II > III > I
d II > I > III

100.

1
2
I

3
4
1

II

2
3
4

I
II
III
Among these compounds, the order of
heats of hydrogenation is
a I > II > III
b III > II > I
c II > I > III
d II > III > I

98.

I

II

III
Among these compounds, the order of
heats of hydrogenation is
a II > III > I
b III > II > I
c I > II > III
d II > I > III

1
2
III

3

4
Examine these canonical structures of
naphthalene. Now, which of the following
statements is true about C1-C2 and C2-C3
bond lengths?
a Both bonds are of same length
b C1-C2 bond is longer than C2-C3
bond
c C2-C3 bond is longer than C1-C2
bond
d It is not possible to judge the bond
length because given structures are
tautomers