resonance (q.b.) 13th.pmd

8/17/2019 Resonance (Q.B.) 13th.pmd

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QUESTION BANK

RESONANCE

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3/12

Q.6 For the following compounds, arrange the labelled proton in increasing order of their ease of deprotonation.

Q.7 Which is stronger acid, A or B and why?

(A) (B)

Q.8 Discuss the following observations:

(a) C–Cl bond in vinyl chloride is stronger than in chloroethane.

(b) Carbon-carbon bond length in ethene is shorter than in CH2 = CHOCH

3

(c) CH3SH is stronger acid than CH

3OH

(d) CH3CH

2NH

2 is stronger base than CH

2 = CHNH

2.

Q.9 Discuss the basic strenght of two nitrogens in benzimidazole.

Benzimidazole

Q.10 In the following structure, which is better site of protonation and why-oxygen or nitrogen?

H

Q.11 Compare the C–N bond-length in the following species:

(i) (ii) (iii)

Q.12 Rank the following in increasing order of basic strength, explaining reason for your choice:

(I) (II) (III)


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4/12

Q.13 Answer the following:

(i) Which of the indicated H is abstracted rapidly by bromine radical and why?

(ii) One of the indicated proton Ha or Hb, is approximately 1030

times more acidic than other, which is moreacidic and why?

Q.14 In each of the following pairs of ions which ion is more stable:

(a) (I) C6H

5 – 2CH

and (II) CH

2=CH– 2CH

(b) (I) CH3 – 2CH

and (II) CH

2 =

CH

(c) (I) and

(d) (I)

33

33

CHCCH|

CHCHCH and

33

33

CHCCH|

CHNCH

Q.15 Consider the given reaction:

+ 3H2

C / Pd

In the above reaction which one of the given ring will undergo reduction?

Q.16 Compare heat of hydrogenation (Decreasing order)

(a) heat of hydrogenation

(i)

(ii)


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(b) and (c) and

(d) and

(e) CH2 = CH – CH and CH

2 = C

Q.17 Which of the following statements is (are) true about resonance.

(a) Resonance is an intramolecular process.

(b) Resonance involves delocalization of both and electrons.(c) Resonance involves delocalization of electrons only.(d) Resonance decreases potential energy of a molecule.

(e) Resonance has no effect on the potential energy of a molecule.(f) Resonance is the only way to increase molecular stability.

(g) Resonance is not the only way to increase molecular stability.

(h) Any resonating molecule is always more stable than any nonresonating molecule.

(i) The canonical structure explains all features of a molecule.

(j) The resonance hybrid explains all features of a molecule.

(k) Resonating structures are real and resonance hybrid is imaginary.

(l) Resonance hybrid is real and resonating structures are imaginary.

(m) Resonance hybrid is always more stable than all canonical structures.

Q.18 Resonance energy will be more if

(a) canonical structures are equivalent than if canonical structures are non-equivalent.(b) molecule is aromatic than if molecule is not aromatic.

Q.19 A canonical structure will be more stable if

(a) it has more number of bonds than if it has less number of bonds.(b) the octets of all atoms are complete than if octets of all atoms are not complete.

(c) it involves cyclic delocalization of (4n + 2) – electrons than if it involves acyclic delocalization of (4n + 2) – electrons.

(d) it involves cyclic delocalization (4n) – electrons than if it involves acyclic delocalizationof (4n) – electrons.

(e) +ve charge is on more electronegative atom than if +ve charge is on less electronegative atoms.

(f) –ve charge is on more electronegative atom than if –ve

Q.20 In which of the following molecules resonance takes place through out the entire system.

(a) (b) (c) (d)

(e)

3

3

COOCH

|COOCH

(f)


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6/12

Q.21 Which of the following groups cannot participate in resonance with other suitable group :

(a) – COOH (b) — OCO (c) – COCl (d)

3NH

(e)2HC

(f) 2HC

(g)

3

2

CH|NCH

Q.22 Which of the following group can participate in resonance with other suitable group :

(a) OCH2 (b) 2

—

2 HCCH (c) 22 HCCH

(d)

(e) (f) – BH2

(g)3PhP

Q.23 In which of the following lone-pair indicated is involved in resonance :

(a) (b) (c) (d)

(e) CH2= CH –

2HC (f) CH

2 = CH – CH = HN

Q.24 In which of the following lone-pair indicated is not involved in resonance :

(a) CH2 = CH – HN

– CH

3(b) CH

2 = CH – CH =

O

(c) CH2 = CH –

O – CH = CH2 (d) CH2 = CH – C

N

(e) (f)

Q.25 Identify electron – donating groups in resonance among the following :

(a) – CONH2

(b) – NO2

(c) – OCOCH3

(d) – COOCH3

(e) – CHO (f) – NHCOCH3

Q.26 Identify electron – withdrawing groups in resonance among the following :

(a) – COOH (b) – CONHCH3

(c) – COCl (d) – CN

(e) – O – CH = CH2

(f)

Q.27 Which of the following groups can either donate or withdraw a pair of electrons in resonance depending

upon situation :

(a) – NO2

(b) – NO (c) – CH = CH (d) – CHO

(e) – NH2

(f) – N = NH

Q.28 Which of the following groups can only withdraw a pair of electrons in resonance depending upon

situation :

(a) – Ph (b) (c) (d)

(e)3MeN

(f) – CONH2


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7/12

Q.29 Write the resonance hybrid of each of the following :

(a)2

—

2 HCCHCH (b) (c) (d) 22 HCCHCH

(e) CH2 = CH – CH

2

Q.30 Write the canonical structures of each of the following :

(a) R – CO – CH = CH2

(b) CH3O – CH = CH – 3MeN

(c) RCOCl (d) HCONH2

(e)

Q.31 Write the resonance hybrid of each of the following :

(a) (b) CH2 = CH – CH = O (c) HCCCH

—

2 (d)

(e) CH2 = CH – CH = CH

2

Q.32 Write the canonical structures of each of the following :

(a) (b) —

2 NNCH

(c) CH2 = C = O (d) (e)

Q.33 In which of the following molecules – electron density in ring is minimum :

(A) (B) (C) (D)

Q.34 In which of the following molecules – electron density in ring is maximum :

(A) (B) (C) (D)

Q.35 CH2 = CH – CH = CH – CH

3 is more stable than CH

3 – CH = C = CH – CH

3 because

(I) (II)

(A) there is resonance in I but not in II (B) there is tautomerism in I but not in II

(C) there is hyperconjugation in I but not in II (D) II has more cononical structures than I.

Q.36 Which of the following pairs has higher resonance energy :

(a) CH3COOH and CH

3COONa (b) CH

2 = CH –

O and CH2 = CH – OH

(c) and (d) and

(e) and CH2 = CH – CH = CH – CH = CH

2


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8/12

Q.37 Which of the following pairs has less resonance energy :

( a )

23CO and HCOO

– (b) and CH2 = CH –

2CH

(c) and CH2 = CH – CH = CH

2(d) and CH

2 = CH –

2HC

(e) and


(a) and (b) and

(c) and

(d) CH2 = CH – OH and CH

2 = CH – CH = CH – OH

(e) and

Q.39 Which of the following pairs has less resonance energy :

(a) and (b) and (c) and

(d) and (e) and


(a) and

(b) CH2 = CH – O – CH = CH

2 and CH

2 = CH – NH – CH = CH

2

(c) HNCHCH2

and HNCHHN

(d) CH2 = CH – F and CH2 = CH – Br

(e) and CH2 = CH – 2HC

Q.41

These are three canonical structures of naphthalene. Examine them and find correct statement among the

following :

(A) All C – C bonds are of same length (B) CI – C2 bond is shorter than C2 – C3 bond.(C) C1 – C2 bond is longer than C2 – C3 bond (D) none.


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9/12

Q.42 Which of the following is (are) the correct order of bond lengths :

(A) C – C > C = C > C C > C N (B) C = N > C = O > C = C(C) C = C > C = N > C = O (D) C – C > C = C > C C > C – H

Q . 4 3 I d e n t i f y m o r e s t a b l e c a n o n i c a l s t r u c t u r e i n e a c h o f t h e f o l l o w i n g p a i r s :

( a ) (b) HCCCHCHCHC22

(c) (d)

(e) OCHCHCHOCHCHHC 22

(f)

Q.44 Identify less stable canonical structure in each of the following pairs :

(a)3232 CHOCHCHOHC

(b)

(c) (d)

(e)

Q.45 In which of the following pairs, indicated bond is of greater strength :

(a) BrCHCH 23 and

ClCHCH 23

(b) BrCHCHCH3 and

Br|

CHCHCH 33

(c) and ClCHCH 23

(d)

22 CHCHCHCH and 3222 CHCHCHCH

(e)

22 CHCHCHCH and 22 NOCHCH

(f) and


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10/12

Q.46 In which of the following pairs, indicated bond having less bond dissociation energy :

(a) and 22 CHCH (b)

CHCCH3 and

CHHC

(c) and

(d) and (e) and

(f) and

Q.47 Which of the following has longest C – O bond :

(A) (B) (C) (D)

Q.48 CH2 = NH

I II III IV

Among these compounds, the correct order of C – N bond lengths is :

(A) IV > I > II > III (B) III > I > II > IV (C) III > II > I > IV (D) III > I > IV > II

Q.49 C1 – C2 bond is shortest in

(A) (B) (C) (D)

Q.50 Among the following molecules, the correct order of C – C bond lenght is

(A) C2H

6 > C

2H

4 > C

6H

6 > C

2H

2(B) C

2H

6 > C

6H

6 > C

2H

4 > C

2H

2(C

6H

6 is benzene)

(C) C2H

4 > C

2H

6 > C

2H

2 > C

6H

6(D) C

2H

6 > C

2H

4 > C

2H

2 > C

6H

6

Q.51 CH3O – CH = CH – NO

2I

CH2 = CH – NO

2II

CH2 = CH – Cl III

CH2 = CH

2IV

Which of the following is the correct order of C – C bond lengths among these compounds :

(A) I > II > III > IV (B) IV > III > II > I (C) I > III > II > IV (D) II > III > I > IV

Q.52 In which of the following molecules resonance is equivalent :

( A ) H C O O

(B) CH2 = CH – CH = CH

2

(C) (D)


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11/12

Q.53 CH2 = CH – CH = CH – CH = CH

2I

CH2 = CH – HC

– CH = CH – 2HC

II

2HC

– CH = CH – CH = CH – 2HC

III

Among these three canonical structures (through 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

Q.54 For 1-methoxy-1, 3-butadiene, which of the following resonating structure is the least stable?

(A)CH

2 — HC

–CH = CH – O – CH

3(B)

CH

2 — CH = CH – CH =

O – CH

3

(C) H2C = CH – HC

– HC

– O – CH

3(D) H

2C = CH – HC

– CH =

O – CH

3

Q.55 Among the following pairs identify the one which gives higher heat of hydrogenation :

(a) and (b) and

(c) CH3 – CH = CH – CH

3 and CH

3 – CH

2 – CH = CH

2

(d) and

Q.56 Which of the following statements would be true about this compound :

(A) All three C – N bonds are of same length.

(B) Cl – N and C3 – N bonds are of same length but shorter than C5 – N bond.

(C) Cl – N and C3 – N bonds are of same length but longer than C5 – N bond.

(D) Cl – N and C3 – N bonds are of different length but both are longer than C5 – N bond.

Q.57 Write resonating structures of complex formed when an electrophile (E) attacks on (i) and (ii) position of naphthalene. Also state which is more stable.

Q.58 Among the following pairs identify the one which gives higher heat of hydrogenation :

(a) and (b) and

(c) CH3 – CH = CH – CH

3 and CH

3 – CH

2 – CH = CH

2

(d) and


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12/12

Q.59 Number the following compounds in order of increasing acidity of indicated proton giving mechanistic

reasoning:

I II III IV

Q.60 From the following pair, select the stronger acid providing clear reasoning:

(a) O2N COOH or COOH

(b) or (c) or

ANSWER KEY

Q.1 a = Resonance form, b= A, c = C, d= A & B, e=B&C, f = B, g = B, h = B

Q.2 (a) are resonance form Q.3 (a) 4658, (b) 4638, (c) 4632, (d) 4656, (e) 5293

Q.4 (a) i , (b) i , (c) ii, (d) i, (e) ii Q.5 (a) ii < iv < i < iii, (b) iii

resonance (q.b.) 13th.pmd

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