Question Set A

CBSE 12th Standard Chemistry Haloalkanes and Haloarenes HOT Questions Study Material - QB365

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Questions:

Generic 2

5 x 2 = 10

How is t-butyl alcohol obtained from acetone?

Explain why is OH group in phenols more strongly held as compound to OH group in alcohols.

How will you distinguish between the following pairs by chemical reactions?
(i) CH₃OH and C₂H₅OH
(ii) Phenol and methanol
(iii) 1-Propanol and 2-methyl-2-propanol
(iv) Ethanol and 1-Propanol?

Explain why the following pairs of compound do not show optical activity.

a 1 : 1 mixture of I and II.

Which will react faster in S_N2displacement, 1-bromopentane or 2-bromopentane, and why?

Generic 3

5 x 3 = 15

Predict the products of the following reactions:
(i)

(ii)

(iii)

(iv)

Elimination of H₂O from carbonyl and amine group and formation of C= N bond.

(i) How can diethyl ether be prepared from the following?
(a) Ethyl iodide (b) Ethyl alcohol
Write the chemical equation in each case.
(ii) Howis ethyl alcohol obtained from molasses?

An alkyl halide X, of formula ${ C }_{ 6 }{ H }_{ 13 }Cl$ on treatment with potassium tertiary butoxide, gives the isomeric alkenes Y and Z $\left( { C }_{ 6 }{ H }_{ 12 } \right) $. Both alkenes on hydrogenation give 2, 3-dimethylbutane. Predict the structures of X, Y and Z.

Convert:
(i) Acetophenone to ethylbenzene
(ii) Ethanal to 2-aminoethanoic acid
(iii) Methyl chloride to ethanoic acid.

10.

Write structures of the products of the following reactions:
(i)

(ii)

(iii)

Generic 5

2 x 5 = 10

11.

An organic compound (A) with molecular formula C₆H₆O gives a characteristic colour with aqueous FeCl₃ solution. When (A) is treated with CO₂ and NaOH at 410K under pressure, it gives compound (B) which upon acidification gives compound (C). Compound (C) reacts with acetyl chloride to give (D) which is a popular pain killer, Deduce the structures of (A), (B), (C) and (D) and explain all the reactions involved.

12.

(a) Write chemical equations to illustrate the following name bearing reactions:
(i) Cannizzaro's reaction
(ii) Hell-Volhard-Zelinsky reaction
(b) Give chemical tests to distinguish between the following pairs of compounds
(i) Propanal and Propanone
(ii) Acetophenone and Benzophenone
(iii) Phenol and Benzoic acid

Multiple Choice Question

5 x 1 = 5

13.

Which of the following reagents converts C₆H₅COCHO to C₆H₅CHOHCOOH ?

(a)

Aq. NaOH

(b)

Acidic Na₂SO₃

(c)

Na₂CrO₄/H₂SO₄

(d)

NaNO₂/HCI

14.

Decreasing order reactivity in Williamson's ether synthesis of the following is:
I.Me₃CCH₂Br
II.CH₃CH₂CH₂Br
III.CH₂=CHCH₂Cl
IV.CH₃CH₂CH₂Cl.

(a)

III > II > IV > I

(b)

I > II > IV III

(c)

IIC > III > IV > I

(d)

I > III > II > IV

15.

Phenol, when it first reacts with concentrated sulphuric acid and then with concentrated nitric acid, gives

(a)

nitrobenzene

(b)

2,4,6-trinitrobenzene

(c)

o-nitrophenol

(d)

p-nitrophenol

16.

Which of the following reactions is an example of nucleophilic substitution reaction ?

(a)

2 RX + Na $\longrightarrow$ R^__R + 2 NaX

(b)

RX + H₂ $\longrightarrow$ RH + HX

(c)

RX + Mg $\longrightarrow$ RMgX

(d)

RX + KOH $\longrightarrow$ ROH + KX

17.

Which of the following are arranged in the decreasing order of dipole moment?

(a)

CH₃Cl, CH₃Br, CH₃F

(b)

CH₃Cl, CH₃F, CH₃Br

(c)

CH₃Br, CH₃Cl, CH₃F

(d)

CH₃Br, CH₃F, CH₃Cl

Fill up / 1 Marks

3 x 1 = 3

18.

Formic acid does not undergo HVZ reaction because it does not have ___________

19.

Pentan-2-one can be distinguished from pentan-3-one by __________ test.

20.

Esters on treatment with excess of Grignard reagents followed by acid hydrolysis gives -----------

Assertion and reason

2 x 1 = 2

21.

Assertion : SN² reaction of an optically active aryl halide with an aqueous solution of KOH always gives an alcohol with opposite sign of rotation.
Reason : SN² reactions always proceed with inversion of configuration.
Codes:
(a) If both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion.
(b) If both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.
(c) If the Assertion is correct but Reason is incorrect.
(d) If both the Assertion and Reason are incorrect.

22.

In the following questions. an Assertion (A) is followed by a corresponding Reason (R) Use the following keys to choose the appropriate answer.
Assertion (A) neo pentyl chloride is formed when neo pentyl alcohol reacts with HCl.
Reason (R) neo pentyl alcohol is a primary alcohol.
Codes:
(a) Both (A) and (R) are correct, (R) is the correct explanation of (A).
(b) Both (A) and (R) are correct, (R) is not the correct explanation of (A).
(c) (A) is correct; (R) is incorrect.
(d) (A) is incorrect; (R) is correct.

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Answers Key:

Generic 2

5 x 2 = 10

The OH group in phenol is more strongly held because of double bond character between C and O due to resonance.

1-Propanol will not react with I₂/NaOH to give iodoform.

It is racemic mixture which contains equal amounts of dexto and laborotatory substances. If half of the total number of molecules rotates the plane-polarised light towards left, remaining half of the molecules rotates towards right such that net optical rotation is zero.

1-Bromopentane will react faster in S_N2 because it is 1^o (primary) halide and has less stearic hidrance.

Generic 3

5 x 3 = 15

(i) Cyclopentanone reacts with hydroxyl amine to form oxime.

(ii) $\text { a } \beta$unsaturated aldehyde reacts with semicarbazide (H₂NCONHNH₂) to form semicarbazone.

(iii) Cyclohexanone reacts with 2,4-dinitro phenyl hydrazine to form 2,4-dinitro phenyl hydrazone.

(iv) Acetophenone reacts with ethyl amine to form an imine

$\begin{equation} \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{l}+\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} \longrightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3}+\mathrm{HI} \end{equation}$

It is given that
$\underset { (X) }{ { C }_{ 6 }{ C }_{ 3 }CI } \overset { { ({ C }H_{ 3 }) }_{ 3 }COK }{ \longrightarrow } \ { \underset { (Y)\ and\ (Z) }{ Twoisomeric\\ alkenes\ of\ \\ formula\ { C }_{ 6 }{ H }_{ 12 }\ } }\overset { { H }_{ 2 } }{ \longrightarrow } \underset { 2,3-Dimethylbutance }{ { C }H_{ 3 }-\overset { \underset { | }{ { C }H_{ 3 } } }{ CH } -\overset { \underset { | }{ { C }H_{ 3 } } }{ CH } -{ C }H_{ 3 } } $
The two alkenes forming 2, 3-dimethyl butane are:
$\underset { (Y) }{ { C }H_{ 3 }-\overset { \underset { | }{ { C }H_{ 3 } } }{ C } =\overset { \underset { | }{ { C }H_{ 3 } } }{ C } -{ C }H_{ 3 } } \quad and\quad \underset { (Z) }{ { C }H_{ 2 }=\overset { \underset { | }{ { C }H_{ 3 } } }{ C } -\overset { \underset { | }{ { C }H_{ 3 } } }{ CH } -{ C }H_{ 3 } } $

10.

Generic 5

2 x 5 = 10

11.

(i) Since compound (A) with M.F. C₆H₆O gives characteristic colour with FeCI₃, therefore, (A) must be phenol.
(ii) Since compound (A), i.e., phenol reacts with CO₂ and NaOR under pressure (i.e., Kolbe's reaction) to give compound (B) which on acidification gives compound (C), therefore, (B) must be sodium salicylate and (C) must be salicylic acid.
(iii) Since (C) reacts with acetyl chloride to form a popular pain killer (D), therefore, (D) must be aspirin.

12.

(a) (i) Clemmensen Reduction
The carbonyl group of aldehydes and ketones is reduced to the CH₂ group on treatment with zinc amalgam and concentrated hydrochloric acid. This is known as Clemmensen reduction.

(ii) Hell-Volhard-Zelinsky (HVZ )reaction.
Carboxylic acids having a $straight alpha$ hydrogen are halogenated at the $straight alpha$ position on treatment with chlorine or bromine in the presence of small amount of red phosphorus to give $straight alpha$ halocarboxylic acids. The reaction is known as Hell-Volhard-Zelinsky reaction.

(b) (i) Propanal (CH₃CH₂CHO) can be distinguished from propanone (CH₃COCH₃) by iodoform test.
Being a methyl ketone, propanone on treatment with I₂/NaOHundergoes iodoform reaction to give a yellow ppt. of iodoform.
(ii) (ii) Benzaldehyde (C₆H₅CHO) and acetophenone (C₆H₅COCH₃) can be distinguished by iodoform test.
(iii) Acetophenone, being a methyl ketone on treatment with I₂ /NaOH undergoes iodoform reaction to give a yellow ppt. of iodoform. On the other hand, benzaldehyde does not give this test.

Multiple Choice Question

5 x 1 = 5

13.

(a)
Aq. NaOH

14.

15.

16.

(d)
RX + KOH $\longrightarrow$ ROH + KX

17.

(b)
CH₃Cl, CH₃F, CH₃Br

Fill up / 1 Marks

3 x 1 = 3

18.

()
$\alpha$-hydrogen

19.

()
Iodoform

20.

()
sec- or tert-alcohols

Assertion and reason

2 x 1 = 2

21.

(d) If both the Assertion and Reason are incorrect.
Explanation:
Assertion is false, because aryl halides do not undergo nucleophilic substitution under ordinary conditions. This is due to resonance, because of which the carbon– chlorine bond acquires partial double bond character, hence it becomes shorter and stronger and thus cannot be replaced by nucleophiles. However Reason is true.

22.

(d) 2- chloro -2 methylbutane (not neo pentyl chloride) is formed when neo pentyl alcohol reacts with HCI. The initially formed primary carbocation undergoes rearrangement to form more stable tertiary carbocation. The reaction is given as follows

Therefore, (A) is incorrect but (R) is correct statement.