Haloalkanes and Haloarenes — Page 2 | NEET Chemistry

Q11

Which one is most reactive towards S N 1 reaction?

A C 6 H 5 CH(C 6 H 5 )Br

B C 6 H 5 CH(CH 3 )Br

C C 6 H 5 C(CH 3 )(C 6 H 5 )Br

D C 6 H 5 CH 2 Br

Correct Answer

Option C

Solution

S N 1 reactions involve the formation of carbocations, hence higher the stability of intermediate carbocation, more will be reactivity of the parent alkyl halide.

The tertiary carbocation formed from (c) is stabilized by two phenyl groups and one methyl group, hence most stable.

Q12

The correct order of increasing reactivity of C

-

X bond towards nucleophile in the following compounds is

A I < II < IV < III

B II < III < I < IV

C IV < III < I < II

D III < II < I < IV

Correct Answer

Option A

Solution

The order of reactivity is dependent on the stability of the intermediate carbocation formed by cleavage of C—X bond.

The 3° carbocation formed in III will be more stable than its 2° homologue (IV) which in turn will be more stable than arenium ion (I).

Moreover, the aryl halide has a double bond character in the C—X bond which makes the cleavage more difficult.

On other hand II will be more reactive than I due to presence of electron withdrawing —NO 2 group.

C—X bond becomes weak and undergoes nucleophilic substitution reaction So Correct order is I < II < IV < III.

Q13

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

A 2RX + 2Na

\to

R

-

R + 2NaX

B RX + H 2

\to

RH + HX

C RX + Mg

\to

RMgX

D RX + KOH

\to

ROH + KX

Correct Answer

Option D

Solution

In nucleophilic substitution, a nucleophile provides an electron pair to the substrate and the leaving group departs with an electron pair.

Q14

In a S N 2 substitution reaction of the type R

-

Br + Cl

-

\overset{{DMF}}\longrightarrow

R

-

Cl + Br

-

which one of the following has the highest relative rate?

A

B CH 3 CH 2 Br

C CH 3 CH 2 CH 2 Br

D

Correct Answer

Option B

Solution

S N 2 mechanism is followed in case of primary and secondary halides i.e., S N 2 reaction is favoured by small groups on the carbon atom attached to halogens so CH 3 – X > R – CH 2 – X > R 2 CH – X > R 3 C – X

Q15

Which of the following is not chiral?

A 2-Hydroxypropanoic acid

B 2-Butanol

C 2,3-Dibromopentane

D 3-Bromopentane

Correct Answer

Option D

Solution

Due to absence of asymmetric carbon atom.

Q16

Which of the following undergoes nucleophilic substitution exclusively by S N 1 mechanism?

A Ethyl chloride

B Isopropyl chloride

C Chlorobenzene

D Benzyl chloride

Correct Answer

Option D

Solution

S N 1 reaction is favoured by heavy groups on the carbon atom attached to halogen i.e Benzyl > allyl > tertiary > primary > secondary > primary > alkyl halides

Q17

Which of the following is least reactive in a nucleophilic sibstitution reaction?

A (CH 3 ) 3 C

-

Cl

B CH 2

=

CHCl

C CH 3 CH 2 Cl

D CH 2

=

CHCH 2 Cl

Correct Answer

Option B

Solution

Chlorine of vinyl chloride (CH 2 ==CHCl) is non-reactive substitution reaction because it shows the following resonating structure due to +M effect of –Cl atom.

In second Cl–atom has positive charge and partial double bond character with C of vinyl group, so it is more tightly attracted towards the nucleus and it does not get replaced by nucleophile in nucleophilic substitution reaction.

Q18

Reactivity order of halides for dehydrologenation is

A

R - F > R - Cl > R - Br > R - {\rm I}

B

R - {\rm I} > R - Br > R - Cl > R - F

C

R - {\rm I} > R - Cl > R - Br > R - F

D

R - F > R - {\rm I} > R - Br > R - Cl

Correct Answer

Option B

Solution

atomic radii order : I > Br > Cl > F F, Cl, Br, I belongs to the same group orderly.

Atomic radii go on increasing as the nuclear charge increases from top to bottom in a group.

The decreasing order of bond length C – I > C – Br > C – Cl > C – F.

The order of bond dissociation energy R – F > R – Cl > R – Br > R – I.

During dehydrohalogenation C – I bond breaks more easily than C – F bond.So reactivity order of halides

R - {\rm I} > R - Br > R - Cl > R - F

Q19

obtained by chlorination of n-butane will be

A meso form

B racemic mixture

C d-form

D

l

-form

Correct Answer

Option B

Solution

Chlorination of n-butane takes place via free radical formation.

Q20

An organic compound A(C 4 H 9 Cl) on reaction with Na/diethyl ether gives a hydrocarbon which on monochlorination gives only one chloro derivative then, A is

A t-butyl chloride

B s-butyl chloride

C iso-butyl chloride

D n-butyl chloride

Correct Answer

Option A

Solution

2C 4 H 9 Cl+2Na

\overset{{Ethanol}}\longrightarrow

C 4 H 9 - C 4 H 9 + 2NaCl In C 4 H 9 - C 4 H 9 i.e., (H 3 C) 3 C—C(CH 3 ) 3 there are primary hydrogens only.

Thus, only one chloro derivative is possible.