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Halkoarenes & Haloarenes NEET Questions — Free MCQ Practice

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The compound that will react most readily with gaseous bromine has the formula ?

A.

C₃H₆

B.

C₂H₂

C.

C₄H₁₀

D.

C₂H₄

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Which one is the most reactive toward S_N1 reaction?

A.

C₆H₅CH(C₆H₅)Br

B.

C₆H₅CH(CH₃)Br

C.

C₆H₅C(CH₃)(C₆H₅)Br

D.

C₆H₅CH₂Br

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Explanation

Key Idea S_N1 reaction involves the formation of carbocation intermediate. More the stability of carbocation, more is the reactivity of alkyl/aryl halides towards S_N1 reaction.

The intermediate carbocations formed by given halides are as:

(a) C₆H₅CH(C₆H₅)Br $\to (C_{6} H_{5})_{2} \overset{+}{C} H$ + Br^-

(b) C₆H₅CH(CH₃)Br $\to C_{6} H_{5} \overset{+}{C} H ({CH}_{3})$ + Br^-

(c) C₆H₅C(CH₃)(C₆H₅)Br $\to (C_{6} H_{5})_{2} \overset{+}{C} ({CH}_{3})$ + Br^-

(d) C₆H₅CH₂Br $\to C_{6} H_{5} \overset{+}{C} H_{2}$ + Br^-

The order of stability of these carbocations is as

$(C_{6} H_{5})_{2} \overset{+}{C} ({CH}_{3}) > (C_{6} H_{5})_{2} \overset{+}{C} H > C_{6} H_{5} \overset{+}{C} H ({CH}_{3}) > C_{6} H_{5} \overset{+}{C} H_{2}$

Thus, C₆H₅(CH₃)(C₆H₅)Br is most reactive towards S_N1 reaction

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Reactivity order of halides for dehydrohalogenation is

A.

R-F>R-Cl>R-Br>R-I

B.

R-I>R-Br>R-Cl>R-F

C.

R-I>R-Cl>R-Br>R-F

D.

R-F>R-I>R-Br>R-Cl

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Explanation

(b) F,Cl,Br and I are the elements of VII A group. In a group atomic radii increases from top to bottom and the bond dissociation energy decreases as

R-F>R-Cl>R-Br>R-I

So, during dehydrohalogenation R-I bond breaks more easily than R-I bond. Hence, order of reactivity will be

R-I>R-Br>R-Cl>R-F

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(CH₃)₃CCI + (CH₃)₃CO – K⁺ → Product

A.

S_N Product will be more

B.

E₂ Product will be more

C.

both will be the same

D.

None of these

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Explanation

Sterically hindered base leads to elimination products.

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In E₂ elimination, some compounds follow Hofmann's rule which means:

A.

the double bond goes to the most substituted carbon

B.

the compound is resistant to elimination

C.

no double bond is formed

D.

the double bond goes mainly towards the least substituted carbon

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Explanation

(d) Follow elimination rules.

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The hydrolysis of alkyl halides by aqueous NaOH is best termed as:

A.

electrophilic substitution reaction

B.

electrophilic addition reaction

C.

nucleophilic addition reaction

D.

nucleophilic substitution reaction

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Explanation

$(d) R - X \overset{NaOH}{\to} R - OH + NaX R - X \overset{{OH}^{-}}{\to} R - OH + X^{-} this is a nucleophilic substitution reaction$

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S_N1 reaction on optically active substrates mainly gives:

A.

retention in configuration

B.

inversion in configuration

C.

racemic product

D.

no product

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Explanation

(c) S_N1 mechanism gives rise to 50% inversion as it involves front seat as well as back seat substitution. this leads to racemic products

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Amongst the following which of the above are true for S_N2 reaction?

(i) The rate of reaction is independent of the concentration of the nucleophile

(ii) the nucleophile attacks the carbon atom on the side of the molecule opposite to the group being displaced.

(iii) The reaction proceeds with simultaneous bond formation and bond rupture.

A.

(i), (ii)

B.

(i),(iii)

C.

(i),(ii),(iii)

D.

(ii),(iii)

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Explanation

(d) Follow characteristics of S_N2 mechanism

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The S_N1 mechanism for substitution reaction by nucleophile is favoured by:

A.

low concentration of nucleophile

B.

weak nature of nucleophile

C.

polar solvent

D.

all of the above

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Explanation

(d) These are characteristics of S_N1 mechanism.

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Which one is the elimination reaction?

A.

CH₃CH₃+Cl₂ $\to$ CH₃CH₂Cl+ HCl

B.

CH₃Cl+ KOH(aq.) $\to$ CH₃OH +KCl

C.

CH₂=CH₂ +Br $\to$ CH₂BrCH₂Br

D.

C₂H₅Br+ KOH(alc.) $\to$ C₂H₄ + KBr+H₂O

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Explanation

(d) Elimination reactions involves removal of a molecule (HBr here) from a substrate.

Halkoarenes & Haloarenes MCQs for NEET — Chemistry Questions with Answers

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