Question

Which one of the following halogen compounds is difficult to be hydrolyzed by S_N1 mechanism?

• A. Tertiary butyl chloride
• B. Isopropyl chloride
• C. Benzyl chloride
• D. Chlorobenzene
• E. Allyl chloride

Answer 1

The correct option is D

Chlorobenzene

Explanation for correct option

(D) Chlorobenzene

S_N1 mechanism

• S_N1 stands for Substitution nucleophilic unimolecular reaction.
• S_N1 reactions are generally carried out in polar protic solvents such as water, and alcohol.
• In this reaction, the rate of reaction depends upon the concentration of only one reactant.
• This reaction occurs in two steps.
• In the first step formation of carbocations takes place.
• The carbocation thus formed is then attacked by the nucleophile in step 2 to complete the reaction.

Chlorobenzene

• Since electrons are delocalized by resonance, the $\mathrm{C}-\mathrm{Cl}$ bond in chlorobenzene is highly stable.
• The resonating structure is as follows:

• Furthermore, the $\mathrm{C}-\mathrm{Cl}$ bond has a double bond character, making hydrolysis through the S_N1 process extremely difficult.

The explanation for incorrect options

• The halogen compound is difficult to be hydrolyzed by S_N1 mechanism is Chlorobenzene.
• Hence all other options (A), (B), (C), and (E) are incorrect.

Therefore, the correct option is (D) Chlorobenzene.