Question

Which of the following is least reactive towards nucleophilic substitution (\( S_N1 \))?

• A. Benzyl chloride
• B. Methyl chloride
• C. Chlorobenzene
• D. Allyl chloride

Hint:

Aryl halides and vinyl halides are generally very unreactive toward nucleophilic substitution because the lone pair on the halogen delocalizes into the \( \pi \) system, strengthening the bond.

Answer 1

The Correct Option is C

Concept: \( S_N1 \) reactivity depends on the stability of the carbocation intermediate formed after the leaving group departs. 
Step 1: Evaluate carbocation stability for each option.

Benzyl chloride: Forms a benzyl carbocation (\( C_6H_5CH_2+ \)), which is highly resonance-stabilized.

Allyl chloride: Forms an allyl carbocation (\( CH_2=CH-CH_2+ \)), which is resonance-stabilized. Methyl chloride: Forms a methyl carbocation (\( CH_3+ \)), which is unstable but can still undergo substitution (usually via \( S_N2 \)).

Chlorobenzene: The \( C-Cl \) bond has partial double bond character due to resonance, and the phenyl carbocation (\( C_6H_5+ \)) is extremely unstable. 

Step 2: Identify the least reactive species.
Chlorobenzene is inert to nucleophilic substitution under ordinary conditions because the \( sp2 \) hybridized carbon holds the chlorine more tightly, and the resulting phenyl cation is not stabilized by resonance.