Which of the following undergoes nucleophilic substitution exclusively by $\mathrm{S}_{\mathrm{N}} 1$…
Which of the following undergoes nucleophilic substitution exclusively by $\mathrm{S}_{\mathrm{N}} 1$ mechanism?
Ethyl chloride
Isopropyl chloride
Chlorobenzene
Benzyl chloride
Solution
More stable the carbocation, more the tendency to undergo $\mathrm{S}_{\mathrm{N}} 1$ reaction. Thus, order of carbocation to undergo $\mathrm{S}_{\mathrm{N}} 1$ reaction is:
Benzyl $>$ Allyl $ > 3^{\circ} > 2^{\circ} > 1^{\circ} > -\mathrm{CH}_3$
Related Theory
The rate of reaction depends on the concentration of the substrate in $S_N$ l reaction While In $S_N$, the rate of reaction depends on the concentration of both the substrate and the nucleophile.
! Caution
Benzyl carbocation is stabilised by resonance so it will undergo $S_N$ I reaction.