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Which one is most reactive towards $S_N1$ reaction?
A
$C_6H_5CH_2Br$
B
$C_6H_5CH(C_6H_5)Br$
C
$C_6H_5CH(CH_3)Br$
D
$C_6H_5C(CH_3)(C_6H_5)Br$
Detailed Solution
In an $S_N1$ reaction the rate-determining step is the ionisation of the C–Br bond to form a carbocation; the more stable the carbocation, the faster the reaction.
$C_6H_5CH_2Br$ gives $C_6H_5CH_2^+$: a primary benzylic cation stabilised by one phenyl group.
$C_6H_5CH(CH_3)Br$ gives $C_6H_5\overset{+}{C}H(CH_3)$: a secondary benzylic cation (one phenyl + one methyl).
$C_6H_5CH(C_6H_5)Br$ gives $(C_6H_5)_2CH^+$: a secondary cation stabilised by resonance with two phenyl groups.
$C_6H_5C(CH_3)(C_6H_5)Br$ gives $(C_6H_5)_2\overset{+}{C}(CH_3)$: a tertiary cation stabilised by resonance with two phenyl groups and by the +I and hyperconjugation effects of the methyl group. It is the most stable.
Hence $C_6H_5C(CH_3)(C_6H_5)Br$ is the most reactive towards $S_N1$.
$C_6H_5CH_2Br$ gives $C_6H_5CH_2^+$: a primary benzylic cation stabilised by one phenyl group.
$C_6H_5CH(CH_3)Br$ gives $C_6H_5\overset{+}{C}H(CH_3)$: a secondary benzylic cation (one phenyl + one methyl).
$C_6H_5CH(C_6H_5)Br$ gives $(C_6H_5)_2CH^+$: a secondary cation stabilised by resonance with two phenyl groups.
$C_6H_5C(CH_3)(C_6H_5)Br$ gives $(C_6H_5)_2\overset{+}{C}(CH_3)$: a tertiary cation stabilised by resonance with two phenyl groups and by the +I and hyperconjugation effects of the methyl group. It is the most stable.
Hence $C_6H_5C(CH_3)(C_6H_5)Br$ is the most reactive towards $S_N1$.