Question: 68. The compound that will undergo \mathrm{S}_{\mathrm{N}}{ }^{1} reaction with the fastest rate is :
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Answer: Option (4)
Explanation:
The rate of an \mathrm{S}_{\mathrm{N}}{ }^{1} reaction depends on the stability of the carbocation formed after the leaving group departs.
In option (1), the bromine is attached to a primary alkyl carbon,
so the carbocation formed is primary and very unstable.
In option (2), the bromine is attached to a cyclohexyl carbon,
which forms a secondary carbocation that is moderately stable.
In option (3), bromobenzene does not undergo \mathrm{S}_{\mathrm{N}}{ }^{1} reaction because the C–Br bond has partial double bond character due to resonance with the benzene ring, preventing carbocation formation.
In option (4), the bromine is attached to a benzylic carbon that forms a secondary benzylic carbocation, which is highly stabilized by resonance with the benzene ring.
Due to maximum carbocation stability, option (4) undergoes the \mathrm{S}_{\mathrm{N}}{ }^{1} reaction at the fastest rate.