Reaction of C_6H_5CH_2Br with aqueous sodium hydroxide follows.

The correct answer is Option (1) → \(S_N1\) mechanism

An \( S_N1 \) reaction goes through a carbocation, so it is fastest for halides whose carbocation is most stable; an \( S_N2 \) reaction is a one-step backside attack that is fastest for unhindered (methyl and primary) halides. Benzyl bromide is a primary halide, but the benzyl carbocation is stabilised by resonance: the positive charge is delocalised over the ortho and para carbons of the ring. So benzylic halides show high reactivity towards the \( S_N1 \) reaction, and water, a polar protic solvent, helps the ionisation.

The C–Br bond first ionises slowly, and the carbocation is then captured by the hydroxide ion:

    \( \displaystyle \ce{C6H5CH2Br -> C6H5CH2+ + Br-}\quad (\text{slow}) \)

    \( \displaystyle \ce{C6H5CH2+ + OH- -> C6H5CH2OH}\quad (\text{fast}) \)

    \( \displaystyle \ce{C6H5CH2Br + NaOH(aq) -> C6H5CH2OH + NaBr} \)

So the reaction follows the \( S_N1 \) mechanism.

Why the other options are incorrect

• Option (2) \(S_N2\) mechanism — Incorrect. \( S_N2 \) is typical of primary alkyl halides whose carbocation would be unstable, such as 1-bromobutane; for benzyl bromide the resonance-stabilised carbocation opens the faster ionisation route.

• Option (3) Mechanism is dependent on temperature of the reaction — Incorrect. The mechanism is decided by the structure of the halide (stability of the carbocation) and the solvent, not by the temperature.

• Option (4) Depends on concentration of aqueous sodium hydroxide — Incorrect. In the \( S_N1 \) route the hydroxide ion enters only after the slow step, so the rate does not depend on its concentration: rate = k[\( \ce{C6H5CH2Br} \)].

NCERT (2026-27 edition):
Substitution Nucleophilic Unimolecular (S_N1) – “… Since the rate of reaction depends upon the slowest step, the rate of reaction depends only on the concentration of alkyl halide and not on the concentration of hydroxide ion. Further, greater the stability of carbocation, greater will be its ease of formation from alkyl halide and faster will be the rate of reaction. … For the same reasons, allylic and benzylic halides show high reactivity towards the S_N1 reaction.”

Note: The same/similar question has been asked in different years by NTA as per the details given below: