Which of the given alkylhalide would undergo racemisation in \(S_N1\) reaction?

\(C_6H_5CH2Cl\)

The correct answer is option 4. \(C_6H_5CH2Cl\).

We already know that the \(S_N1\) reaction - A Nucleophilic Substitution in which the Rate Determining Step involves 1 component. \(S_N1\) reactions are unimolecular, proceeding through an intermediate carbocation. \(S_N1\) reactions give racemization of stereochemistry at the reaction centre.

All the molecules undergoing \(S_N1\) reaction show racemisation as there is formation of 2 types of product in equal amounts.

The reaction occurs only if the products formed are stable. The Reaction Rate Increases with Substitution at Carbon. Out of all these 4 alkyl halides, \(C_6H_5CH_2Cl\) where, benzyl chloride has the strongest tendency to undergo \(S_N​1\) here thus leading to formation of racemic mixture as it contains benzylic carbocation which is most stable.

So, the correct answer is option 4. \(C_6H_5CH2Cl\).