JEE Main 2025 Jan 28 Shift 2, Chemistry Q9: Preparation of Alkyl Halides (Halo Alkanes) & Aryl Halides (Halo Arenes)

Step 1 (HCl addition): Addition of HCl to the styryl alkene follows Markovnikov regioselectivity. The benzylic carbocation is stabilised by resonance with the aromatic ring, so Cl ends up on the benzylic carbon. The major product (A) is the benzylic chloride $p{\text{-CH}}_3{\text{-C}}_6{\text{H}}_4{\text{-CHCl-CH}}_2{\text{-CH}}_3$.

Step 2 (AgCN treatment): Silver cyanide is a predominantly covalent (soft) reagent. With AgCN, nitrogen is the more nucleophilic site, so the product is the isocyanide (not the nitrile). Cl is displaced and replaced by $-NC$.

$\therefore$ B is the benzylic isocyanide $p{\text{-CH}}_3{\text{-C}}_6{\text{H}}_4{\text{-CH(NC)-CH}}_2{\text{-CH}}_3$.