SN1 vs SN2 mechanisms in nucleophilic substitution

Steps:

• SN2 is a concerted bimolecular process with a single transition state involving the nucleophile and substrate.
• SN1 is a two-step unimolecular process starting with slow formation of a carbocation intermediate.
• Evaluate options: A claims secondary always SN2 (false, can be both); B misstates SN1 initial step (not bimolecular); C assigns SN1 to primary chloroethane (incorrect, primaries favor SN2); D describes SN2 transition state accurately.
• Select D as it matches SN2 definition.

Why D is correct:

• In SN2, the transition state is a pentacoordinate species with partial bonds to both the incoming cyanide nucleophile (CN⁻) and departing halide, resulting in a short-lived negative charge.

Why the others are wrong:

• A: Secondary halogenoalkanes can react via both SN1 and SN2, depending on solvent and conditions.
• B: SN1 initial step is unimolecular, involving only the halogenoalkane dissociating to form a carbocation; cyanide acts later.
• C: Primary halogenoalkanes like chloroethane favor SN2 due to low steric hindrance and no stable carbocation.

Final answer: D