SN2 mechanism for primary alkyl halide

Steps:

• Classify substrate: 1-chloro-2-methylbutane is a primary alkyl chloride.
• Assess nucleophile and solvent: CN⁻ is a strong nucleophile; ethanol is polar protic but favors SN2 for primaries.
• Rule out SN1: Primary halides resist carbocation formation due to instability.
• Conclude mechanism: Concerted SN2 with backside attack transition state.

Why A is correct:

• A shows the SN2 transition state with pentacoordinate carbon, partial bonds to CN⁻ and Cl⁻, per SN2 definition of concerted substitution.

Why the others are wrong:

• B depicts a carbocation intermediate, characteristic of SN1, unlikely for primary substrate.
• C illustrates frontside attack, violating SN2 inversion stereochemistry.
• D represents a free radical species, irrelevant to nucleophilic substitution.

Final answer: A