SN2 Transition State in Primary Alkyl Halide Substitution

Steps:

• Identify the substrate: 1-chloro-2-methylbutane is a primary alkyl chloride with beta branching.
• Assess reaction conditions: NaCN provides CN⁻, a strong nucleophile, favoring bimolecular substitution.
• Predict mechanism: Primary halides undergo SN2, not SN1, due to low carbocation stability.
• Determine key feature: SN2 involves a single transition state with partial bonds to entering group and leaving group.

Why A is correct:

• A depicts the SN2 transition state, a trigonal bipyramidal structure with simultaneous CN⁻ approach and Cl⁻ departure, per the SN2 mechanism definition.

Why the others are wrong:

• B shows a carbocation intermediate, characteristic of SN1 but unlikely for primary halides.
• C represents an SN1 transition state, which requires rate-determining carbocation formation absent here.
• D illustrates an SNi or frontside attack, not favored in SN2 due to steric and electronic factors.

Final answer: A