Does the reaction below proceed via an SN1 or SN2 reaction mechanism?

Question

BlackTom AI · Accepted Answer

A benzyl (benzylic) bromide undergoes nucleophilic substitution by CN− via an SN2 pathway.

1) Relevant concepts and facts
- Substrate: benzylic bromide (the carbon bearing Br is directly attached to an aromatic ring). This position is primary (or primary-like) but the adjacent aromatic ring can stabilize a positive charge (benzylic carbocation).
- Nucleophile: CN− is a strong, small, and highly nucleophilic species.
- Mechanistic preferences: Primary (and benzylic primary) centers typically undergo SN2 when a strong nucleophile is present. SN1 is favored by weak nucleophiles and by very stabilized carbocations under polar protic conditions.

2) Step-by-step reasoning
- Step 1 (concerted attack): CN− approaches the electrophilic carbon bearing Br from the backside relative to the C–Br bond.
- Step 2 (transition state): A single, concerted transition state forms in which the C–CN bond is partially formed while the C–Br bond is partially broken.
- Step 3 (product formation): Bromide departs as Br− simultaneously as CN− bonds to the carbon, giving benzyl cyanide (and NaBr when paired with Na+).

3) Intermediate/transition steps
- No stable carbocation intermediate is formed. The process is bimolecular (one molecule of substrate + one CN− in the rate-determining step), characteristic of SN2. The key “intermediate” is the single transition state with partial bonds.

4) Why SN2 is correct
- Strong nucleophile (CN−) and a primary/benzylic carbon favor a direct bimolecular displacement (SN2). The benzylic position is sterically accessible for backside attack, so SN2 is both fast and favorable. Although benzylic carbocations are resonance-stabilized, their formation (SN1) is not necessary when a strong nucleophile is present.

5) Why SN1 is incorrect
- SN1 requires formation of a carbocation intermediate and is typically favored by weak nucleophiles and polar protic solvents. Here, the presence of CN− (strong nucleophile) and a primary benzylic substrate leads to direct displacement rather than ionization to a carbocation. If SN1 occurred, one would expect evidence of a carbocation (e.g., racemization if chiral) and different kinetics (unimolecular rate law).

6) Conclusion
The reaction proceeds by an SN2 mechanism. This matches the provided correct answer: b. SN2.

Does the reaction below proceed via an SN1 or SN2 reaction mechanism?单项选择题

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