Question

Which of the following compounds will show retention in configuration on nucleophilic substitution by $OH^-$ ion?

• A. A.
• B. B.
• C. C.
• D. D.

Answer 1

Answer: (B)

B.

Answer 2

Compound B, 1-bromo-1-phenylethane, is a chiral secondary benzylic halide. It undergoes nucleophilic substitution via an SN1 mechanism due to the resonance stabilization of the carbocation intermediate by the phenyl ring. While SN1 reactions typically lead to racemization, retention of configuration can occur in benzylic systems due to ion-pairing effects, where the leaving group shields one face, allowing nucleophilic attack from the same side.