Question

Which statement best explains the partial racemization seen in some cases of $S_{N}1$ reactions?

• A. The C - X bond of the alkyl halide dissociates completely, and the dissociated and solvated carbonium ion does not become entirely planar but retains its configurational information in part.
• B. The alkyl halide reacts as an intimate ion pair or a solvent-separated ion pair, and the carbonium ion never really becomes fully planar; therefore, it retains its stereochemical information content.
• C. The alkyl halide reacts as an intimate ion pair or a solvent-separated ion pair, and one face of the planar carbonium ion is covered up by the leaving group.
• D. The partial racemization must be the result of contributions by the S${}_N$2 reaction path.

Answer 1

The correct option is C The alkyl halide reacts as an intimate ion pair or a solvent-separated ion pair, and one face of the planar carbonium ion is covered up by the leaving group.

During $S_{N}1$ mechanism, the alkyl halide forms an intimate ion pair or solvent separated ion pair. The probability of attack on the front side and the back side is not equal as one face of the planar carbonium ion is covered by the leaving group. Hence, partial racemization is observed.