Which transformation converts an alcohol into a good leaving group that can be displaced in SN2?

• A. Hydroxylation
• B. Hydrogenation
• C. Tosylate formation
• D. Esterification

Answer: (C)

Transforming an alcohol into a good leaving group for SN2 hinges on making the leaving group depart as a stable anion. An alcohol’s OH is a poor leaving group, so it’s not favorable to displace it in SN2. Converting the alcohol to a tosylate does the trick: reaction with p-toluenesulfonyl chloride forms an alkyl tosylate (ROTs). The leaving group that leaves is the p-toluenesulfonate anion, a highly stabilized species thanks to resonance and the electron-withdrawing sulfonyl group. This makes departure in an SN2 step much more favorable.

Other options don’t achieve this improvement. Hydroxylation adds another OH, not a better leaving group. Hydrogenation changes saturation rather than the leaving-group ability. Esterification creates an ester, but the group that would depart in SN2 is not a good leaving group, so SN2 displacement remains unlikely.