When an optically active alcohol reacts with thionyl chloride () in the presence of pyridine, the reaction proceeds via which mechanistic pathway, and what is the stereochemical outcome?

Question

When an optically active alcohol reacts with thionyl chloride ($SOCl_2$) in the presence of pyridine, the reaction proceeds via which mechanistic pathway, and what is the stereochemical outcome?

Prashant Kotian · Accepted Answer

Correct answer: B. The reaction of an alcohol with pure proceeds via the Internal Nucleophilic Substitution () mechanism, resulting in retention of configuration. However, when the base pyridine is added, it neutralizes the byproduct to form pyridinium chloride. The free ion acts as a strong nucleophile and performs a backside attack on the intermediate chlorosulfite, shifting the mechanism to and resulting in an inversion of configuration.

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