In the synthesis of alkyl chlorides via Darzen's procedure, the reaction's stereochemical fate is he — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

In Darzen's method with $SOCl_2$ alone, the intermediate alkyl chlorosulphite decomposes via an intramolecular attack ($S_Ni$), yielding complete retention of configuration. However, when pyridine is added, it neutralizes the $HCl$ generated in the first step, forming pyridinium chloride. The freely available, highly nucleophilic chloride ions in the solution attack the carbon bonded to the chlorosulphite group from the backside ($S_N2$). This rapid bimolecular attack displaces the $SO_2Cl^-$ group before $S_Ni$ can occur, resulting in total inversion of configuration.