Please elaborately explain the SNi Mechanism.

Dear aniket,

This reaction mechanism is supported by the observation that addition of  pyridine to the reaction leads to inversion. The reasoning behind this finding is that pyridine reacts with the intermediate sulfite replacing chlorine. The dislodged chloride has to resort to nucleophilic attack from the rear as in a regular nucleophilic substitution.

In the complete picture for this reaction the sulfite reacts with a chlorine ion in a standard SN2 reaction with inversion of configuration. When the solvent is also a Nucleophile such as dioxanetwo successive S_N2 reactions take place and the stereochemistry is again retention. With standard SN1 reaction conditions the reaction outcome is retention via a competing SNi mechanism and not racemization and with pyridine added the result is again inversion.

Approved