# Why does pyridine make such a big change from SNi to SN2?

When an alcohol is treated with SOCl2 alone, retention through SNi takes place, whereas adding pyridine makes the reaction follow SN2 mechanism and inversion takes place. What concept lies behind this occurrence?

• Either the C-O bond of the alkyl chlorosulfite brakes, a (tight) ion pair of carbo-cation and chlorosulfit anion is formed. The later decomposes into $\ce{SO2}$ and $\ce{Cl-}$, the latter reacting with the carbo-cation. This yields the rentention of configuration observed.