The rate law for this transformation is given by:
Rate = k[ROH][SOCl2]
Now the mechanism of this reaction was determined by isolating the intermediate ROSOCl which then decomposes to give the product. In this reaction the formation of intermediate proceeds with retention of configuration as none of the bonds with the chiral center are broken. The decomposition of the intermediate takes place in an intimate ion pair (11) where the nucleophile can attack only from the same side from where the OSOCl moiety has departed (Scheme 9).
Scheme 9
The evidence for this mechanism comes from the addition of pyridine to generate the naked Cl− anion that leads the reaction to proceed by SN2 pathway leading to inversion. The role of pyridine is to combine with either the ROSOCl to form ROSOpy+ or HCl generated as a byproduct of the original SNi reaction to form pyridinium cation, thereby allowing the free chloride ion to attack from the back leading to inversion of configuration (Scheme 10).
Scheme 10