Reaction of thionyl chloride with chiral 2º-alcohols: Mechanisms

Since the reaction proceeds through a backside attack (S_N2 ), there is inversion of configuration at the carbon

The mechanism for formation of acid chlorides from carboxylic acids is similar. The conversion of caboxylic acids to acid chlorides is similar, but proceeds through a [1,2]-addition of chloride ion to the carbonyl carbon followed by [1,2]-elimination to give the acid chloride, SO2 and HCl.

 The PBr₃ reaction is thought to involve two successive S_N2-like steps:

Notice that these reactions result in inversion of stereochemistry in the resulting alkyl halide.