Preparation of acyl chlorides

This page discusses the methods of swapping the -OH group in the -COOH group of a carboxylic acid for a chlorine atom to make acyl chlorides (acid chlorides) using thionyl chloride. In the examples below, consider the conversion of acetic acid to acetyl chloride to be typical of these types of reactions.

.  It reacts with carboxylic acids to produce an acyl chloride, giving off sulfur dioxide and hydrogen chloride gases. For example:

CH3COOH+SOCl2→CH3COCl+SO2+HCl

The separation is simplified to an extent because the by-products are both gases. Fractional distillation is still required to separate the acyl chloride from any excess acid or thionyl chloride.