Hydrolysis of acid halides is a nucleophilic acyl substitution reaction, where an acid halide reacts with water to form a carboxylic acid.

Since acid halides are very reactive, the hydrolysis does not require an acid or a base catalyst.

The mechanism proceeds in three steps. The first step is the nucleophilic attack by water at the carbonyl carbon to form a tetrahedral intermediate.

In the second step, the carbon-oxygen π bond is re-formed, and the chloride ion departs as a leaving group.

Lastly, deprotonation forms the corresponding carboxylic acid as the final substitution product.

In this reaction, hydrochloric acid is formed as a by-product, which often leads to unwanted side reactions.

So, a weak base like pyridine is added to the reaction mixture to neutralize the hydrochloric acid, thereby preventing any side reaction.