Carboxylic acids, when treated with SOCl₂ or PCl₅, result in acid chlorides. Here, the –OH group of the carboxylic acid is replaced by –Cl.

The reaction mechanism begins with a nucleophilic attack of the carboxylic acid on thionyl chloride to generate an intermediate that immediately loses a chloride ion.

Subsequent deprotonation forms a reactive chlorosulfite derivative bearing a good leaving group.

The derivative then undergoes a nucleophilic attack by the chloride ion to form a tetrahedral intermediate.

Finally, with the departure of the leaving group, acid chloride is formed as a product.

Treatment of carboxylic acids with PCl₅ also produces acid chloride along with phosphorus oxychloride that bears a highly stable P=O bond which drives the reaction forward.

In acid chlorides, the presence of both electron-withdrawing atoms makes the carbonyl carbon highly electrophilic and reactive. Acid chlorides can undergo nucleophilic acyl substitution via the addition–elimination mechanism to generate acid derivatives.