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.

Carboxylic acids react with SOCl₂ or PCl₅ to form acid chlorides. Amongst the carboxylic acid derivatives, acid chlorides are the most reactive and synthetically important derivatives. They are useful reagents for Friedel–Crafts acylation of some aromatic compounds.

An alternative reagent for converting a carboxylic acid to an acid chloride is phosphorus pentachloride. The mechanism involves the attack by a carboxylic acid at the phosphorus center of PCl₅ while eliminating a chloride ion. Subsequently, the attack by this chloride ion at the carbonyl center results in an acid chloride, along with the formation of phosphorus oxychloride. The formation of a highly stable P=O bond drives the reaction forward.

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