A carboxylic acid derivative is a compound that can be hydrolysed to form the parent carboxylic acid.

Examples of carboxylic acid derivatives include esters, acyl chlorides, acid anhydrides, and amides.

Acyl chlorides are also called acid chlorides.

Acyl chlorides are made directly from the reaction of their parent carboxylic acid with thionyl chloride (SOCl₂). The reaction produces the acyl chloride as the only liquid product as the other products (SO₂ and HCl) are gases.

For example, the equation for the preparation of ethanoyl chloride from ethanoic acid using SOCl₂ is:

CH₃COOH + SOCl₂ ➔ CH₃COCl + SO₂ + HCl

Acyl chlorides react with alcohols to form esters and hydrogen chloride as a by-product.

For example, the reaction between ethanoyl chloride and ethanol produces ethyl ethanoate and HCl:

CH₃COCl + C₂H₅OH ➔ CH₃COOC₂H₅ + HCl

Acid anhydrides react with alcohols to form esters and carboxylic acids.

For example, the reaction between ethanoic anhydride and ethanol produces ethyl ethanoate and ethanoic acid:

(CH₃CO)₂O + C₂H₅OH ➔ CH₃COOC₂H₅ + CH₃COOH

Acyl chlorides react with phenols to form phenyl esters and hydrogen chloride as a by-product.

For example, the reaction between ethanoyl chloride and phenol produces phenyl ethanoate and HCl:

CH₃COCl + C₆H₅OH ➔ CH₃COOC₆H₅ + HCl

Acyl chlorides undergo hydrolysis with water to form carboxylic acids. This is a vigorous reaction that produces steamy hydrogen chloride fumes.

For example, the reaction between ethanoyl chloride and water produces ethanoic acid and HCl:

CH₃COCl + H₂O ➔ CH₃COOH + HCl

Acyl chlorides react with ammonia to form primary amides and ammonium chloride as a by-product.

For example, the reaction between ethanoyl chloride and ammonia produces ethanamide and NH₄Cl:

CH₃COCl + 2NH₃ ➔ CH₃CONH₂ + NH₄Cl.

Acyl chlorides react with primary amines to form secondary amides and an ammonium salt as a by-product.

For example, the reaction between ethanoyl chloride and methylamine produces N-methylethanamide and CH₃NH₃Cl:

CH₃COCl + 2CH₃NH₂ ➔ CH₃CONHCH₃ + CH₃NH₃Cl.

Acyl chlorides are named by removing the -oic acid suffix from the parent carboxylic acid and replacing it with the suffix -oyl chloride.

For example, the acyl chloride of ethanoic acid is ethanoyl chloride.