Acetyl chloride, also known as ethanoyl chloride, is an acid chloride derived from acetic acid. It has the formula H₃C-COCl and it belongs to the class of organic compounds called acyl halides. The chemical structure of acetyl chloride is shown at right. At room temperature and pressure, it is a clear colorless liquid. Acetyl chloride does not exist in nature, because contact with water would hydrolyze it into acetic acid and hydrogen chloride.

It is artificially produced by the reaction of thionyl chloride with acetic acid

H₃C-COO-H + O=SCl₂ → H₃C-COCl + SO₂ + H-Cl

and is used as a reagent for acetylation in the synthesis or derivatization of chemical compounds. Examples of acetylation reactions include acylation processes such as esterification (see below) and the Friedel-Crafts reaction).

H₃C-COCl + HO-CH₂-CH₃ → H₃C-COO-CH₂-CH₃ + H-Cl

Frequently such acylations are carried out in the presence of a base such as pyridine, which helps to promote the reaction and also removes the HCl as a pyridinium salt.

Acetylation is the introduction of an acetyl group via acylation using a reactant such as acetyl chloride or acetic anhydride. An acetyl group is an acyl group having the formula
-COCH₃.

For further information on the types of chemical reactions compounds such as acetyl chloride can undergo, see acyl halide.