An acyl chloride is a chemical compound made from an oxoacid^[1] or carboxylic acid by replacing a hydroxyl group (−OH) with a chlorine atom.

There are different kinds of acyl chlorides. They have different chemical formulas.

Acyl chlorides are the most important type of acyl halide, a group that includes similar compounds with other halogen atoms.

Organic acyl chlorides can be produced by the reaction of the parent carboxylic acid with thionyl chloride, SOCl₂:

R−COOH + SOCl₂ → R−COCl + SO₂ + HCl

Acyl chlorides are the most reactive derivatives of their parent carboxylic acid, which makes them useful in preparing other related compounds. These reactions usually have hydrochloric acid as a byproduct. Some of these reactions include:

• With water to make the parent acid:

RCOCl + H₂O → RCOOH + HCl

• With carboxylate to make an acid anhydride (see anhydride):

RCOCl + R'COO⁻ → RC(=O)−O−C(=O)R' + Cl⁻

• With alcohol to make an ester:

RCOCl + R'OH → RCOOR' + HCl

• With amine to make an amide:

RCOCl + H₂NR' → RC(=O)−NHR' + HCl

These reactions all have similar mechanisms. The carbon atom in an acyl chloride is an electrophile because of the bonds to oxygen and chlorine, and the other ingredients all act as nucleophiles. The reaction occurs in two steps: addition of the nucleophile to the acyl chloride's carbon, followed by elimination of hydrochloric acid or chloride.^[2]

The hydrochloric acid byproduct can be difficult to handle: many bases that would neutralize the byproduct are also nucleophiles that can react with acyl chlorides.

In organic chemistry, this is the functional group −C(=O)Cl.