In organic chemistry, an acid chloride (or acyl chloride) is an organic chemical compound which is a very reactive derivative of a carboxylic acid. In its molecule, an acyl chloride has the reactive functional group -CO-Cl. An acyl chloride has the general formula RCOCl where R is an organic radical group. See the general chemical structure of an acyl chloride at right.

Acyl chlorides are a subset of acyl halides and undergo many of the chemical reactions mentioned under Acyl halide. A specific example of an acyl chloride is acetyl chloride (IUPAC name: ethanoyl chloride) CH₃COCl. See the simple diagram at left.

Acid chlorides are very reactive, the presence of the chlorine on the carbonyl carbon makes it a very good electrophile, since both the oxygen and chlorine are strongly electron withdrawing, giving the carbon a large partial positive charge. This means even a weak nucleophile can attack the carbon. Since a chloride ion is a good leaving group, this is a very useful compound synthetically, it can be used to prepare any carboxylic acid derivative: an acid anhydride, an ester or an amide by reacting acid chlorides with: a salt of a carboxylic acid, an alcohol or an amine respectively. Acid chlorides will also react with water or a Gilman reagent (lithium diorganocopper compound).

Synthesis

Acyl chlorides are often prepared by reacting a carboxylic acid with thionyl chloride (R-COOH + SOCl₂ -> R-COCl + SO₂ + HCl). The SO₂ and HCl generated are both gases which can leave the reaction vessel, driving the reaction forward, they are also both toxic gases. Acyl chlorides can also be prepared using certain phosphorus chloride reagents.