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General chemical structure of an acyl chloride

In organic chemistry, an acyl chloride (or acid chloride) is an organic compound which is a reactive derivative of a carboxylic acid. As part of its molecular structure, 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.

Example of an acyl chloride:
acetyl chloride

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.

Acyl 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, it will be readily replaced with the nucleophile. Acyl chlorides 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). In most of these reactions, HCl (hydrogen chloride or hydrochloric acid) is also formed from the chlorine in the acyl chloride combining with a hydrogen from the reactant.

For example:

R-COCl + HO-H → R-COOH + HCl

Acyl chlorides must also be reacted in anhydrous conditions to prevent the hydrolysis of acyl chloride by the moisture in the air.

With carbon nucleophiles such as Grignard reagents, acyl chlorides generally react first to the ketone and then with a second equivalent to the tertiary alcohol. A notable exception is the reaction of acyl halides with certain organocadmium reagents which stops at the ketone stage. Acid chlorides of aromatic acids are generally less reactive those of alkyl acids and thus somewhat more rigorous conditions are required for reaction.

Because acyl chlorides are such reactive compounds, they are generally toxic and special precautions should be taken while handling them. They are lachrymatory chemicals because they can react with water at the surface of the eye producing hydrochloric and organic acids irritating to the eye. Similar problems can result if one inhales acyl chloride vapors.

Acyl chlorides are often prepared by reacting a carboxylic acid with thionyl chloride.

R-COOH + SOCl₂ → R-COCl + SO₂ + HCl

The sulfur dioxide (SO₂) and hydrogen chloride (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 such as phosphorus trichloride or phosphorus pentachloride:

R-COOH + PCl₅ → R-COCl + POCl₃ + HCl