Acyl chloride

In organic chemistry, an acyl chloride (or acid chloride) is an organic compound with the functional group −C(=O)Cl. Their formula is usually written R−COCl, where R is a side chain. They are reactive derivatives of carboxylic acids (R−C(=O)OH). A specific example of an acyl chloride is acetyl chloride, CH3COCl. Acyl chlorides are the most important subset of acyl halides.

Nomenclature

Where the acyl chloride moiety takes priority, acyl chlorides are named by taking the name of the parent carboxylic acid, and substituting -yl chloride for -ic acid. Thus:

acetic acid (CH3COOH) → acetyl chloride (CH3COCl)

benzoic acid (C6H5COOH) → benzoyl chloride (C6H5COCl)

butyric acid (C3H7COOH) → butyryl chloride (C3H7COCl)

(Idiosyncratically, for some trivial names, -oyl chloride substitutes -ic acid. For example, pivalic acid becomes pivaloyl chloride and acrylic acid becomes acryloyl chloride. The names pivalyl chloride and acrylyl chloride are less commonly used, although they are arguably more logical.)

Acyl chlorides of dicarboxylic acids, called diacyl chlorides (sometimes diacid chlorides or dioyl chlorides), follow similar rules, retaining either their common name or a multiplicative prefix before -yl and chloride.

oxalic acid or ethanedioic acid ((COOH)2) → oxalyl chloride or ethanedioyl dichloride ((COCl)2)

adipic acid or hexanedioic acid (C4H8(COOH)2) → adipoyl chloride or hexanedioyl dichloride (C4H8(COCl)2

phthalic acid or benzene-1,4-dicarboxylic acid (C6H4(COOH)2) → phthaloyl chloride or benzene-1,4-dicarbonyl dichloride (C6H4(COCl)2)

When other functional groups take priority, acyl chlorides are considered prefixes — chlorocarbonyl-:

acetic acid (CH3COOH) → (chlorocarbonyl)acetic acid (ClOCCH2COOH)

Properties

Lacking the ability to form hydrogen bonds, acyl chlorides have lower boiling and melting points than similar carboxylic acids.