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.

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:

(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,^{[citation needed]} although they are arguably more logical.^[opinion])

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.

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

Lacking the ability to form hydrogen bonds, acyl chlorides have lower boiling and melting points than similar carboxylic acids. For example, acetic acid boils at 118 °C, whereas acetyl chloride boils at 51 °C. Like most carbonyl compounds, infrared spectroscopy reveals a band near 1750 cm⁻¹.

The simplest stable acyl chloride is acetyl chloride; formyl chloride is not stable at room temperature, although it can be prepared at –60 °C or below.

Acyl chlorides hydrolyze (react with water) to form the corresponding carboxylic acid and hydrochloric acid: