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A carboxylic acid such as ethanoic acid has the structure:

There are a number of related compounds in which the -OH group in the acid is replaced by something else. Compounds like this are described as acid derivatives.

Acid chlorides  are one example of an acid derivative. In this case, the -OH group has been replaced by a chlorine atom.

The acyl group is a hydrocarbon group attached to a carbon-oxygen double bond:

For UK A level purposes, the “R” group is normally restricted to an alkyl group. It could, however, equally well be a group based on a benzene ring.

The easiest way of thinking about the names is to see the relationship with the corresponding carboxylic acid:

If you have something substituted into the hydrocarbon chain, the carbon in the -COCl group counts as the number 1 carbon.

For example, 2-methylbutanoyl chloride is:

An acyl chloride is a colourless fuming liquid. The strong smell of ethanoyl chloride is a mixture of the smell of vinegar (ethanoic acid) and the acrid smell of hydrogen chloride gas.

Solubility in water

Acyl chlorides can’t be said to dissolve in water because they react (often violently) with it. The strong reaction means that it is impossible to get a simple aqueous solution of an acyl chloride.

Boiling points

Taking ethanoyl chloride as typical:

Ethanoyl chloride boils at 51°C. It is a polar molecule, and so has dipole-dipole attractions between its molecules as well as van der Waals dispersion forces.

However, it doesn’t form hydrogen bonds. Its boiling point is therefore higher than, say, an alkane of similar size (which has no permanent dipoles), but not as high as a similarly sized alcohol (which forms hydrogen bonds in addition to everything else.)

Reactivity of acyl chlorides

Acyl chlorides are extremely reactive, and in their reactions the chlorine atom is replaced by other things.

In each case, in the first instance, hydrogen chloride gas is produced as steamy acidic fumes. However, in some cases the hydrogen chloride goes on to react with one of the substances in the reaction mixture.

Taking ethanoyl chloride as typical, the initial reaction is of this kind:

The reactions involve things like water, alcohols and phenols, or ammonia and amines. All of these particular cases contain a very electronegative element with an active lone pair of electrons - either oxygen or nitrogen.