An important group of reactions of carboxylic acids begin with nucleophilic addition to the carbonyl group, such as acyl transfer reactions. In beginning with nucleophilic addition to the carbonyl group, they are similar to certain reactions of aldehydes and ketones that also begin by nucleophilic addition to the carbonyl group. Another group of reactions of carboxylic acids are similar to certain reactions of aldehydes and ketones because they involve tautomerism of an α hydrogen. The Hell-Volhard-Zelinsky reaction (α-halogenation) is an example from this second group.