Alpha halogenation is the reaction of an enolizable aldehyde or enolizable ketone with a halogen, usually chlorine or bromine, in the presence of an acid catalyst to give an alpha-haloaldehyde or alpha-haloketone.
Notice that the reaction consumes one molecule of acid and generates two, meaning overall it generates one molecules of acid, making the reaction autocatalytic (autocatalysis). That is, the reaction can be carried out in the absence of an acid; after an incubation period, the reaction would accelerate.
It is important that only one molar equivalent of the halogen be used and the halogen be added dropwise to the reaction mixture so as to minimize multiple halogenation.
Alpha halogenation can be carried out in the presence of a base catalyst, but in the presence of a base multiple halogenation is much harder to prevent. (See haloform reaction.)