When a nucleophilic atom bearing a good leaving group attacks a carbonyl group, an adjacent Rgroup can migrate to the new atom, inserting it into the R-acyl bond. This mechanism can insert O, NH, or CH2 groups into the acyl bond with informative stereospecificity in the case of the Beckmann rearrangement of oximes. Although the migrating groups are formally anionic, relative migratory aptitudes show that they give up electron density during rearrangement. Acid dissociation of protons a to a carbonyl group to form enolates, and the ease of forming enols, gives α-carbons nucleophilic reactivity under both basic and acidic conditions. This explains H/D exchange and racemization as well as halogenation and alkylation of α-carbons.