Formation of isomeric monohalo-n-alkanoic acids in the reactions of γ- and δ-n-alkanolactones with hydrogen halides

In 57% hydriodic acid or 48% hydrobromic acid, under reflux, n-alkanoic γ- or δ-lactones of from 5 to 18 carbon atoms undergo facile ring opening to monohalo-n-alkanoic acids. With both reagents the ratio of acid to lactone at equilibrium varies widely, ranging from about 1:3 for γ-valerolactone (C5) to 3:1 or more for γ-octanolactone and the larger members of the series. Extensive scrambling of the halogen atoms accompanies the formation of the haloacids, whereby mixtures of monohalo isomers substituted at all positions from C-4 to the penultimate carbon are found. In an 18 h reaction with HBr, for example, γ-caprolactone was converted into a 1:3 mixture of 4- and 5-bromohexanoic acids, and γ-decanolactone into a 1:1.4:1.4:1.4:2.3:2.3 mixture of 4-, 5-, 6-, 7-, 8-, and 9-bromodecanoic acids. By contrast, the γ-lactones containing 14 or 16 carbon atoms gave only 10:1 mixtures of the 4- and 5-bromoacids, and at the level of 18 carbon atoms rearrangement was no longer evident; i.e., γ-octadecanolactone affo...