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 afforded only 4-bromooctadecanoic acid. Similar isomer distributions were obtained for the iodoacid homologs. Hydrochloric acid (37%) was far less effective in opening the lactone rings, and also in inducing rearrangement of the chlorine atoms introduced. Differences in entropy and in solvation appear to be the main factors contributing to the variations observed among isomeric lactones.