Reaction of γ-nonanolactone with deuterobromic acid

Abstract

Series of isomeric monohaloalkanoic acids may be prepared from γ- or δ-alkanoic lactones by reaction with hydrogen halides. For example, γ-nonanolactone in 48% hydrogen bromide under reflux affords a 1:1.5:1.7:2.2:2.4 mixture of 4-, 5-, 6-, 7-, and 8-bromononanoic acids. In an examination of possible ways whereby such products are formed, it has been found that the use of 48% deuterium bromide leads to the introduction of up to 14 atoms of the isotope per bromoacid. According to 1H, 2H, and 13C nuclear magnetic resonance observations, however, the major components of the isomer mixture contain 8–10 atoms of deuterium confined mainly to carbons 4–8, i.e., close to full deuteration of each of the carbons. This indicates that a prominent mechanism governing the formation of the bromoacid mixture involves repeated elimination and addition of HBr (2HBr) along a strictly limited segment of the carbon chain. Initiated at the C-4 bridge of the lactone and characterized, hypothetically, by a succession of carbocation intermediates, the halogenation reactions appear to include a repetitive migration of transient alkene functions from C-4 through C-8, and an accompanying series of "zipperlike" hydride (deuteride) shifts in the inverse direction. Keywords: deuterohaloalkanoic acids, carbocation rearrangements.