The mass spectra of carboxylic acids—V: Carboxyl‐carboxyl interaction in the fragmentation of some cycloalkene‐1,2‐dicarboxylic acids

The fragmentation mechanisms of the six isomeric cyclohexene‐1,2‐dicarboxylic acids are discussed. Only the 1‐cyclohexene acid, by virtue of the major sequential losses of H 2 O and CO 2 from the molecular ion, is readily distinguishable from its isomers, all of whose mass spectra are closely similar. In contrast to cis and trans cyclohexane‐1,2‐dicarboxylic acids, whose mass spectra were markedly different, the cis and trans cyclohexene‐1,2‐dicarboxylic acids fragment in a similar fashion. The mass spectra of 1‐cyclopentene‐1,2‐dicarboxylic acid and 1‐cyclobutene‐1,2‐dicarboxylic acid also exhibit a strong carboxyl‐carboxyl interaction; the fragmentation behaviour of the 1‐cyclopenteneacid is, however, more complex than that of the 1‐cyclohexene and 1‐cyclobutene acids.