Stereochemical applications of mass spectrometry. II—Chemical ionization mass spectra of isomeric dicarboxylic acids and derivatives

Abstract

AbstractThe H2 and CH4, chemical ionization mass spectra of the cis dicarboxylic acids, maleic and citraconic acid, show much more extensive loss of H2O from [MH]+ than the trans isomers, fumaric acid and mesaconic acid. Similarly, esters of maleic acid show a much more facile loss of ROH (R=alkyl or phenyl) from [MH]+ than do esters of fumaric acid. Similar differences are observed in the chemical ionization mass spectra of the isomeric phthalic and isophthalic acids and derivatives, where the ortho isomers show more extensive fragmentation of [MH]+ than the meta isomers. The facile fragmentation of [MH]+ for the cis and ortho isomers is attributed to ROH elimination involving interaction between the two carboxylate functions and forming the stable cyclic anhydride structure for the fragment ion. By contrast ROH elimination from [MH]+ for the trans and metu isomers requires a symmetry‐forbidden [1,3]‐H migration in the carboxyl protonated species and cannot lead to the cyclic anhydride structure. The chemical ionization mass spectra of cis and trans cyclohexane‐1,2‐dicarboxylic acids are essentially identical and show extensive fragmentation of the [IMH]+ ion. Experiments using deuterium labelling show extensive carboxyl group interactions for both isomers. The chemical ionization mass spectra of maleanilic and phthalanilic acids and of the related anhydrides and imides also are reported, as are the electron impact mass spectra of diphenyl maleate, diphenyl fumarate, diphenyl phthalate, maleanilic acid and phthalanilic acid.