The intriguing behaviour of (ionized) oxalacetic acid investigated by tandem mass spectrometry

Tandem mass spectrometry based experiments on commercial oxalacetic acid (OAA) samples confirm the indirect evidence from the elegant study of Flint et al. (J. Org. Chem. 57 (1992) 7270) that solid OAA exists solely in the (Z)-enol form of HOOCC(H)C(OH)COOH. It is further shown that: (1) the samples contain a minor impurity in the low percent range, assigned as a dehydration product of 4-hydroxy-4-methyl-2-ketoglutaric acid; (2) careful evaporation of solid OAA samples yields mass spectra representative of the (Z)-enol form. The (E)-enol is not present in the solid, nor is its ion generated in the gas phase via isomerization of the ionized (Z)-enol form. However, even under gentle sample introduction conditions, a partial ketonization of the neutral (Z)-enol takes place. The resulting keto OAA molecules are either ionized intact or decarboxylate to yield a mixture of α-hydroxyacrylic acid, CH2C(OH)COOH and its keto isomer pyruvic acid, CH3C(O)COOH. From computational quantum chemistry (CBS-4) and experimental data, ionic enthalpies of formation of 95 and 109 kcal/mol were derived for CH2C(OH)COOH·+ and CH3C(O)COOH·+, respectively; and (3) under less controlled sample introduction conditions, thermal dehydration may take place to yield C4H2O4 molecules whose mass spectral characteristics are indistinguishable from those of ionized hydroxymaleic anhydride.