Competing fragmentation processes of O-acetyl-substituted carboxylate anions subjected to collision-induced dissociation.

Carboxylic acids containing an O-acetyl substituent were studied using tandem mass spectrometry (MS/MS). Decarboxylation was observed for deprotonated O-acetylmandelic acid, whereas deprotonated acetoxyacetic acid and acetylsalicylic acid fragmented by two competing pathways. In the lower energy process, the product ion was formed by intramolecular proton abstraction and subsequent neutral loss of ketene (CH₂=C=O) from the O-acetyl group. At higher collision energies, nucleophilic displacement of the O-acetyl group by the carboxylate group of acetoxyacetate yielded acetate (CH₃CO₂^-) as the more abundant product ion. The relative energetics computed for the reaction pathways of acetoxyacetate were consistent with the product ion spectra. Overall, the observation of both the loss of ketene and the formation of acetate ion are characteristic of an O-acetyl group in the precursor carboxylate ion undergoing collision-induced dissociation. The different fragmentation behavior exhibited by O-acetyl mandelate was attributed to the charge stabilizing properties of the phenyl substituent that facilitated decarboxylation. Thus, the fragmentation processes observed depended on the structures of the O-acetyl-substituted carboxylate ions and the associated intramolecular interactions.