Collision of molecular anions of benzenedicarboxylic esters with oxygen in a triple quadrupole mass spectrometer

A series of symmetrical phthalate, isophthalate, and terephthalate ester molecular anions were reacted with oxygen in the collision cell of a triple quadrupole mass spectrometer to produce characteristic [M−R] − fragments that can be used to identify these compounds. The [M−R] − /M −· intensity ratios decreased for homologous esters in the following order: phthalates > isophthalates ≫ terephthalates. Based on the [M−R] − ion intensities for different alkyl substituents, on 18 O 2 labeling experiments, and on the reactivity of bis( t -butylcyclohexyl)phthalates, it was concluded that M — R anions are generated through an S N 2 nucleophilic displacement at the alpha carbon of the saturated alkyl substituent. For the phenyl ester, the reaction proceeds through attack at the carbonyl carbon.