Fragmentation Reactions of Some Aliphatic Esters in the NCI(F-) and NCI(NH2-) Mass Spectra

Article abstract of DOI:10.1002/oms.1210190305

The NCI(F^-) and NCI(NH2^-) mass spectra of a series of aliphatic acetates and of methyl and ethyl trimethylacetate have been obtained.The formation of fluoroenolate ions CH2COF^- and of carboxamide anions RCONH^- (R=CH3, (CH3)3C), respectively, is observed besides formation of ^- ions and carboxylate ions RCOO^- (R=CH3, (CH3)3C).The relative intensities of the different anions depend on the structure of the ester molecules and on the primary reactant anions.Usually, the NCI(NH2^-) spectra of the acetates are dominated by ^- ions (^- ions in the case of trideuterioacetates) fragmenting unimolecularly by elimination of an alcohol.The carboxylate ions are important fragments, too, but carboxamide ions are only observed with large intensities in the NCI(NH2^-) spectra of the trimethylacetates.The NCI(F^-) spectra show much larger intensities of carboxylate ions and fluoroenolate ions.The mechanisms of the fragmentation reactions are discussed.The results indicate that most or even all of the fragment ions in the NCI(F^-) mass spectra of aliphatic esters are formed by addition-elimination reactions via a tetrahedral intermediate, while competition between direct proton abstraction and addition-elimination reactions occurs in the NCI(NH2^-) mass spectra because of the higher basicity of NH2^- resulting in an early transition state for direct proton abstraction.