Hydrogen migrations in mass spectrometry. IV—formation of [C6H7]+ in the chemical ionization mass spectra of alkylbenzenes

Using specific deuterium labelling the mechanisms of the olefin elimination reactions leading to formation of C₆H₇]+ in the H₂ and CH₄ chemical ionizatin mass spectra of ethylbenzene and n-propylbenzene (and to C₂H₅C₆H₆]+ in the CH₄ chemical ionization mass spectra) have been investigated. The results show that the reaction does not occur by specific migration of H from the β position of the alkyl group to the benzene ring. For ethylbenzene 23–29% of the migrating H originates from the α-position, while for n-propylbenzene H migration from all propyl positions is observed in the approximate ratio, position 1:position 2:position 3=0.30:0.22:0.48. It is proposed that the results can be explained on the basis of competing H migration from each alkyl position involving cyclic transition states of different ring sizes, rather than by H randomization within the alkyl chain.