MASS-SPECTROMETRIC INVESTIGATIONS ON PHENYLACETIC ACID-DERIVATIVES .4. LOSS OF ORTHO-SUBSTITUENTS FROM IONIZED PHENYL-2-PROPANONES UPON ELECTRON-IMPACT

In the gas phase, the phenyl-2-propanone molecules 2a-4a lose upon electron impact chloro-, bromo-, and iodo-radicals specifically at the ortho-position of the phenyl group giving rise to strong (M-Hal.)+-ions (70/12 eV; 1st and 2nd FFR) of identical structure as confirmed by their MIKE-CAD-spectra. The daughter ions at m/z 133 from o-chlorophenyl-2-propanone (2a) and 2,2-dimethyl-2,3-dihydro[b]furane (11) are structurally similar but not identical (similarity index 99.8). The collisionally activated (2nd FFR) (M-Br.)+-ions from o-bromophenyl-2-propanone (3a) and 1-bromo-1-phenyl-2-propanone (12) produce virtually congruent spectra. The most important subsequent fragmentation of the (M-Hal.)+-ions from 2a4a is the loss of CO which incorporates the C-atom of the carbonyl group exclusively (C-13 labelling). Mechanistic aspects of the fragmentation sequences are discussed (Figs. 5 and 8).