Fragmentations of Protonated Acetophenones via Intermediate Ion-Molecule Complexes

Article abstract of DOI:10.1002/oms.1210220711

Protonated acetophenones, substituted with a methoxymethyl group in the para and meta positions, have been generated by electron impact induced fragmentation of the correspondingly substituted 2-phenylpropan-2-ols.The metastable ions, formed in the second field-free region of a VG ZAB2F mass spectrometer, react unimolecularly by elimination of CH3OH, formation of CH3CO⁺ and CH3O⁺=CH2 ions, loss of CH3COOCH3, and loss of CH2O.The mechanisms of these fragmentations have been elucidated with the aid of deuterated analogues of the protonated acetophenones.It is shown that these reactions are initiated by an endothermic transfer of the proton at the carbonyl group of the protonated acetophenones to the benzene ring.A further migration of the proton to the ether O atom of the methoxymethyl side-chain leads eventually to the elimination of CH3OH.Protolytic bond cleavages of either side-chain gives rise to the CH3CO⁺ and CH3O⁺=CH2 ions.At low internal energies both these ions may be trapped by the aromatic neutral fragment in ion-molecule complexes.Reactions within these complexes result in the energetically favourable losses of CH3COOCH3 and CH2O, respectively.With respect to these reactions, the protonated acetophenones behave analogously to the correspondingly substituted and protonated benzaldehydes.