SITE OF GAS-PHASE METHYLATION OF ALKYL PHENYL KETONES

The CH3+ adducts of benzaldehyde, acetophenone, propiophenone and butyrophenone were prepared using CH3F-CH4 and CH3Cl-CH4 mixtures under chemical ionization conditions and the unimolecular and collision-induced fragmentation reactions of their adduct ions were studied For comparison, the fragmentation reactions of the protonated p-methylphenyl alkyl ketones were studied as models of the ring-methylated species and the fragmentation reactions of [C6H5C(R)=OCH3](+), produced by electron impact ionization of the appropriate methyl ether, were studied as models of the O-methylated species. From these comparisons it is concluded that methyl cation addition occurs predominantly at the carbonyl oxygen under our experimental conditions. However, for methyl-cationated acetophenone there is extensive migration of the methyl group from the oxygen to the ring prior to fragmentation of the adduct.