Gas-phase cationic methylation of biphenyl and methylbiphenyls. A mass spectrometric and radiolytic study

Alkylation of biphenyl and methylbiphenyls by Me2F+ and Me@+ ions has been studied in the gas phase by a combination of radiolytic and mass spectrometric techniques, including chemical ionization and collisionally induced dissociation spectrometry. The results, in particular, the disproportionately high extent of ortho substitution, conform to a mechanistic model envisaging, in addition to direct substitution, the preliminary formation of an electrostatically bound adduct between the symmetric, bidentate electrophile and both rings of the substrate. Formation of the adduct increases the local concentration of the electrophile, hence the rate of alkylation at the ortho positions of biphenyl.