Gas phase methylation of the dihydroxybenzenes

Abstract

Tandem mass spectrometry has been used to determine the site of gas-phase methylation of o-, m-, and p-dihydroxybenzene under chemical ionization conditions. The reagent ions were CH 3ClCH 3 + generated from methyl chloride, and CH 3FCH 3 + generated from methyl fluoride. Ring methylation is favored over substituent methylation by approximately 3:1 with both reagent gases. However, the behavior of the meta compound with methyl chloride is exceptional, yielding almost exclusive methylation in the ring. The less reactive dimethylchloronium ion apparently reacts more selectively than its fluorine analog and therefore preferentially generates the thermodynamically more stable adduct. The enhanced stability of the ring bonded form in the meta case follows from simple resonance considerations. The pressure of the reagent gas generally had a significant effect on the ratio of ring-to-substituent methylation. This is accounted for in terms of the approach to equilibrium achieved at elevated pressure. In addition to the ring- and substituent-bonded structures, the presence of a loosely bound ion is also indicated from the collision-induced dissociation spectra of the ion/molecule products. This is particularly evident in the products of reaction of catechol and hydroquinone with methyl fluoride at low pressure. In methylation, as well as in the accompanying halomethylation reactions, isomer identity is retained in the adduct ions.