Gas-phase alkylation and halogenomethylation by free dialkylhalogenonium and halogenomethylium ions

Abstract

Dimethylhalogenonium ions from the γ-radiolysis of methyl halides were allowed to react in the gas phase with selected substrates, containing either a π- or an n-type nucleophilic centre. The reactions were studied at CH₃X pressures ranging from 100 to 760 Torr, in the presence of a thermal radical scavenger (O₂) and variable concentrations of an added base (NH₃). Dimethylfluoronium ions readily react with all the substrates investigated, yielding methylated products, whose isomeric composition appreciably depends on the pressure of the gaseous system. Their attack on toluene leads to an intramolecular distribution (para/0.5 meta= 2.5) typical of a gas-phase electrophilic displacement process, whereas their substrate selectivity reveals the influence of steric hindrance for the most encumbered substrates. On the other hand, dimethylchloronium and dimethylbromonium ions sharply discriminate between n- and π-type basic centres, showing much greater selectivity than dimethylfluoronium ions. The reactivity and selectivity features of other electrophiles, formed in minor concentrations from the radiolysis of the halides, i.e. the halogenomethylium ions, were also examined. The mechanisms of the gas-phase alkylations are discussed, and compared with those of different gaseous electrophiles and with those of Friedel–Crafts alkylating agents. It is concluded that the behaviour of condensed-phase methylating species and gas-phase dimethyl-fluoronium ions display significant analogies, traced to their common mode of attack on aromatic substrates.