Gas-phase alkylation and halomethylation of free dialkylhalonium and halomethylium ions

Dimethylhalonium ions Me2X+ (X = F, Cl, Br) were prepd. in the gas phase by γ-radiolysis of MeX, and their reactions were studied with substrates contg. either a π- or n-type nucleophilic center, e.g. benzene, toluene, xylene, MeOH. The reactions were studied at MeX pressures 100-760 torr, in the presence of O2 as a thermal radical scavenger, and variable concns. of added NH3. Me2F+ ions react readily with all the substrates studied, giving methylated products whose isomeric compn. depends on the pressure of the gaseous system. Attack by Me2F+ on toluene leads to an intramol. distribution (para/0.5 meta = 2.5) typical of a gas-phase electrophilic displacement reaction, whereas the selectivity towards substrates shows the influence of steric hindrance for the most encumbered substrates. Me2Cl+ and Me2Br+ ions sharply discriminate between n- and π-type basic centers, showing much greater selectivity than Me2F+. The reactivity and selectivity features were also studied for other electrophiles, e.g. +CH2X, formed in minor concns. from the radiolysis of MeX. The mechanisms of the gas-phase methylations are compared with those of different gaseous electrophiles and with those of Friedel-Crafts alkylating agents. The behavior of condensed-phase methylating species and gas-phase Me2F+ ions show significant analogies, traced to their common mode of attack on arom. substrates.