Liquid-phase reactions of free methyl cations from the decay of methane-t4. Alternative alkylation sites in halobenzenes.

The reaction of free methyl cations, generated from the decay of multitritiated methane dissolved in liquid halobenzenes, gives the corresponding labeled halotoluenes together with methyl halides, toluene, and minor amounts of decomposition products. The attack of the reagent on the n-type nucleophilic centers displays a remarkable selectivity, following the trend Br > CI > F, in agreement with the HSAB principle. Ring alkylation is characterized by an appreciable positional selectivity, which is in contrast with the scarce substrate discrimination. The reactivity ratios deduced from the competition experiments depend on the composition of the system, the selectivity of the reagent increasing with the concentration of halobenzenes. This trend is explained by the intervention of methylphenylhalonium ions, which, besides being intermediates for the formation of methyl halides. are also methylating agents and are more selective than free methyl cations.