Gas-phase aromatic substitution. Isomerization of gaseous arenium ions from the attack of D2T+ on dihalobenzenes

The isomerization of gaseous, unsolvated arenium ions from the attack of radiolytically formed D2T+ on dihalobenzenes was investigated in the gas phase at atm. pressure, irradiating with the 60Co γ radiation homogeneous systems contg. an excess of D2 gas labeled with DT and low concns. of the arom. substrate and a thermal radical scavenger (O2). The isomeric composition of the tritiated products from RC6H4R1 (R = R1 = F, Cl; R = F, R1 = Cl, Br) was detd. Gas-phase tritiodeprotonation of dihalobenzenes by D2T+ was accompanied by isomerization, the rate of which depended on the nature of the substituents. The migratory aptitude of the halogens increased in the order: F << Cl < Br. The position of the substituents was also significant as the meta isomer of a given dihalobenzene exhibited less extensive halogen migration than did the ortho and para isomers. The isomerization process is intramol. and involved consecutive 1→2 halogen shifts of the gaseous arenium ions formed from the exothermic triton transfer from D2T+ to dihalobenzenes. The mechanism of the isomerization was compared with those of similar reactions promoted in soln. by strong Broensted acids.