Abstract
The mercuration and thallation of benzene and substituted benzenes was studied with mercuric and thallic trifluoroacetate, respectively, in trifluoroacetic acid. With the shortest reaction time (1 sec) at 0°, the relative rate of mercuration of toluene compared to that of benzene was 17.5, with the isomer distribution in toluene of: ortho, 17.4%; meta, 5.9%; and para, 76.7%. The isomer distribution in toluene varied with the reaction time, significantly more at 25° than at 0°. The competitive thallation of benzene and toluene with thallic trifluoroacetate in trifluoroacetic acid at 15° showed the relative rate, toluene/benzene, to be 33, with the isomer distribution in toluene of: ortho, 9.5%; meta, 5.5%; and para, 85.0%. With increasingly higher reaction temperatures in both mercuration and thallation reactions of aromatics, isomerization (both intramolecular and intermolecular) within the relevant ortho- and para-metallated intermediate ions and/or of the isomers becomes more important. Competitive rates and isomer distributions of mercuration and thallation of benzene and substituted benzenes were also determined. The predominant para substitution in both mercuration and thallation of methylbenzenes reflects, besides some steric factors, the strong stabilizing effect of para methyl groups on the arenium ion intermediates. Under predominantly kinetically controlled conditions, no anomalous increase in the amount of meta substitution was observed.
Keywords: electrophilic substitution, selectivity
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