Thallation durene

Indeed the diversion to side products during thallation coincides with the direct obsovation of the arene radical cation as a transient intermediate both by UV-visible and ESR spectroscopy. A similar dichotomy between the products of mercuration and thallation exists with durene, albeit to a lesser degree. Finally no discrepancy is observed with mesitylene, nuclear substitution occuiring exclusively in both mercuration and thallation. Such a divergence between mercuration and thallation can be reconciled by the formulation in Scheme 6 if they difier by the extent to which di sive separation (ki) occurs in equation (31). All factors being the same, diffusive separation of the radical pair from thallium(III) should... 

The study of mercuration and thallation provides a shaip focus on the experimental delineation of stepwise and concerted mechanisms for arene activation. Thus the unequivoc demonstration of arene radical cations as key intermediates in thallation, particularly of durene and pentamethylbenzene, is consistent with a stepwise (electron-transfer) mechanism for arene activation (compare Scheme 6 and equation 39). 

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