Thallation reactions reversibility

Aromatic thallation has been shown to be a reversible electrophilic substitution reaction with an energy of activation of approximately 27 kcal/mole and an extremely large steric requirement 153). The consequence of the latter feature of aromatic thallation is that there is a significant preference for para substitution in thallation of simple monosubstituted benzeno id compounds. It will be seen by examination of Table VI that the amount of para substitution increases as the size of the substituent increases (for... 

The thallium is initially introduced into the position that is most reactive toward electrophiles, but the reaction is reversible, so that heating gives a mixture having a composition governed by the relative thermodynamic stability of the various possible isomers. In some cases, then, thallation followed by isomerization can lead to a product with an orientation different from that obtained by direct electrophilic substitution." The arylthallium compounds can also be converted to phenols via a procedure that involves oxidation and hydrolysis" ... 

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