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ABSTRACT 
This thesis deals "with a kinetic investigation on the 
thallation of simple aromatic compounds in two solvent systems. 
(i) In trifluoroacetic acid at 25°C the reaction of thallium 
trifluoroacetate with benzene and toluene is too rapid for observa­
tion ty conventional kinetic procedures. It has been found that the 
addition of acetic acid and a wide range of other diluents causes a 
lowering of the rate, and the reaction has been followed in trifluoro­
acetic acid:acetic acid mixtures. Toluene in this system is found to 
react with the thallium species at a rate seven times greater than 
benzene and shows an isomer ratio 10 percent ortho, 3 percent meta, 
and 87 percent para. A primary isotope effect for both toluene and 
benzene has been recorded. Enhanced positional and substrate selec­
tivity has been observed for a variety of electrophilic aromatic 
substitution reactions in trifluoroacetic acid. The activation para­
meters of thallation are of interest in this connection since they 
are sensitive both to temperature and dilution ty acetic acid. It 
has been found that activation energy decreases to low values for 
solutions with small amounts of acetic acid but at the same time the 
entropy of activation becomes less favourable. 
(ii) In 80 percent acetic acid 20 percent water catalysis of 
the reaction ty perchloric and sulphuric acid has been studied. 
Catalysis does not depend on the acidity of the medium but is consis-
tant with a scheme dependent on an equilibrium in which successive 
acetate anions are replaced ty the anions of the catalyst. All the 
species so produced are effective electrophiles but the one which has 
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all the acetate anions replaced is the most reactive The isomer 
distribution for sulphuric acid cata3ysis has also been studied and 
shows that there is increased selectivity towards para substitution 
at higher catalyst concentrations. 
Thallation shows evidence of reversibility in both trifluoro-
acetic acid and in mineral acid catalysed reactions in acetic acid-
water. In general the mechanism of thallation appears quite similar 
to that of the corresponding mercuration reactions.