Permercuration of the
benzoic acids XC6H4CO2H (X = o-Me, F, Cl, or Br; m- Me, F, Cl, Br, CF3,
NO2 or OMe; or p-Me, F, Cl, Br, CF3 or NO2) and 2,6- X2C6H3CO2H
(X = Me, Cl, or Br) with molten mercuric
trifluoroacetate at c. 180-245° followed by bromodemercuration gave the
corresponding perbromobenzoic acids XC6Br4CO2H
or 2,6-X2C6Br3CO2H, together with
the corresponding perbromobenzenes C6Br5X or m-X2C6Br4
which were formed owing to decarboxylation under permercuration conditions.
Similar treatment of the acids XC6H4CO2H (X =
o-NO2, CF3, or OMe; or p-OMe) and 2,6-F2C6H3CO2H
gave only the appropriate perbromobenzenes. Possible mechanisms for
permercuration induced decarboxylation are proposed on the basis of the effect
of substituents on yields of perbromobenzoic acids and perbromobenzenes.
Decarboxylation occurs more widely under permercuration conditions than on
pyrolysis of mercuric carboxylates. Regiospecific mercuration meta to the
carboxyl group and not decarboxylation occurred on thermal decomposition of mercuric
p-methoxybenzoate.