Synthesis of perbromobenzoic acids and perbromobenzenes from aromatic carboxylic acids by permercuration and bromodemercuration

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.