SELECTIVE SYNTHESIS OF BENZOPHENONES BY PHASE TRANSFER CATALYZED MONOCHROMATE OXIDATION OF BENZHYDROLS IN NON-POLAR MEDIA

Abstract

Selective synthesis of benzophenone and para substituted benzophenones from benzhydrol and corresponding para substituted benzhydrols has been carried out by using acidified monochromate ions as an oxidant in organic medium under phase transfer catalysis. The oxidation reaction was studied at 50˚C by magnetic stirring for about two hours in ethyl acetate and toluene as solvents using tetrabutylphosphonium bromide, tetrabutylammonium bromide, tetrabutylammonium hydrogen sulphate, cetyltrimethylammonium bromide and tricaprylmethylammonium chloride as phase transfer catalysts. The reaction is found to be highly selective since no traces of benzoic acids have been detected and the yield of corresponding benzophenones is above 90%. The products obtained on oxidation were precipitated as 2,4-dinitrophenylhydrazone and the recrystallized products were characterized by melting point and by infra-red analysis. From the results obtained, it was found that ethyl acetate is the better solvent and tricaprylmethylammonium chloride is the suitable phase transfer catalyst for this oxidation. The reaction is practically very difficult to proceed in the absence of phase transfer catalyst which proves the significance of phase transfer catalysis technique.