Green synthesis of benzaldehydes by the selective oxidation o benzyl alcohols using hexacyanoferrate (III) and bromate under phase transfer catalysis

Abstract

The selective oxidation of benzyl alcohols and substituted benzyl alcohols was carried out in greener solvents like ethyl acetate and toluene by hexacyanoferrate (III) and bromate ions under phase transfer catalysis. Various phase transfer catalysts like tetrabutylphosphonium bromide, tetrabutylammonium bromide, tetrabutylammonium hydrogen sulphate, cetyltrimethylammonium bromide and tricaprylmethylmmonium chloride were used. A pinch of sodium tungstate dihydrate is added as a co-catalyst when bromate is used an oxidant and hexacyanoferrate (III) is used in presence of mineral acids. The products obtained were precipitated as their 2,4- dinitrophenylhydrazone, purified, weighed and the yield was found to be > 90 %. The products were recrystallized in ethanol and analyzed by melting point and by various spectral techniques. It was proved that corresponding benzaldehydes were formed and there was no further oxidation to corresponding acids. All the catalysts were found effective in bringing out the reaction but based on yield and ease of reaction, the order of reactivity is tricaprylmethylammonium chloride > tetrabutylphosphonium bromide > tetrabutylammonium bromide > tetrabutylammonium hydrogen sulphate > cetyltrimethylammonium bromide. The selective oxidation of benzyl alcohols is very difficult to proceed in organic solvents without the aid of a phase transfer catalyst. Yield of products is found to be slightly more in ethyl acetate than that in toluene due to difference in polarity.