Green synthesis
of benzaldehydes by the selective oxidation o benzyl alcohols using hexacyanoferrate
(III) and bromate under phase transfer catalysis
Bashpa P. and Bijudas K.
Res. J. Chem. Environ; Vol. 26(1); 28-33;
doi: https://doi.org/10.25303/2601rjce2833; (2022)
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.
