Selective Oxidation of Benzyl Halides to Benzaldehydes by Phase Transfer Catalyzed Monochromate Ions in Non-Polar Solvents

Benzyl chloride, benzyl bromide and their para substituted derivatives were selectively oxidized to corresponding benzaldehydes by acidified monochromate in toluene and ethyl acetate with the help of phase transfer catalysts like tetrabutylphosphonium bromide (TBPB), tetrabutylammonium bromide (TBAB), tetrabutylammonium hydrogen sulphate (TBAHS), cetyltrimethylammonium bromide (CTMAB) and tricaprylmethylammonium chloride (TCMC). The reaction was carried out by simple magnetic stirring for about 2 h at 60 ºC. The products were recrystallized and analyzed by infrared and UV-visible spectral techniques. Benzaldehyde and substituted benzaldehydes were formed in good yield (> 90%) on oxidation of benzyl chlorides and benzyl bromides. The reaction is proved to be highly selective due to the absence of acids or any other products during the reaction. All the phase transfer catalysts were highly effective in bringing out the reaction in both the solvents. This is highly significant as the oxidation reaction will not occur in non-polar solvents without the presence of phase transfer catalysts.