Selective production of benzaldehyde by permanganate oxidation of benzyl alcohol using 18-crown-6 as phase transfer catalyst

The oxidation of benzyl alcohol to benzaldehyde with potassium permanganate (KMnO4) was studied in a batch reactor using 18-crown-6 (crown ether) as phase transfer catalyst in a solid–liquid system. Benzene was used as the solvent and KMnO4 was taken as the solid reactant. KMnO4 is not soluble in benzene in normal condition but the addition of 18-crown-6 forms a complex with KMnO4 and makes it soluble in benzene. The oxidation reaction was studied at different degrees of agitation, temperature, catalyst concentration and mole ratio of benzyl alcohol to KMnO4. The reaction usually occurs in two steps where the first step is the oxidation of benzyl alcohol to benzaldehyde and the second step is the formation of benzoic acid from benzaldehyde. In the chosen reaction condition benzaldehyde was obtained as the only product for all the cases. However, at higher temperature and substantial amount of catalyst concentration benzoic acid was obtained along with benzaldehyde. The initial rate was found to increase with increase in concentration of catalyst (18-crown-6) and benzyl alcohol. A semi-empirical model for the reaction was depicted to illustrate the proposed mechanism. The activation energy and frequency factor of the reaction were found to be 9.149 kJ/mol and 1.85 × 106 respectively.

The oxidation of benzyl alcohol to benzaldehyde by potassium permanganate has been carried out in organic phase using 18-crown-6 as catalyst. Benzaldehyde was found to be the only product as the chosen reaction condition. However with increased catalyst concentration and temperature the formation of benzoic acid along with benzaldehyde has been noted. A kinetic model has been proposed.Figure optionsDownload as PowerPoint slide