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

Abstract

The oxidation of benzyl alcohol to benzaldehyde with potassium permanganate (KMnO 4) 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 KMnO 4 was taken as the solid reactant. KMnO 4 is not soluble in benzene in normal condition but the addition of 18-crown-6 forms a complex with KMnO 4 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 KMnO 4. 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 × 10 6 respectively.