Kinetic Studies of Transition Metal Ion Catalyzed Oxidation of Some Fragrance Alcohols

摘要

The controlled oxidation of the aliphatic alcohols 2-propanol, 2-butanol and 3-methyl-1 -butanol to the corresponding carbonyl compounds has been carried out using Ce (IV) in acidic medium in the absence and presence of transition metal ions of the first series. The aliphatic alcohols are widely used as diluents in the perfumery industry. The oxidation reaction was monitored under pseudo unimolecular conditions with respect to [Ce (IV)] in the temperature range 303-318 K. Aliquots of the reaction were withdrawn at regular time intervals, quenched using ice and the unreacted oxidant was estimated titrimetrically using standard ferrous ammonium sulphate with ferroin as indicator. The pseudo first order rate constants were determined from the linear plots of log (a-x) versus time. It was observed that the rate increases with alcohol concentration but decreases with Ce(IV) concentration. This has been attributed to the formation of unreactive dimeric [Ce(IV)J3 at higher concentration of Ce(IV). Potassium sulfate was used to study the effect of ionic strength on the oxidation rate. The thermodynamic activation parameters were determined from the effect of temperature on the oxidation rate. The Ru(VIII), Os(VIII) and Cr(VI) ions have been exhaustively used to catalyse a variety of organic reactions. In the present study, relatively low-cost metal ions of the first transition series have been used as effective catalysts for the oxidation of the fragrance alcohols under study. The reaction mechanism suggested for the oxidation process involves intermediates with hypervalent ions i.e., M(III). The catalytic efficiency of the metal ions is based on the stability of the complexes formed as reaction intermediates which in turn depends on the charge density of the metal ions involved. We have observed some discrepancies as the experimentally determined sequence of catalytic efficiency of metal ions does not follow the theoretically expected sequence. Suitable reaction mechanisms have been suggested for the oxidation of the alcohols in the absence and presence of transition metal ions. In the absence of metal ions, the oxidation rates of aliphatic alcohols the sequence: 2-propanol > 2-butanol > 3-methyl-1 -butanol. The relative rates of oxidation of alcohols have been discussed and explained on the basis of structures. steric factors and isomeric characteristics of the perfumery alcohols under study.