A Comparative Study of Oxidation Rates of Some Industrially Important Alcohols Using Inorganic Salts

Abstract

A variety of organic oxidants have been used for the oxidation of alcohols to aldehydes/ketones under different experimental conditions. The quantitative aspects of the oxidation of alcohols have been exhaustively reported but there are few reports of the kinetic studies of the oxidations of alcohols. This paper describes the kinetics of the controlled oxidation of the secondary cyclic alcohols, borneol, isoborneol and menthol using the inorganic salts, KBrO₃, KIO₃ and KIO₄ in acidic medium. The stereoisomers, borneol and isoborneol are used in the preparation of perfumes. Menthol also finds use in medicine as a local anaesthetic and counter irritant. The oxidation rates of the perfumery alcohols were determined using pseudo first order kinetics with respect to the oxidant i.e., [alc.] >> [ox.]. The progress of the reaction was continuously monitored by titrimetric estimation of the unreacted oxidant during the course of the reaction. The oxidation rates increased with [alc.] but decreased with increasing [ox.] K₂SO₄ was used to determine the influence of ionic strength on the rate of reaction. The oxidation rate of the alcohols was found to be independent of ionic strength as borne out by the reaction mechanism suggested. From the variation of oxidation rate with temperature the thermodynamic activation parameters were evaluated and explained in terms of the dynamics of the oxidation process. For all the inorganic oxidants used, the oxidation rates showed the same sequence: borneol > isoborneol > menthol. The sequence has been explained on the basis of steric hindrance, isomeric and structural features of the alcohols under investigation. Suitable reaction mechanisms have been given on the basis of the formation of the halic acids, HBrO₃, HIO₃ and HIO₄ in acidic medium. Halic acids are strong acids as well as strong oxidizing agents.