Kinetics of Pindolol Oxidation by Peroxodisulfate

The kinetics of oxidation of pindolol by peroxodisulfate (PDS) in sulfuric acid and 40%(v/v) methanol + water solvent has been investigated. The pH profile of the rate constant was also investigated. It has been found that the reaction proceeds only in the pH range 0 to 4. A mechanism was proposed for the oxidation reaction; the reaction starts by the attack of peroxodisulfate (PDS) to form an indoleninic species that can rearrange into an indoxyl species. Once the indoxyl intermediate is formed, it can follow different paths leading to different products, depending on the acidity and PDS concentration. At low acid concentration, oxipindolol is formed whereas at high acid concentration dioxipindolol forms instead. Dioxipindolol can dimerize to form the indigo form of pindolol. A polar mechanism was proposed for the oxidation reaction, and the thermodynamic parameters were determined.