Kinetics and mechanism of oxidation of some phenols by potassium nitrodisulphonate (Fremy’s radical) in aqueous acetate-methanol medium

Abstract

Oxidation of phenols by potassium nitrosodisulphonate (PNOS) in aqueous acetate-methanol medium <i>p</i>H 8 shows first order dependence each on [PNDS] and [phenol]. Under the experimental conditions PNDS has been found to be quite stable. However, the little self-decomposition of PNDS found on standing for longer periods has been prevented by the addition of sulphamate ion. Increase in ionic strength of the medium has been found to increase the rate f oxidation. Increase in polarity of the solvent also has increased the reaction rate and the plot of log (rate) versus I/D has been found to be linear with a negative slope. Rate of oxi dation increases with increase in <i>p</i>H, reaching a maximum at a <i>p</i>H equal to ~ the <i>pK_a </i> of phenol. On the basis of kinetic results, a mechanism in which a direct reaction between phenoxide ion and PNDS leading to the electron transfer from phenoxide ion has been proposed. The electron-donating substituent in the <i>para</i>-position of the phenyl ring is found to accelerate the rate of electron transfer reaction supporting the above mechanism.