Kinetic and Mechanistic Patterns of Oxidation of Selenium(Iv)

Abstract

Kinetic and mechanistic patterns of oxidation of selenium(IV) by one-electron oxidants like neptunium(VII), cerium(IV), cobalt(III) and manganese(III), by two-electron oxidants like hydrogen peroxide, chloramine-T, N-chlorosuccinimide, and N-bromosuccinimide, and metal ion-catalyzed oxidations involving Ag(I), Ru(III) and Os(VIII) are critically reviewed. The important conclusions arising from this review are: 1. Selenium(IV) dimerises in acid medium and dimeric selenium(IV) is about four times more reactive than the monomeric form. 2. Oxidation of selenium(IV) by one-electron oxidants proceeds, in general, through two successive one-electron steps (oxidation by cobalt(III) being an exception), while that by two-electron oxidants takes place in a simultaneous two-electron step. The same trend also seems to persist in metal-ion catalyzed oxidations. 3. Oxidation of selenium(IV) by oxygenated two-electron oxidants, like hydrogen peroxide and periodate, seems to involve an oxygen atom transfer from the oxidant to the substrate. 4. Oxidation of selenium(IV) by periodate takes place in acid, neutral as well as in alkaline medium, the rate being maximal at pH 8. 5. Oxidation of selenium(IV) by the multi-electron oxidant, permanganate, is another reaction that takes place in acid, neutral and alkaline media. The rate-determining step involves a one-electron change in all three media, but the stoichiometry is different, being 5:2 in acid, 3:2 in neutral and 1:2 in alkaline medium.