A novel kinetic and mechanistic study of the oxidation of pinacol by acid bromate—homogeneous catalysis of maganous ions

✍ Scribed by Ch. Sanjeeva Reddy; E. V. Sundaram

• Publisher: John Wiley and Sons
• Year: 1983
• Tongue: English
• Weight: 511 KB
• Volume: 15
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.550150402

✦ Synopsis

A detailed kinetic study of the Mn(1I)-catalyzed and -uncatalyzed oxidation of pinacol by bromate has been carried out in aqueous acetic acid media containing Hg(I1) ions. The uncatalyzed reaction exhibits 1.5 order that is, 0.5 order in [pinacol] and 1.0 in [bromate]. A decrease in k 1 by increasing [bromate] has been accounted for due to the formation of Br205, which is inactive toward reduction. Mn(I1)-catalyzed oxidation follows first order in [oxidant], 0.5 order in [manganous ion], and variable order with respect to [pinacol]. At lower [pinacol] (0.005-0.025M) the order is 0.5, but a t higher concentration (0.03-0.15M) it becomes negative (-1.0). These observations can be accounted for qualitatively by the formation of 1:l and 1:2 Mn(I1)-pinacol complexes of which only 1:l is active toward bromate oxidation. At higher (pinacol] the ratio of 1:2 and 1:l complexes reached 98.2. All reactions were accelerated with acidity, and the rate constant follows the ho function. Participation of H20 in the rate-limiting step and a free-radical mechanism were proposed for the manganous-ion-catalyzed reaction, whereas for the uncatalyzed reaction this was not true. The effects of NaC104, N Q z 0 7 , and the dielectric constant of the media are also in accordance with the proposed mechanism.