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A novel kinetic and mechanistic study of the oxidation of pinacol by acid bromate—homogeneous catalysis of maganous ions
Author(s) -
Reddy Ch. Sanjeeva,
Sundaram E. V.
Publication year - 1983
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.550150402
Subject(s) - chemistry , pinacol , bromate , catalysis , reaction rate constant , inorganic chemistry , acetic acid , aqueous solution , reaction mechanism , catalytic cycle , medicinal chemistry , ion , photochemistry , kinetics , organic chemistry , physics , quantum mechanics
A detailed kinetic study of the Mn(II)‐catalyzed and ‐uncatalyzed oxidation of pinacol by bromate has been carried out in aqueous acetic acid media containing Hg(II) 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 Br 2 O 5 , which is inactive toward reduction. Mn(II)‐catalyzed oxidation follows first order in [oxidant], 0.5 order in [manganous ion], and variable order with respect to [pinacol]. At lower [pinacol] (0.005–0.025 M ) the order is 0.5, but at higher concentration (0.03–0.15 M ) it becomes negative (−1.0). These observations can be accounted for qualitatively by the formation of 1:1 and 1:2 Mn(II)–pinacol complexes of which only 1:1 is active toward bromate oxidation. At higher [pinacol] the ratio of 1:2 and 1:1 complexes reached 98.2. All reactions were accelerated with acidity, and the rate constant follows the h 0 function. Participation of H 2 O 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 NaClO 4 , Na 4 P 2 O 7 , and the dielectric constant of the media are also in accordance with the proposed mechanism.