Kinetics and mechanism of electron transfer reactions. Osmium(VIII) catalyzed oxidation of mannitol by hexacyanoferrate(III) in aqueous alkaline medium

Abstract

The kinetics of electron transfer from mannitol to hexacyanoferrate(III), catalyzed by osmium(VIII), has been studied in alkaline medium. The substrate order is complex, whereas it is one with respect to the catalyst. The rate is independent of the concentration of oxidant. Also, the rate increases with increasing concentration of hydroxide ion in a complex manner. A kinetic rate law corresponding to the proposed mechanism has been suggested as follows: $$- { 1 \over 2} {\hbox{d}[\hbox{Fe}(\hbox{CN})_{6}^{3-}] \over \hbox{d}t}={( k_1 K_2 + k_2 K_1 K_3 [\hbox{OH}^{-}] [\hbox{Os}^{\rm VIII}] [\hbox{Mtol}]) \over 1 + K_1 [\hbox{OH}^{-}] + K_2[\hbox{Mtol}]+K_1 K_3[\hbox{Mtol}][\hbox{OH}^{-}]}$$ where [Mtol] is for mannitol. The kinetic parameters have been evaluated and the value of K1 is in agreement with the value determined spectrophotometrically.