Oxidation of cyclic ketones by alkaline hexacyanoferrate(III) catalyzed by rhodium(III)

The kinetics of the reaction between cyclopentanone and cyclohexanone with hexacyanoferrate(III) ion catalyzed by rhodium(III) chloride in alkaline medium has been investigated at four temperatures. The data show that the reaction follows first order kinetics with respect to hydroxide ion and cyclic ketone concentrations while the rate shows direct proportionality with respect to low concentrations of hexacyanoferrate(III) ion, which tends to become zeroth order at higher [oxidant]. Increase in rhodium(III) chloride concentrations increases the rate in the beginning but after reaching to a maximum further increase in catalyst concentration retards the rate, thus showing a peculiar nature. In both cases, decrease in rate after a fixed maximum is attributed to a particular pH of the medium below which un-reactive species of catalyst retards the velocity. Change in pH of the medium shows that decreasing pH decreases the rate with breaks at specific points. Increase in ionic strength of the medium has a positive effect, while increase in hexacyanoferrate(II) decreases the reaction velocity. Various thermodynamic parameters were calculated.

Graphical abstractIn the oxidation of cyclic ketones by alkaline hexacyanoferrate(III) catalyzed by rhodium(III) rate decreases after a fixed catalyst concentration due to transformation of reactive species of catalyst into the un-reactive one at lower pH of the medium. Hexacyanoferrate(III) is reduced before the slow step with dicarboxylic acid as the oxidation products. Final rate law may be given as-Figure optionsDownload full-size imageDownload as PowerPoint slide