Oxidation of ketones by cerium(IV) in presence of iridium(III) chloride

Kinetic data, in iridium(III) chloride catalyzed oxidation of ethyl methyl ketone (EMK) and methyl propyl ketone (MPK) by cerium(IV) perchlorate in aqueous perchloric acid medium, suggest the formation of complex C1 between cerium(IV) and organic substrate in the first equilibrium step, which in turn gives rise to another complex C2 with the catalyst. This second complex in the rate-determining step gives rise to the intermediate products. Interestingly IrCl3, which is considered to be a sluggish catalyst in alkaline media, was found to surpass the catalytic efficiency of even osmium and ruthenium in acidic media. Rate decreases in the beginning at low acid concentrations, but after reaching to a minimum it becomes directly proportional to acid concentrations. Probably on increasing the acid concentrations hydrolyzed species of ceric perchlorate gradually converts into the un-hydrolyzed species, which then accelerates the rate at higher [H+], resulting in the observed peculiar effect of hydrogen ions on the rate. Initial concentrations of cerium(IV) and acid determine the extent of reduction of cerium(IV) by water. Order of the reaction shows direct proportionality with respect to the oxidant and ketones at their low concentrations, but tends to become zeroth order at their higher concentrations. Rate of the reaction shows direct proportionality with respect to [IrCl3] while change in ionic strength of the medium does not affect the reaction velocity. Parameters such as the energy of activation, free energy of activation and entropy data suggest that methyl propyl ketone forms the activated complex more easily compared to ethyl methyl ketone.

Graphical abstractIridium trichloride in acidic medium was found more effective than RuCl3 or OsO4 in the oxidation of ketones by ceric perchlorate. Data indicate conversion of hydrolysed to unhydrolysed species of cerium with increasing acid concentrations. Dip in acid graph precisely coincides with reported data. Reduction of cerium(IV) by water was also verified. Given rate law explains all experimental findings. Figure optionsDownload full-size imageDownload as PowerPoint slide