Oxidation of Cyclic Alcohols by Hexacyanoferrate(III) in Alkaline Medium in the Presence of Rhodium(III) Chloride as a Homogeneous Catalyst

ABSTRACT

Un-catalyzed oxidation of cyclic alcohols viz. cyclopentanol and cyclohexanol by hexacyanoferrate(III) in aqueous alkaline medium does not proceed to measurable extent even after 98 hr. Addition of traces of RhCl3, used as a homogeneous catalyst accelerates the reaction resulting in the formation of corresponding dicarboxylic acids. Correlating this result with previously reported results in the un-catalyzed oxidation of cyclopentanone to dicarboxylic acid indicates that hexacyanoferrate(III) in alkaline medium is incapable to oxidize cyclopentanol. Addition of catalyst initiates the initial conversion of cyclic alcohol into cyclic ketone which in turn is oxidized to dicarboxylic acid. Rate follows direct proportionality at low concentrations of catalyst, reaching to a maximum with increasing catalyst concentrations but further increase in RhCl3 concentration retards the reaction velocity. As the amount of catalyst is increased in the reaction mixture it also increases the amount of hydrochloric acid in which RhCl3 was prepared, resulting in the decrease in pH of the medium. At a particular pH, reactive species of catalyst Rh(OH)3(H2O)3, converts into the un-reactive RhCl3(H2O)3 and thus the reaction velocity decreases. Rate of the reaction shows first order kinetics in [OH-] and [cyclic alcohol], but it is one changing to zero order in oxidant concentrations. Increase in chloride ions and hexacyanoferrate(II) concentrations respectively increases and decreases the reaction velocity. Entropy, free energy and enthalpy values were calculated.

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