Reactions of Ethylene with Rhodium (III) Chloride. Part III. The Catalytic Oxidation of Ethylene to Acetaldehyde in Aqueous Hydrochloric Acid Solutions Containing Pentachloroaquorhodate(III)
In the presence of iron(III) or other oxidants, aqueous acid chloride solutions of RhCl5(H2O)2− catalyze under mild conditions the oxidation of ethylene to acetaldehyde. The kinetics of the reaction measured by gas-uptake techniques indicate the presence of both ethylene dependent and independent paths. Besides fully protonated anions, hydroxy species such as RhCl5 (OH)3− and RhCl4(OH)(H2O)2−, although present in very small concentrations, are significantly reactive towards ethylene. A mechanism, based on that postulated for a similar palladium(II) system in the well-known Wacker process, is presented. Under our reaction conditions the slow steps in the rhodium system involve formation of π-complexes in the ethylene dependent paths and the formation of tetrachlororhodate(III) complexes in the ethylene independent paths. Iron(III) regenerates the rhodium(III) catalyst by oxidation of the rhodium(I). Rate constants are estimated for the various reaction paths.