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Reaction of rhodium III chlorides with ethylene in aqueous HC1 solution. Kastner, Michael Robin


In the presence of iron (III) and other oxidants, HCl solutions of RhCl₃.3H₂O under mild conditions catalyze 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 steps. 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 Pd(II) system is presented. This involves the rearrangement of a rhodium (III) hydroxy π - ethylene complex to a σ - complex, followed by the production of acetaldehyde and rhodium (I). However, unlike in the Pd(II) system where the rate determining step is the conversion of the π - to σ-C₂H₄ complex, the rate determining step in the Rh(III) system is thought to involve the production of the π- complex. Iron (III) oxidizes Rh(I) back to Rh(III), giving the net reaction: C₄H₄ + H₂O + 2Fe(III) Rh(III) ⇥ CH₃CHO + 2H+ + 2Fe(II)

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