Selective Ethylene Oxidation by the Wacker Process

The Wacker process was the first organometallic catal5Tic oxidation [15, 16]. It was developed 1959 by Smidt and co-workers at the Wacker Consortimn for Industrial Electrochemistry in Munich and is mainly used for the production of acetaldehyde from ethylene and oxygen (Eq. 3-5) 

The process proceeds by homogeneous catalysis on PdCl2. It had been known much eaher that solutions of Pd complexes stoichiometrically oxidize ethylene to acetaldehyde, but the crucial discovery was the exploitation of this reaction in a catalytic cycle. A closed-cycle process was developed in which an excess of the oxidizing agent Cu re-oxidizes the palladium formed in the process without its depositing on die reactor walls. The Cu formed in the redox process is re-oxidized to Cu by oxygen. The reaction steps are described by Equations 3-6 to 3-8. 

CH2=CH2 + H2O + PdCl2 CH3CH0 + Pd + 2HC1 Pd + 2CuCl2 PdCl2 + 2CuCl 

A mechanistic study of the Wacker process involving detailed stereochemical investigations showed that CO bond formation occurs with trans stereochemistry that is, the ethylene molecule is not attacked intramolecularly by a coordinated water molecule. Instead, an additional, uncomplexed water molecule attacks the double bond. 

The formation of B by addition of water is followed by two further steps in which the coordinated alcohol is isomerized. First, a j -hydride elimination gives C, and then an insertion reaction forms D. The elimination of the product acetaldehyde and H gives Pd , which is oxidized back to Pd by Cu V02. With the exception of this last step, the oxidation state of palladium in all steps of the cycle is +2 [7]. 

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