Ethene Wacker process

Wacker process The oxidation of ethene to ethanal by air and a PdClj catalyst in aqueous solution. The Pd is reduced to Pd in the process but is reoxidized to Pd " by oxygen and Cu. ... 

A common property of coordinated alkenes is their susceptibility to attack by nucleophiles such as OH , OMe , MeC02, and Cl , and it has long been known that Zeise s salt is slowly attacked by non-acidic water to give MeCHO and Pt metal, while corresponding Pd complexes are even more reactive. This forms the basis of the Wacker process (developed by J. Smidt and his colleagues at Wacker Chemie, 1959-60) for converting ethene (ethylene) into ethanal (acetaldehyde) — see Panel overleaf. 

In the Wacker process, the coordinated ethene undergoes nucleophilic attack by OH-. In the course of the redox reaction, palladium(II) is reduced... 

Acetaldehyde is the product of the Wacker process. At the end of the fifties oxidation of ethene to ethanal replaced the addition of water to acetylene, because the acetylene/coal-based chemistry became obsolete, and the ethene/petrochemistry entered the commercial organic chemicals scene. The acetylene route involved one of the oldest organometallics-mediated catalytic routes started up in the 1920s the catalyst system comprised mercury in sulfuric acid. Coordination of acetylene to mercury(II) activates it toward nucleophilic attack of water, but the reaction is slow and large reactor volumes of this toxic catalyst were needed. An equally slow related catalytic process, the zinc catalysed addition of carboxylic acids to acetylene, is still in use in paint manufacture. 

Catalytic Oxidation of Ethene to Acetaldehyde and Acetic Acid. -Evnin et al120 studied Pd-doped V2 Os catalysts for the vapor-phase oxidation of ethene to acetaldehyde in a heterogeneous type of Wacker process. From a mechanistic study they establish a redox mechanism with Pd both as the site of the ethene oxidation and of the reoxidation of the catalyst. On the basis of the role of the V4+ ions proposed by these authors, Forni and Gilardi121 substantiated this mechanism by adding tetra- and hexa-valent dopants to the V2 05 and studying the effects on the catalytic reaction. 

In my opinion (and I am convinced that Ernst Otto Fischer would have agreed), Walter Hafner was one of the best Ph.D. students, probably the best Fischer ever had. After he finished his doctoral research, he joined the Consortium filr Elektrochemische Industrie, a research subsidiary of Wacker-Chemie in Munich, where he laid the foundations for the palladium-catalyzed oxidation of ethene to acetaldehyde, the Wacker process. This process has been licenced to various chemical companies all over the world and initiated an evergrowing area of synthetic organic chemistry. Walter Hafner retired in 1992 and died in 2004. 

Suzuki reaction the palladium-catalyzed reaction of an aryl or vinyl halide with an aryl boronic acid to give an arylated or vinylated arene. In some cases, primary alkyl halides can react in place of the aryl or vinyl halides Wacker process the palladium-catalyzed oxidation of ethene to acetaldehyde by oxygen... 

The oxidation of ethene by palladium salts in water to give acetaldehyde has been known for 100 years see Oxidation Catalysis by Transition Metal Complexes). It is often called the Wacker Process, after Wacker Chemie GmbH, which first developed the process. The key steps in this oxidation are shown in Scheme 2. Palladium catalyzes the nucleophilic addition of water to ethene, leading to the reduction of Pd to Pd°. Then the palladium is reoxidized back to Pd with Cu salts, giving Cu which in turn is oxidized by oxygen. 

The point has been made that the conditions of p-chloroethanol formation are not the same as used for the Wacker oxidation. Cu Pd chlorine-bridged dimers are likely reactants under higher [Cl ] reaction conditions, which may lead to a different reaction mechanism. However, a second stereochemical study also obtained results consistent with trans hydroxypaUadation. When cfr-l,2-dideuteroethene is oxidized in water with PdCl2 under a CO atmosphere, the product is tran5 -2,3-dideutero-jS-propiolactone (Scheme 37). The reaction conditions were, once again, not identical with standard Wacker process conditions, since the solvent was acetonitrile water, the temperature was —25°C, the bis-ethene PdCl2 complex was used, and there was no excess Cl present. Nevertheless, it is clear that, under many reaction conditions, a trans addition of water onto ethene coordinated to Pd is the favored reaction stereochemistry. 

We now study one cycle in detail to illustrate the notations. Figure 26.2 shows a simplified catalytic cycle for the Wacker process which converts ethene to acetaldehyde (equation... 

Wacker process /wak-er/ An industrial process for making ethanal (and other carbonyl compounds). To produce ethanal, ethene and air are bubbled through an acid solution of palladium(II) chloride and cop-per(ll) chloride (20-60°C and moderate pressure) ... 

Wacker process A process for the manufacture of ethanal by the air oxidation of ethene. A mixture of air and ethene is bubbled through a solution containing palla-dium(II) chloride andcopper(II) chloride. The Pd + ions form a complex with the ethene in which the ion is bound to the pi electrons in the C=C bond. This decreases the electron density in the bond, making it susceptible to nucleophilic attack by water molecules. The complex formed breaks down to ethanal and palladium metal. The Cu + ions oxidize the palladium back to Pd +, being reduced to Cu+ ions in the process. 

Oxidation of ethene with O2 in aqueous solution catalyzed by Pd(II) chlorides was developed into an industrial-scale process for production of acetaldehyde (212). This reaction, known as the Wacker process, requires the presence of CuCl2 and can be described in a very simplified way by the following equations. 

One factor which influences the preference for heterogeneous catalysts in industry is the relative ease of separation of products from catalyst. Homogeneous catalysts often require a solvent. If this is an organic chemical, additional cost is incurred. There are also the problems of product separation and solvent recovery. The homogeneously catalysed Wacker process for oxidation of ethene (p. 380) does not suffer from these disadvantages because the product, acetaldehyde, is very volatile and water (containing some acetic acid) can be used as the solvent. 

Fig. 12.13 Suggested mechanism for the oxidation of ethene to acetaldehyde (Wacker process).

The Wacker oxidation was discovered by Smidt and co-workers at Consortium fiir Electrochemie (a subsidiary of Wacker Chemie and Farbwerken Bayer). It is actually a combination of known reactions and thus not a catalytic reaction in the strictest sense (Scheme 1). The first and most basic reaction, the oxidation of ethene in aqueous solution was first discovered by Phillips in 9AP The precipitation of palladium metal from a palladium(II) chloride solution was used as a test for olefins. However, it was the discovery by Smidt and co-workers that the Pd(0) formed could be regenerated by cupric chloride that made the reaction a commercial success. The final step, the oxidation of CuCl to CuCl2 is one of the fastest reactions in inorganic chemistry, The three reactions add up to the simple air oxidation of ethene to ethanal. At one point over two billion pounds a year of ethanal was produced by the Wacker process. Presently, the Monsanto acetic acid process has largely replaced the Wacker procednre.t" ... 

In the presence of a palladium catalyst [commonly Pd(II)Cl2] oxygen (O2) reacts with alkenes to yield carbonyl compounds. This process (the Wacker Process) utilizes the plant growth regulator ethene (ethylene, CH2=CH2) as the exclusive raw material for the commercial production of ethanal (acetaldehyde, CH3CH=0). The overall reaction is shown in Scheme 6.8 and it is important to note that only a trace... 

Scheme 6.8. An abbreviated cartoon depiction of what might be occurring in the oxidative conversion of ethene (CH2=CH2) to acetaldehyde (CH3CH=0) by the Wacker Process. Unspecified ligands to Pd might include H2O, Cl, and H.

Interestingly, it is often convenient during the manufacturing process of ethanoic anhydride [acetic anhydride, (CH3C0)20] to generate the ethanal (acetaldehyde, CH3CHO) itself on the same site in a separate, palladium(II) chloride (PdCy, catalyzed oxidation of ethene (ethylene, CH2=CH2) as shown in Scheme 9.133 (the Wacker process). ... 

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