Wacker Smidt Process

FIGURE 14.21 Wacker (Smidt) Wacker (Smidt) Process 

In the first step of the mechanism, the tetrachloropalladate ion dissociates chloride and coordinates to the alkene. In the second step, water replaces chloride. In the third step, water acts as a nucleophile and attacks the coordinated alkene (recall 

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The Wacker-Smidt process—hereafter known simply as the Wacker oxidation, reaction, or process—enjoyed considerable success, yet its use has declined dramatically over the past 10 years for at least two reasons.49 First, manufacturing plants are expensive to build and maintain because they must be constructed to withstand a corrosive environment. Second, another procedure that yields acetic acid directly from synthesis gas was developed and now supplants the Wacker-Smidt process. This newer route also uses homogeneous catalysis involving Rh and Ir complexes and will be described in Section 9-5. 

The Wacker-Smidt process utilizes a Pd catalyst to convert ethylene gas into acetaldehyde, which is then oxidized to make acetic acid. The catalytic cycle is shown in Figure 19.32. 

The Wacker or Smidt process, used to synthesize acetaldehyde from ethylene, involves a catalytic cycle that uses PdCl4. A brief outline of a cycle proposed for this process is shown in Figure 14-17. The fourth step in this cycle is substantially more complex than that shown in the figure and has been the subject of much study. ... 

For a good discussion of the Wacker-Smidt and related processes, see P. Wiseman, An Introduction to Industrial Organic Chemistry, Wiley New York, 1976, pp. 97-103. A process using palladium-impregnated silica, developed later, is superior to the homogeneously-catalyzed process described in equation 9.17. 

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. 

Although the oxidation of ethylene to acetaldehyde was known for a number of years,506 its utility depended on the catalytic regeneration of Pd(0) in situ with cop-per(II) chloride discovered by Smidt and coworkers.507 508 Air oxidation of Cu(I) to Cu(n) makes a complete catalytic cycle. This coupled three-step transformation is known as the Wacker process [Eqs. (9.97)-(9.99)]. The overall reaction [Eq. (9.100)] is the indirect oxidation with oxygen of alkenes to carbonyl compounds ... 

Discovered by Phillips in 1894,382 the oxidation of ethylene to acetaldehyde by palladium(ll) salts in an aqueous solution was developed into a commercial process about 60 years later by Smidt and coworkers at Wacker Chemie.383,384 These researchers succeeded in transforming this stoichiometric oxidation by a precious metal (equation 150) into a catalytic reaction through the reoxidation of the resulting Pd° by molecular oxygen in the presence of copper salts (equations 151-152). 

The palladium-catalyzed oxidation of ethylene to acetaldehyde (the Wacker process) was discovered by Smidt and co-workers514-518 in 1959. This process combines the stoichiometric reduction of Pd(II) with reoxidation of metal in situ by molecular oxygen in the presence of copper salts. The overall reaction constitutes a palladium-catalyzed oxidation of ethylene to acetaldehyde by molecular oxygen ... 

Smidt combined this reaction with a redox system, which led to a catalytic process ( Wacker process ) ... 

The Wacker process is carried out in an aqueous medium containing hydrochloric acid. In addition to ethylene, Smidt and coworkers carried out the oxidation of other alkenes in an acidic aqueous solution of PdCh to prepare carbonyl compoimds. After this report, a few studies on the oxidation of higher alkenes were carried out in organic media. In general, terminal alkenes are converted to methyl ketones rather than aldehydes (equation 1). 

Research to convert ethylene directly to acetaldehyde was begun in 1956 at the Consortium fiir Elektrochemische Industrie G.m.b.H., a subsidiary of Wacker Chemie G.m.b.H., under the direction of J. Smidt. The results of this research were summarized (34, 35) in two fundamental publications and in numerous patents. Smidt and co-workers first surveyed the open literature to determine what approaches had been used to oxidize ethylene and what the resulting oxidation products were. Table III summarizes the pertinent literature findings up to 1956. None of the processes published offered much promise for converting ethylene to acetaldehyde directly. In their initial experiments, Smidt et al. (34) passed mixtures of ethylene, oxygen, and hydrogen over a catalyst of palladium deposited on activated carbon, obtaining traces of acetaldehyde. They also found that the acetaldehyde yield was increased when... 

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)... 

Smidt, J. Sieber, R. Angew. Chem. Int. Ed. 1962, 1, 80-88. Wacker is not a person, but a place in Germany where Wacker Chemie developed this process. Since Hoechst AG later refined the reaction, this is sometimes called Hoechst-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" ... 

Smidt and coworkers Wacker process acetaldehyde from ethylene catalyzed by PdCb/CuCh complexes in water... 

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