Wacker oxidation of ethene

Other methods for the preparation of acetic acid are partial oxidation of butane, oxidation of ethanal -obtained from Wacker oxidation of ethene-, biooxidation of ethanol for food applications, and we may add the same carbonylation reaction carried out with a cobalt catalyst or an iridium catalyst. The rhodium and iridium catalysts have several distinct advantages over the cobalt catalyst they are much fester and fer more selective. In process terms the higher rate is translated into much lower pressures (the cobalt catalyst is operated by BASF at pressures of 700 bar). For years now the Monsanto process (now owned by BP) has been the most attractive route for the preparation of acetic acid, but in recent years the iridium-based CATTVA process, developed by BP, has come on stream. 

Perhaps the best-known example of oxidation by the Group 10 metals is the Wacker oxidation of ethene 31 (R = H) to ethanal 32 (R = H) (Scheme 1.9). 

The Wacker oxidation of ethene to ethanal is an important industrial process for the oxygenation of a hydrocarbon feedstock. Essentially, the same process may be used to convert 1-alkenes to methyl ketones. The stoichiometry of the process is shown in Figure 23.23. The reaction is catalytic in both palladium and copper the ultimate oxidant is (inexpensive) molecular oxygen. A proposed mechanism is shown in Figure 23.24 the main controversy has been as to whether the attack of water on the coordinated alkene is external or via prior coordination of the water to palladium. Current thinking is that external attack predominates in high concentrations of chloride ion and internal attack when [CT] is low. Different details of mechanisms under the two conditions are supported by different reaction kinetics. 

The Wacker oxidation of ethene is an important commercial process. An adaptation of the process is widely used in total synthesis to convert 1-alkenes to methyl ketones and, in this context, is often referred to as the Wacker-Tsuji reaction. Some examples of the reaction are shown in Figure 23.25 note that in the third example, the reaction is selective for the 1-alkene. The use of the process in a steroid synthesis is shown in Figure 23.26. 

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. ... 

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 a recent review it was argued that such additives of copper, benzoquinone, and HPMOV are not really needed all that is needed is the presence of oxidation-resistant ligands that prevent palladium metal formation [15]. Indeed, activation of the C-H bond is not as slow as, for example, the Wacker reaction of ethene in which reoxidation of palladium must be performed by copper oxidation, although in this catalytic system the additives may still play a role in stabilizing the intermediate low-valent palladium species and thus prevent catalyst decomposition. This thesis was corroborated by the work of de Vos and Jacobs, who showed that addition of benzoic acid to the oxidative arylation reaction in the presence of oxygen led to superior results in the coupling of a variety of substituted arenes with acrylates, cinnamates, and ,/f-unsaturated ketones. Very good yields and TON up to 762 were obtained at 90 °C. A mixture of the o, m, and p isomers is obtained if substituted arenes are used [16]. 

To overcome the problems encountered in the homogeneous Wacker oxidation of higher alkenes several attempts have been undertaken to develop a gas-phase version of the process. The first heterogeneous catalysts were prepared by the deposition of palladium chloride and copper chloride on support materials, such as zeolite Y [2,3] or active carbon [4]. However, these catalysts all suffered from rapid deactivation. Other authors applied other redox components such as vanadium pentoxide [5,6] or p-benzoquinone [7]. The best results have been achieved with catalysts based on palladium salts deposited on a monolayer of vanadium oxide spread out over a high surface area support material, such as y-alumina [8]. Van der Heide showed that with catalysts consisting of H2PdCU deposited on a monolayer vanadium oxide supported on y-alumina, ethene as well as 1-butene and styrene... 

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. 

In this context it is also worth mentioning that Showa Denko has developed a new process for the direct oxidation of ethene to acetic acid using a combination of palladium(II) and a heteropoly acid [104]. However, the reaction probably involves heteropoly acid-catalyzed hydration followed by palladium-catalyzed aerobic oxidation of ethanol to acetic acid rather than a classical Wacker mechanism. 

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. 

Francis, J. W., Henry, P. M. Oxidation of olefins by palladium(ll). Part XIV. Product distribution and kinetics of the oxidation of ethene by PdCl3(pyridine)- in aqueous solution in the presence and absence of CUCI2 a modified Wacker catalyst with altered reactivity. J. Mol. Catal. A Chemical 1995, 99, 77-86. 

The oxidation of ethene to ethanal by oxygen and a solution of a paUadium(II) salt in aqueous hydrochloric acid is an important industrial process (the Wacker reaction). The palladium(II) is simultaneously reduced to the metal, but the reaction is made catalytic by addition of copper(II) chloride in the presence of air or oxygen, whereby the palladium is continuously re-oxidized to palladium(II) (5.111). 

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" ... 

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). ... 

The Wacker oxidation is an industrial process using a Pd/Cu catalyst for the air oxidation of ethene to ethanal. 

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