The oxidation of ethylene to acetaldehyde

Strictly speaking this reaction does not occur catalytically however, the mechanism is of interest in the context of the reactions described above. 

Treatment of aqueous solutions of palladium(ll) chloride with ethylene gives an ethylene-palladium complex which is readily hydrolysed forming acetaldehyde and palladium metal. The palladium metal may be reoxidized by copper(II) chloride, in a continuous process [116]. 

As shown above, the oxidation of the ethylene is thought to proceed via nucleophilic attack of a hydroxide on the ethylene. Palladium(ll) chloride will also catalyse the oxidation of propylene to acetone and cis- and trans-butenes to methylethyl ketone [116a]. Kinetic studies on these reactions suggest that the rate-determining step is the addition of hydroxide to the co-ordinated olefin, viz.  

A similar reaction is the formation of monoacetate derivatives from olefins, e.g. 

Similar mechanisms are thought to be important in vinylation reactions which are catalysed by Pd(ll) chloride [117]. In a typical reaction, vinyl acetate is formed from ethylene and acetic acid in the presence of Pd(II)Cl2 and Na2HP04. A mechanism involving ethylene-palladium intermediates is suggested by the reaction 

As shown above, the oxidation of the ethylene is thought to proceed via nucleophilic attack of a hydroxide ion on the ethylene. P adium(II) 

The oxidation of ethylene by palladium catalysts in acetic add solution containing sodium acetate gives vinyl acetate. This reaction is the basis of the industrial preparation of vinyl acetate. The reaction may proceed as follows, 

The trimerization of butadiene and relatad raactions catalyzed by some n-allyl complexes 

If the complex, 10.6, is treated with a compound which acts as a non-labile ligand to the metal, then instead of trimerization only dimerization of butadiene occurs, since the attachment of a third butadiene molecule is prevented. The reaction is represented below. The intermediate complex, 10.8, has been isolated when Do = tris(2-biphenyl) phosphite. 

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The Wacker process for the oxidation of ethylene to acetaldehyde with PdCb/CuCb at 100°C (212°F) with 95 percent yield and 95 to 99 percent conversion per pass. 

The oxidation of ethylene to acetaldehyde by palladium chloride in water has been known since the nineteenth century.80 However, the reaction requires the use of a stoichiometric amount of PdCl2, resulting in Pd(0) deposit. Anderson, in 1934, observed a similar reaction (but... 

The oxidation of ethylene to acetaldehyde by dioxygen catalyzed by palladium and cupric salts found important technological application. The systematic study of this process was started by Smidt [245] and Moiseev [246]. The process includes the following stoichiometric stages [247,248] ... 

The Wacker process (Eq. 1) was developed nearly 50 years ago [1-3] and represents one of the most successful examples of homogeneous catalysis in industry [4-9]. This palladium-catalyzed method for the oxidation of ethylene to acetaldehyde in aqueous solution employs a copper cocatalyst to facilitate aerobic oxidation of Pd° (Scheme 1). Despite the success of this process, certain features of the reaction have Umited the development of related aerobic oxidation reactions. Many organic molecules are only sparingly sol-... 

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 ... 

The oxidation of ethylene to acetaldehyde in the gas phase, carried out at rather low temperatures (100—200°C), is very similar to the Wacker liquid phase process. One of the main steps in this process is... 

WACKER REACTION. The oxidation of ethylene to acetaldehyde in the presence of palladium chloride and cupric chloride. 

Fig. 64. Redox cycle for the oxidation of ethylene to acetaldehyde. V5+ (oxidized heteropolyanion) represents vanadium in the oxidized heteropolyanion. (From Ref. 368.)...

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). 

This is the oxidation of ethylene to acetaldehyde, catalysed by PdCl2 and CuCl2 ... 

Activation of olefins to nucleophilic attack, by 7r-complex formation at soft metal centers (Section III.D), can also occur with heterogeneous catalysts. Thus, the oxidation of ethylene to acetaldehyde or vinyl acetate, as described earlier for homogeneous Pd(II) catalysts, can also be carried out heterogeneously in either the liquid or gas phase.512... 

Fig Proposed mechanism for the oxidation of ethylene to acetaldehyde in the Wacker process. Chloride ligands have been omitted. The oxidation number ofpalladium is + 2 at all stages of this cycle except the upper left where eductive elimination of acetaldehyde gives Pd (0), which is oxidised by Cu (II). The complete cycle for the reoxidation of Cu (I) is not shown. 

The oxidation of ethylene to acetaldehyde using PdCb and CuCb as catalysts undo- an oxygm atmosphere is well known as the Wacker process (Scheme 1), and is one of the most important industrial processes employing transition metal catalysts.This industrial oxidation reaction of ethylene involves the following three stoichiometric reactions. These sequential oxidation and reduction reactions constitute a catalytic cycle. 

Copper chloride complexes can be used as catalysts in a number of organic reactions. Examples include the Wacker process, which is the oxidization of ethylene to acetaldehyde by oxygen and aqueous Cu and Pd precatalysts (or, alternatively using iron catalysts) plus the synthesis of acrylonitrile from acetylene and hydrogen cyanide using CuCl. Cuprous chloride has also been used as a desulfiuizmg and... 

Nucleophilic addition to metal-activated alkenes as a synthetic method can be traced to the Wacker Process, the oxidation of ethylene to acetaldehyde with Pd and... 

Olefin complexes can often be prepared in solution by adding the olefin to a soluble Pd(II) salt. Thus NaaPdCl, in HOAc, will absorb ethylene reversibly to give solutions of a different color than the original solution of the Pd(II) salt (106). In some cases the intermediate tt complex can be detected in catalytic systems. Thus, in the oxidation of ethylene to acetaldehyde, formation of the intermediate tt complex, according to the following equilibrium... 

No doubt the most studied reaction in Pd(ll) chemistry is the basic Wacker process—the oxidation of ethylene to acetaldehyde in H2O. 

For example, in one version of the Wacker process used for the oxidation of ethylene to acetaldehyde with soluble palladium-copper complexes, a tubular reactor containing ceramic rings is used to promote gas-liquid contacting and to impart a gas-liquid flow pattern that approaches ideal tubular reactor behavior. 

Most of the catalysts employed in the chemical technologies are heterogeneous. The chemical reaction takes place on surfaces, and the reactants are introduced as gases or liquids. Homogeneous catalysts, which are frequently metalloorganic molecules or clusters of molecules, also find wide and important applications in the chemical technologies [24]. Some of the important homogeneously catalyzed processes are listed in Table 7.44. Carbonylation, which involves the addition of CO and H2 to a C olefin to produce a + 1 acid, aldehyde, or alcohol, uses rhodium and cobalt complexes. Cobalt, copper, and palladium ions are used for the oxidation of ethylene to acetaldehyde and to acetic acid. Cobalt(II) acetate is used mostly for alkane oxidation to acids, especially butane. The air oxidation of cyclohexane to cyclohexanone and cyclohexanol is also carried out mostly with cobalt salts. Further oxidation to adipic acid uses copper(II) and vanadium(V) salts as catalysts. The... 

The present chapter is limited to oxidation of lower olefins, especially those with two to five carbon atoms, over solid catalysts in the vapor phase. Patent literature is given scant attention, but journal literature is covered through 1965. Liquid phase oxidation with homogeneous catalysts has recently grown in importance, but such studies are excluded from this chapter. A review of the oxidation of ethylene to acetaldehyde with PdClg solutions is given by Smidt (3). 

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