Wacker oxidation

What is Wacker oxidation?

The Wacker oxidation, or Wacker process, is a chemical reaction that involves the conversion of an alkene into an aldehyde or ketone through the use of oxygen and a platinum catalyst. Although the reaction was first reported by Phillips in 1894. It is an industrial production of acetaldehyde by the oxidation of ethylene with oxygen in the presence of aqueous acidic solution of palladium chloride and cupric chloride. However, this reaction was not named after Phillips, instead it is generally known as the Hoechst-Wacker process, or simply as the Wacker process after the German chemical companies (Farbwerke Hoechst AG and Wacker Chemie GmbH), which initially implemented this reaction for the production of acetaldehyde in the 1970s.

Wacker oxidation
Wacker oxidation

The Wacker oxidation is typically performed by heating an alkene compound with oxygen and a platinum catalyst, such as platinum dioxide or chloroplatinic acid. The platinum catalyst facilitates the incorporation of oxygen into the alkene compound, forming an intermediate compound known as a peroxide..

The peroxide is then converted into the final product through the loss of a proton, resulting in the formation of an aldehyde or ketone. The Wacker oxidation is a useful method for the synthesis of aldehydes and ketones, but it has several limitations, including the need for expensive platinum catalysts and the formation of unwanted byproducts..

Summary

The Wacker oxidation is a valuable method for the synthesis of aldehydes and ketones, but it has several limitations that make it less practical than other methods. Despite this, it remains an important part of the history of chemistry and continues to be studied by chemists today..

Example

The Wacker oxidation is a chemical reaction that converts an alkene to an aldehyde or a ketone using a palladium catalyst and oxygen as the oxidant.

One example of a Wacker oxidation is the reaction of 1-octene with palladium on charcoal and oxygen to form 1-octanal.

The balanced equation for this reaction is:

C8H16 + O2 + Pd/C → C8H14O + H2O

It’s important to note that this reaction is also known as the “palladium-catalyzed” oxidation, and it’s a powerful method to convert alkenes to carbonyl compounds.

Mechanism of reaction

Wacker oxidation involves the initial activation of the olefinic functionality via the coordination of the C-C double bond with the palladium cation, followed by a reversible nucleophilic attack of water in Markovnikov fashion, and a ratelimiting step in which a chloride is cleaved from the σ complex. The resulting 14-e complex undergoes rapid β-elimination to form palladium hydride and vinyl alcohol. Vinyl alcohol can directly tautomerize into methyl ketone, whereas palladium hydride undergoes reductive elimination to form palladium (0), which is then oxidized by Cu(II) to palladium (II) again while Cu(II) is reduced to Cu(I). The oxidation of Cu(I) to Cu(II) by oxygen in the presence of HCl completes the catalytic cycle. Overall, in this reaction, only oxygen and olefins are consumed, and palladium chloride and cupric chloride are regenerated during the reaction cycle.

Reaction mechanism of Wacker oxidation
Reaction mechanism of Wacker oxidation

The mechanism of the Wacker oxidation can be broken down into the following steps:

  • Palladium(II) ions are reduced to Palladium(0) by the alkene in the presence of a reducing agent, such as sodium methoxide.
  • The palladium(0) complex forms a complex with the alkene, called a “palladium-alkene complex”.
  • Hydrogen peroxide is added to the reaction mixture, which is then converted to water and oxygen by the palladium-alkene complex.
  • The oxygen atom from the hydrogen peroxide becomes attached to the alkene, forming a “palladium-alkoxide complex”.
  • The palladium-alkoxide complex then loses a molecule of water, forming the ketone product and regenerating the palladium(0) catalyst.
  • The palladium(0) catalyst can then be reused to oxidize another alkene molecule.

References

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