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Oppenauer Oxidation and Dakin Reaction

Oppenauer OxidationA selective oxidation is used in Oppenauer oxidation to convert secondary alcohols to ketones.

Oppenauer Oxidation

A selective oxidation is used in Oppenauer oxidation to convert secondary alcohols to ketones. Oppenauer is named for the chemist Rupert Viktor Oppenauer. Al(i-Pro)3 and acetone are required for the oxidation reaction to occur. Catalyzing the oxidation of secondary alcohols to ketones in its presence is a metal alkoxide. Meerwein Ponndorf Verley is the reverse of this. Oxidation of allylic alcohols to α, β- unsaturated ketones is a very good method.

Oppenauer Oxidation Mechanism

  • The alcohol is combined with aluminum isopropoxide to form a complex
  • A six-membered transition complex is formed when this complex reacts with ketone
  • A carbonyl carbon is formed after the aluminium-catalyzed hydride shift of the alpha-carbon in alcohol.
  • In an acetone transition state, there are six members.
  • Upon the transfer of hydride, the desired ketone forms.

Oxidation of alcohols to carbonyl compounds is achieved by this reaction. When aluminium alkoxide, typically isopropoxide or t-butoxide, is present, this hydride acceptor can be either acetone or cyclohexanone. During the oxidation, the alcohol is deprotonated by equilibration with the alkoxide and then transferred to the hydride, which is the exact opposite of what happens in Meerwein-Ponndorf-Verley reduction. By using a large excess of the hydride acceptor, this equilibrium is generally displaced to the right.

Disadvantages of Oppenauer Oxidation

In Oppenauer oxidation, chemoelectivity is improved by the use of high temperatures and large quantities of ketone hydride acceptors. However, this method produces aldol condensation products with hydride acceptors, which can significantly impair the efficiency of the oxidation process. Oppenauer-type oxidations, conducted under milder conditions, have been developed to be more efficient. Due to the fact that aluminium compounds involved in oxidation are basic, the product can go through prototropic shifts upon oxidation. C=C also migrates during cholesterol oxidation to give α, β - unsaturated ketones.

Oxidation by Oppenauer is not a good method of preparing aldehydes because the basic medium causes reactions between ketones and aldehydes, e.g., aldol condensation. Optimum Oppenauer type oxidation conditions may be achieved in milder conditions.

Dakin Reaction

Dakin oxidation (or Dakin reaction) is a redox reaction where phenyl aldehydes (2-hydroxybenzaldehyde or 4-hydroxybenzaldehyde) or ketones are oxidized by hydrogen peroxide to form benzenediols and carboxylates. The carbonyl group of the hydrogen peroxide is oxidized, but the hydrogen peroxide is reduced. Although both reactions are named for Henry Drysdale Dakin, they do not have any connection to the Dakin oxidation. It is closely related to the Baeyer-Villiger oxidation.

Redox reactions include the Dakin reaction. Benzaldehyde or ketone, that is ortho- or para-hydroxylated, reacts with hydrogen peroxide in an alkaline solution to form benzenediol and carboxylate ions.

The Dakin reaction results from oxidizing an aldehyde or ketone to the corresponding phenol.

Reaction Mechanism of Dakin Reaction

Applications of Dakin Reactions

Catechol is synthesized by the Dakin reaction. Several catecholamines and derivates of catecholamines are synthesised using catechol. One of the common antioxidants and polymerization inhibitors is 1, 4-tertbutyl catechol.

Dilute hypochlorite solution is Dakin's solution. A strong antiseptic, chlorine is the active ingredient in Dakin's solution. It kills most kinds of bacteria and viruses that may cause tissue and skin infections.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of pharmaguideline.com, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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