Etard Reaction: Mechanism, Application, Limitations and Sample Questions

Etard reaction deals with partial oxidation of aromatic ring-like Benzene by using Chromyl Chloride. Since Chromyl Chloride is a weak oxidizing agent, therefore it is used with a non-polar solvent like Carbon Tetrachloride. The end result is an Aldehyde.

Keyterms: Benzene, Aldehyde, Carbon Tetrachloride, Benzaldehyde, Toluene, Hydrolysis, Organic Compound, Chromyl Chloride

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Etard Reaction

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Etard reaction is the reaction named after French Chemist Alexandre Leon Etard that can be used to prepare Benzaldehyde by using Toluene. When Toluene is reacted with Chromyl Chloride in a nonpolar solvent and then undergoes hydrolysis, it gives Benzaldehyde and this reaction is called Etard reaction. Benzaldehyde is an important organic compound containing a benzene ring with a formyl substituent that is used in cosmetics and perfume for fragrance. 

Etard reaction is broken down into two steps:

1. An unstable brown complex is formed
2. The final product is obtained by hydrolysis

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Etard Reaction Mechanism

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In the reaction, Chromyl Chloride(CrO₂Cl₂) oxides Methyl group(R-CH₃) to a Chromium complex which on hydrolysis gives corresponding Benzaldehyde. Since Chromium Chloride is a weak oxidizing agent and in Etard’s reaction partial oxidation occurs, Toluene gives only Benzaldehyde.

In other words, when Toluene is reacted with catalyst Chromyl Chloride in a non-polar Carbon Tetrachloride solvent, a brown complex is formed which then undergoes hydrolysis to give Benzaldehyde. This reaction is known as the Etard reaction.

The reaction gets completed in two steps:

• The breakdown into unstable brown complex
• Benzaldehyde is formed by hydrolysis

Let’s look at the mechanism stepwise:

• The reaction of Toluene with Chromyl Chloride leads to homolytic cleavage which is the breakdown in such a way that each part gets equal bits of electrons.

• Similarly, the C-H bond of the methyl group is also broken down.
• Toluene breaks to give hydrogen which bonds with the oxygens of two Chromyl Chloride molecules forming two OCr(OH)Cl₂.
• The two OCr(OH)Cl₂ molecules then attach themselves to the carbon of the methyl group of Toluene. This is done with the help of second oxygen which forms an unstable complex compound also referred to as brown complex.

Here our first part of the reaction is completed. This is followed by :

• The hydrolysis of the unstable complex compound. This will result in the exchange of electrons of oxygen with the carbon of Toluene.

The oxygen in the first OCr(OH)Cl₂ breaks the C-O bond and attaches to the H+ of water forming OCr(OH)₂Cl₂.

• The oxygen of the second OCr(OH)Cl₂ will break the O-Cr bond-forming double bond with Carbon.
• The remaining Cr(OH)Cl₂ bonds with OH of water again forming OCr(OH)₂Cl₂.
• The two OCr(OH)₂Cl₂ then get separated to form Benzaldehyde.

Here step two is completed giving the finished product Benzaldehyde thus completing the Etard reaction.

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Application of Etard Reaction

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Etard reaction is very important as it gives Benzaldehyde which is an important organic compound used largely in the chemical industry.

• Benzaldehyde is an aromatic compound that has a major use in adding fragrance to perfumes, soaps and cosmetics.
• It is also used in the food industry as a substitute for almond flavour as it has the same flavour.
• It is also used as bee repellent to extract honey from the honeycomb.
• It is also used as a solvent for several oils, ethers and resins.
• It also acts as a preservative for foods.
• It is used in the formation of phentermine that is used as a drug to treat obesity.

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Limitations of Etard Reaction:

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Etard reaction can be easily accomplished in just two steps but there are certain limitations to it as well.

• Except for Toluene, the reaction is not easy to obtain specific aldehyde if the reagent is changed.
• If strong oxidizing agents are used in the reaction, the reaction will not take place as it will end up in a resulting stable compound giving carboxylic acid instead of aldehyde.

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Things to Remember

1. Etard reaction is the partial oxidation of an aromatic ring attached with a methyl group to obtain an aldehyde.
2. Toluene is reacted with Chromyl Chloride in presence of a non-polar solvent like Carbon Tetrachloride to obtain a complex compound.
3. The complex compound can then be hydrolysed to obtain the final aldehyde product Benzaldehyde.
4. Benzaldehyde is an aromatic compound that is used for its fragrance in perfumes, soaps and cosmetics.
5. Etard reaction only uses Toluene to obtain Benzaldehyde. Any other reactant will not result in obtaining specific aldehyde.
6. Only mild oxidizing agents must be used to carry out the Etard reaction as strong agents will result in the formation of carboxylic acid instead of aldehyde.
7. The solvent used to carry out the Etard reaction must be a non-polar solvent like Carbon Tetrachloride. Other non-polar solvents like Chloroform, Carbon disulfide can also be used.

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