What is the Etard Reaction definition mechanism equation and example
Etard reaction is one of the important name reactions of CBSE Class XII Chemistry. It is named after French Chemist Alexandre Leon Etard. Benzaldehyde can be prepared using toluene by etard reaction. Benzaldehyde is an important organic compound that is highly used in the food industry in place of almonds due to its almond-like flavor. In this reaction partial oxidation of the methyl group bonded to an aromatic ring takes place. To formaldehyde, we require partial oxidation, and this is the reason we use chromyl chloride in non-polar solvent as it’s a weak oxidizing agent. If we use potassium permanganate which is a strong oxidizing agent in place of chromyl chloride, then due to complete oxidation we get carboxylic acid as a product.
The reaction in which the methyl group attached to an aromatic ring undergoes partial oxidation using chromyl chloride and non-polar solvent (such as carbon tetrachloride, carbon disulphide, etc.) to give aldehyde is called Etard Reaction. The reaction is given below –
Etard Reaction -
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The above reaction can also be written in detail as follows –
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Reaction Mechanism of Etard’s Oxidation
In the Etard reaction first weak oxidizing agent chromyl chloride reacts with toluene in presence of non-polar solvent carbon tetrachloride. During this reaction homolytic cleavage of - bonds of chromyl, chloride takes place. In the same way, homolytic cleavage of C-H bonds of methyl group also takes place. It leads to the formation of the Etard complex or chromyl complex. The reaction is given below (By dotted arrows we are showing which atom is bonding with which atom to form Etard complex for your better understanding) –
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Now hydrolysis of the etard complex takes place which leads to the removal of two molecules of Cr(OH)2Cl2 and hence the formation of benzaldehyde (Aldehyde). The reaction is given below -
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Thus, the formation of aldehyde from the methyl group attached to the aromatic ring takes place by direct partial oxidation.
Applications of Etard Reaction
Conversion of toluene into benzaldehyde by oxidation is very useful as benzaldehyde is quite useful in the food industry due to its almond-like flavor. It is used as a precursor for the formation of dyes, perfumes, and many pharma compounds. Aldehydes are more reactive and take part in aldol condensation.
Dyes, perfumes, and many other compounds are found by this reaction as a precursor. Benzaldehyde is very useful in the synthesis of many compounds such as phentermine.
Limitations of Etard Reaction
Although etard reaction is an easy and direct method for conversion of toluene into benzaldehyde. But it has some limitations as well. Obtaining specific aldehyde products by etard reaction using other reagents than toluene is difficult. If we use strong oxidizing agents in the reaction, then they give rise to more stable carboxylic acids.
So, this was all about Etard Reaction for class 11, where we went through the detailed explanation of the mechanism, including Mechanism, Applications, and Limitations. Going through this page will help you clear your concept on the reaction mechanism of Etard’s Oxidation.
FAQs on Etard Reaction in Organic Chemistry
1. What is the Etard reaction in organic chemistry?
The Etard reaction is a chemical reaction in which an aromatic methyl group (–CH3) is selectively oxidized to an aldehyde (–CHO) using chromyl chloride (CrO2Cl2). It is mainly used to convert toluene and its derivatives into corresponding aromatic aldehydes without over‑oxidation to carboxylic acids. This reaction is especially useful in organic synthesis when controlled oxidation of the side chain is required.
2. What is the reagent used in the Etard reaction?
The reagent used in the Etard reaction is chromyl chloride (CrO2Cl2), usually dissolved in a non‑polar solvent like carbon tetrachloride or carbon disulfide. Chromyl chloride acts as a selective oxidizing agent that specifically oxidizes the benzylic methyl group to an aldehyde. The reaction proceeds through formation of a chromyl complex intermediate.
3. What is the product of the Etard reaction of toluene?
The Etard reaction of toluene (C6H5CH3) produces benzaldehyde (C6H5CHO) as the main product. The methyl group attached to the benzene ring is oxidized to an aldehyde group under controlled conditions. Unlike strong oxidizing agents such as KMnO4, the Etard reaction stops at the aldehyde stage instead of forming benzoic acid.
4. What is the mechanism of the Etard reaction?
The mechanism of the Etard reaction involves formation of a chromyl complex followed by hydrolysis to give an aldehyde. The steps are:
- Formation of a chromyl complex between CrO2Cl2 and the benzylic methyl group.
- Selective oxidation of the benzylic carbon.
- Hydrolysis of the intermediate to yield the corresponding aromatic aldehyde.
This mechanism explains why oxidation stops at the –CHO stage instead of proceeding to –COOH.
5. Why does the Etard reaction stop at the aldehyde stage?
The Etard reaction stops at the aldehyde stage because chromyl chloride forms a stable intermediate complex that prevents further oxidation under controlled conditions. Unlike strong oxidants such as KMnO4 or K2Cr2O7, CrO2Cl2 provides mild and selective oxidation. This selectivity makes it useful for preparing benzaldehydes without forming carboxylic acids.
6. What types of compounds undergo the Etard reaction?
Compounds that undergo the Etard reaction are mainly aromatic hydrocarbons containing a benzylic methyl group. These include:
- Toluene and substituted toluenes
- Xylenes (in controlled conditions)
- Other alkyl benzenes with at least one benzylic hydrogen
The presence of a benzylic hydrogen is essential for the oxidation to occur.
7. What is the difference between the Etard reaction and oxidation with KMnO4?
The main difference is that the Etard reaction oxidizes a methyl group to an aldehyde, while KMnO4 oxidizes it completely to a carboxylic acid. Key differences include:
- Etard reaction uses CrO2Cl2; KMnO4 uses potassium permanganate.
- Etard reaction gives benzaldehyde from toluene.
- KMnO4 gives benzoic acid (C6H5COOH).
Thus, the Etard reaction is more selective and milder.
8. What are the conditions required for the Etard reaction?
The Etard reaction requires chromyl chloride in a non‑polar solvent under controlled temperature conditions. Important conditions include:
- Use of solvents like carbon tetrachloride or carbon disulfide.
- Anhydrous conditions during complex formation.
- Subsequent hydrolysis to release the aldehyde.
Careful temperature control prevents over‑oxidation and side reactions.
9. Can the Etard reaction be used for aliphatic hydrocarbons?
No, the Etard reaction is mainly limited to aromatic hydrocarbons with a benzylic methyl group and is not suitable for simple aliphatic hydrocarbons. The reaction requires a benzylic position where the intermediate complex can stabilize through resonance. Aliphatic methyl groups do not provide this stabilization, so the reaction does not proceed effectively.
10. What is the importance of the Etard reaction in organic synthesis?
The Etard reaction is important because it provides a selective method to prepare aromatic aldehydes from methyl-substituted benzenes. Its significance includes:
- Controlled oxidation without forming carboxylic acids.
- Preparation of intermediates like benzaldehyde for dyes, perfumes, and pharmaceuticals.
- Use in laboratory synthesis where mild benzylic oxidation is required.
It remains a classic named reaction in organic chemistry for selective side‑chain oxidation.
