What is Etard Reaction?
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
1. What is the Etard reaction and what is its primary purpose in organic synthesis?
The Etard reaction is a named chemical reaction used for the controlled, partial oxidation of an alkyl group (specifically a methyl group) attached to an aromatic ring to form an aldehyde. Its primary purpose is the direct conversion of toluene (methylbenzene) into benzaldehyde using chromyl chloride as the oxidising agent in a non-polar solvent.
2. What specific reagents and solvent are used in the Etard reaction and why are they chosen?
The key reagents and conditions for the Etard reaction are:
- Oxidising Agent: Chromyl chloride (CrO₂Cl₂). It is chosen because it is a mild oxidising agent, which is crucial for stopping the reaction at the aldehyde stage.
- Solvent: A non-polar, inert solvent like carbon disulphide (CS₂) or carbon tetrachloride (CCl₄). This solvent prevents unwanted side reactions and helps in the formation of the reaction intermediate.
3. What is the intermediate formed during the Etard reaction mechanism?
The intermediate formed during the Etard reaction is a brown chromium complex. When toluene reacts with two molecules of chromyl chloride (CrO₂Cl₂), this stable complex with the formula C₆H₅CH(OCrOHCl₂)₂ is precipitated. The formation of this intermediate is a key step that prevents the methyl group from being over-oxidised. This complex is then hydrolysed (treated with water) to yield the final product, benzaldehyde.
4. In which chapter of the Class 12 Chemistry NCERT syllabus is the Etard reaction discussed?
As per the CBSE syllabus for 2025-26, the Etard reaction is a key topic in Chapter 8: Aldehydes, Ketones and Carboxylic Acids. It is discussed under the section 'Preparation of Aldehydes' from hydrocarbons.
5. Why does the Etard reaction stop at the aldehyde stage and not proceed to form a carboxylic acid?
The reaction stops at the aldehyde stage due to two main reasons:
- Mild Oxidising Agent: Chromyl chloride (CrO₂Cl₂) is not a strong oxidising agent like potassium permanganate (KMnO₄). It has just enough strength to perform the partial oxidation of the methyl group.
- Stable Intermediate: The formation of the stable chromium complex effectively 'protects' the aldehyde group from further oxidation. The aldehyde is only released during the final hydrolysis step, by which time the oxidising agent has already reacted.
If a strong oxidising agent were used, the reaction would proceed directly to form the more stable benzoic acid.
6. How does the Etard reaction differ from the Gattermann-Koch reaction?
While both reactions can produce benzaldehyde, they differ significantly in their starting materials and mechanism:
- Starting Material: The Etard reaction starts with toluene (or another alkylbenzene), modifying an existing side-chain. The Gattermann-Koch reaction starts with benzene itself, adding a formyl group (-CHO) directly to the ring.
- Reagents: The Etard reaction uses chromyl chloride (CrO₂Cl₂). The Gattermann-Koch reaction uses a mixture of carbon monoxide (CO) and hydrogen chloride (HCl) in the presence of a catalyst like anhydrous aluminium chloride (AlCl₃) and cuprous chloride (CuCl).
- Mechanism: The Etard reaction involves the oxidation of a methyl group, whereas the Gattermann-Koch reaction is a type of electrophilic aromatic substitution (formylation).
7. Can the Etard reaction be used for compounds other than toluene? What are its main limitations?
Yes, the Etard reaction can be applied to other aromatic hydrocarbons with a methyl side-chain (like ethylbenzene) to form the corresponding aldehydes. However, it has some key limitations:
- Yields: The reaction often gives moderate to low yields, which can be a drawback for large-scale industrial synthesis.
- Substrate Scope: It is most efficient for converting a -CH₃ group. For longer alkyl chains like in ethylbenzene, the reaction can be less selective and yield can be poor.
- Side Reactions: Careful control of reaction conditions is necessary to avoid the formation of by-products or over-oxidation, especially if the reagents are not pure.
8. What is the importance of the Etard reaction in real-world applications?
The primary importance of the Etard reaction lies in its ability to produce benzaldehyde, a commercially significant compound. Benzaldehyde, also known as 'oil of bitter almond', has several important applications:
- Food Industry: Used as a flavouring agent to impart an almond-like flavour to food products and liqueurs.
- Perfumery: It is a key ingredient in perfumes and cosmetics due to its pleasant almond scent.
- Chemical Industry: It serves as a precursor for synthesising other organic compounds, including certain dyes (like malachite green), pharmaceuticals, and agrochemicals.
