What is Etard's reagent?

Answer

### Step-by-Step Solution:

1. **Definition of Etard's Reagent**:
Etard's reagent is a chemical reagent used for the oxidation of toluene to benzaldehyde. It is specifically a solution of chromyl chloride (CrO2Cl2) in carbon disulfide (CS2).

2. **Chemical Composition**:
The main component of Etard's reagent is chromyl chloride, which has the chemical formula CrO2Cl2. This compound is dissolved in carbon disulfide (CS2) to form the reagent.

3. **Oxidation Reaction**:
When toluene (C6H5CH3) is treated with Etard's reagent, it undergoes an oxidation reaction. Toluene is a benzene ring with a methyl group attached.

4. **Formation of Brown Chromium Complex**:
During the reaction, a brown chromium complex is formed. This complex can be represented as:
- C6H5-CH(OH)-O-CrCl2
This complex is characterized by the presence of a chromium atom bonded to the benzene ring and hydroxyl (OH) groups.

5. **Hydrolysis of the Complex**:
The brown chromium complex undergoes hydrolysis when treated with an acidic solution. This step is crucial as it leads to the formation of the final product.

6. **Final Product - Benzaldehyde**:
The hydrolysis of the brown chromium complex yields benzaldehyde (C6H5CHO), which is the desired product of the reaction. Benzaldehyde consists of a benzene ring with a formyl group (CHO) attached.

### Summary:
Etard's reagent is a solution of chromyl chloride in carbon disulfide, used to oxidize toluene to benzaldehyde through the formation of a brown chromium complex, which is then hydrolyzed to yield benzaldehyde.

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