Methyl group attached to benzene can be oxidised to carboxyl group by reacting with :

Options

A. `Fe_(2)O_(3)`
B. `AgNO_(3)`
C. `KMnO_(4)`
D. `CrO_(3)`

Answer

To solve the question of how a methyl group attached to benzene can be oxidized to a carboxyl group, we will analyze the options provided and determine which reagent can facilitate this transformation.

### Step-by-Step Solution:

1. **Understanding the Structure**:
- We start with toluene (methylbenzene), which has a methyl group (-CH3) attached to a benzene ring.

2. **Identifying the Reaction**:
- The goal is to oxidize the methyl group (-CH3) to a carboxylic acid (-COOH). This means we need a reagent that can facilitate this oxidation.

3. **Analyzing Option A: Fe2O3**:
- Fe2O3 is a mild oxidizing agent. It can oxidize methyl groups, but it typically only converts them to aldehydes (-CHO) and not to carboxylic acids. Therefore, this option is not suitable.

4. **Analyzing Option B: Silver Nitrate**:
- Silver nitrate does not have the capability to oxidize the methyl group to a carboxylic acid. It is not an effective oxidizing agent for this transformation. Thus, this option is also not suitable.

5. **Analyzing Option C: KMnO4**:
- Potassium permanganate (KMnO4) is a strong oxidizing agent. It can oxidize the methyl group all the way to a carboxylic acid, provided there is at least one hydrogen atom on the carbon adjacent to the benzene ring. Therefore, this option is suitable and is likely the correct answer.

6. **Analyzing Option D: CrO3**:
- Chromium trioxide (CrO3) is used in the Jones oxidation, which typically oxidizes alcohols and aldehydes. In the case of toluene, it would convert the methyl group to an aldehyde (-CHO) but not to a carboxylic acid (-COOH). Thus, this option is also not suitable.

7. **Conclusion**:
- Based on the analysis, the only reagent that can oxidize the methyl group attached to benzene to a carboxylic acid is KMnO4. Therefore, the answer is **Option C: KMnO4**.

### Final Answer:
**The methyl group attached to benzene can be oxidized to a carboxyl group by reacting with KMnO4.**

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