Keto-enol tautomerism is observed in

Options

A. `C_(6)H_(5)-overset(O)overset(||)C-H`
B. `C_(6)H_(5)-overset(O)overset(||)C-CH_(3)`
C. `C_(6)H_(5)-overset(O)overset(||)C-C_(6)H_(5)`
D. `C_(6)H_(5)-overset(O)overset(||)C-CH_(2)-overset(O)overset(||)C-CH_(3)`

Explanation

To determine which compounds exhibit keto-enol tautomerism, we need to analyze the structures given in the question. Keto-enol tautomerism occurs when a compound has an alpha hydrogen, which is a hydrogen atom attached to a carbon adjacent to a carbonyl group (C=O). Let's go through the structures step by step.

### Step-by-Step Solution:

1. **Identify the Structures**:
We have four structures to analyze for the presence of alpha hydrogen.

2. **Check for Alpha Hydrogen**:
- **Structure 1: C6H5COOH (Benzoic Acid)**
- In this structure, the carbonyl carbon (C=O) is sp2 hybridized and is part of a carboxylic acid. The adjacent carbon (the one attached to the benzene ring) is also sp2 hybridized, and there are no sp3 carbons present. Therefore, there are no alpha hydrogens.
- **Conclusion**: No keto-enol tautomerism.

- **Structure 2: C6H5C(=O)CH3 (Acetophenone)**
- Here, the carbonyl carbon is sp2 hybridized, and the adjacent carbon (the one attached to the methyl group) is sp3 hybridized. The hydrogen attached to this sp3 carbon is the alpha hydrogen.
- **Conclusion**: This compound can undergo keto-enol tautomerism.

- **Structure 3: C6H5C(=O)C6H5 (Benzophenone)**
- In this case, both the carbonyl carbon and the adjacent carbon (attached to another phenyl group) are sp2 hybridized. There are no sp3 carbons present, hence no alpha hydrogens.
- **Conclusion**: No keto-enol tautomerism.

- **Structure 4: C6H5C(=O)CH2C(=O)CH3**
- Here, the carbonyl carbon is sp2 hybridized, and the adjacent carbon (the one attached to the CH2 group) is sp3 hybridized. The hydrogen on this sp3 carbon is the alpha hydrogen.
- **Conclusion**: This compound can also undergo keto-enol tautomerism.

3. **Final Answer**:
- The compounds that exhibit keto-enol tautomerism are **Structure 2 (Acetophenone)** and **Structure 4**.

### Summary:
Keto-enol tautomerism is observed in the following structures:
- **Structure 2**: C6H5C(=O)CH3
- **Structure 4**: C6H5C(=O)CH2C(=O)CH3