Keto-enol tautomerism is shown by

Options

A. `C_6 H_5 CHO`
B. `C_6 H_5 COCH_3`
C. `C_6 H_5 COC_6 H_5`
D. `C_6 H_5 COCH_2 CO CH_3`

Answer

To determine which compounds exhibit keto-enol tautomerism, we need to analyze the presence of alpha-hydrogens in each compound. Keto-enol tautomerism occurs when a keto form (with a carbonyl group) can interconvert with an enol form (with a double bond and hydroxyl group) through the transfer of a hydrogen atom.

### Step-by-Step Solution:

1. **Understanding Keto-Enol Tautomerism**:
- Keto-enol tautomerism involves the interconversion between a keto form (with a carbonyl group, C=O) and an enol form (with a double bond, C=C, and a hydroxyl group, -OH).
- The presence of alpha-hydrogens (hydrogens on the carbon adjacent to the carbonyl carbon) is crucial for this tautomerization.

2. **Analyzing the Compounds**:
- **Compound A: Benzaldehyde (C6H5CHO)**:
- Structure: C6H5-CO-H
- The carbon adjacent to the carbonyl carbon does not have any hydrogen atoms (it is bonded to three other carbons).
- **Conclusion**: No alpha-hydrogen present, so it does not show tautomerism.

- **Compound B: Acetophenone (C6H5COCH3)**:
- Structure: C6H5-CO-CH3
- The carbon adjacent to the carbonyl carbon (the alpha-carbon) has three hydrogens (from the methyl group).
- **Conclusion**: Alpha-hydrogen is present, so it shows tautomerism.

- **Compound C: Benzophenone (C6H5COC6H5)**:
- Structure: C6H5-CO-C6H5
- The carbons adjacent to the carbonyl carbon do not have any hydrogen atoms (each is bonded to two other carbons).
- **Conclusion**: No alpha-hydrogen present, so it does not show tautomerism.

- **Compound D: 1,3-Diphenyl-2-propanone (C6H5COCH2COC6H5)**:
- Structure: C6H5-CO-CH2-CO-C6H5
- The carbon adjacent to the carbonyl carbon has two alpha-hydrogens (from the CH2 group).
- **Conclusion**: Alpha-hydrogen is present, so it shows tautomerism.

3. **Final Answer**:
- The compounds that show keto-enol tautomerism are **B (Acetophenone)** and **D (1,3-Diphenyl-2-propanone)**.

### Summary:
- **Compound A**: No tautomerism (no alpha-hydrogen).
- **Compound B**: Shows tautomerism (has alpha-hydrogen).
- **Compound C**: No tautomerism (no alpha-hydrogen).
- **Compound D**: Shows tautomerism (has alpha-hydrogen).

**Answer**: Compounds B and D show keto-enol tautomerism.