Identify the compound that exhibits tautomerism

Options

A. 2 - butene
B. lactic acid
C. 2 - pentanone
D. phenol

Answer

To identify the compound that exhibits tautomerism, we need to understand the concept of tautomerism and specifically keto-enol isomerism. Here’s a step-by-step solution:

### Step 1: Understand Tautomerism
Tautomerism is a type of isomerism where two isomers (tautomers) can interconvert, typically involving the relocation of a hydrogen atom and a switch of a double bond. The most common form of tautomerism is keto-enol tautomerism, where a ketone and its corresponding enol form are in equilibrium.

### Step 2: Identify the Structure of Compounds
We need to analyze the given compounds to see if they have the necessary structural features for tautomerism:
- **Keto form**: Contains a carbonyl group (C=O).
- **Enol form**: Contains a hydroxyl group (–OH) and a double bond (C=C).

### Step 3: Analyze Each Compound
1. **2-Butene**:
- Structure: CH3-CH=CH-CH3
- It does not have a carbonyl group (C=O), thus it cannot exhibit tautomerism.

2. **Lactic Acid**:
- Structure: CH3-CHOH-COOH
- While it has a hydroxyl group, the carbonyl group is not in a position to allow tautomerism, and resonance stabilization prevents the formation of an enol. Therefore, it does not exhibit tautomerism.

3. **2-Pentanone**:
- Structure: CH3-CO-CH2-CH2-CH3
- This compound has a carbonyl group (C=O) and alpha hydrogens (hydrogens on the carbon adjacent to the carbonyl). It can undergo tautomerism to form an enol (CH3-C(OH)=CH2). Thus, this compound exhibits tautomerism.

4. **Phenol**:
- Structure: C6H5-OH
- Although it has an –OH group, it does not have a carbonyl group (C=O) and is stabilized in its aromatic form. Therefore, it does not exhibit tautomerism.

### Step 4: Conclusion
From the analysis, the compound that exhibits tautomerism is **2-Pentanone** (Option 3).

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