The compound which is optically active is :

Options

A. 1-butanol
B. 2-butanol
C. 1-propanol
D. 2-methyl-l-propanol

Explanation

To determine which compound is optically active, we need to identify if any of the given compounds contain a chiral carbon atom. A chiral carbon is one that is bonded to four different groups or atoms.

### Step-by-Step Solution:

1. **Identify the Compounds**: The compounds given are:
- (a) 1-butanol
- (b) 2-butanol
- (c) 1-propanol
- (d) 2-methyl-1-propanol

2. **Analyze 1-butanol**:
- Structure: CH3-CH2-CH2-OH
- Carbon atoms: The four carbon atoms in 1-butanol are:
- C1: CH3
- C2: CH2
- C3: CH2
- C4: CH2 (attached to OH)
- None of these carbons are bonded to four different groups.
- **Conclusion**: 1-butanol is **not optically active**.

3. **Analyze 2-butanol**:
- Structure: CH3-CH(OH)-CH2-CH3
- Carbon atoms: The relevant carbon atom is the second carbon (C2):
- C1: CH3
- C2: CH (attached to OH, CH3, and H)
- C3: CH2
- C4: CH3
- C2 is bonded to four different groups (CH3, OH, CH2, and H).
- **Conclusion**: 2-butanol is **optically active**.

4. **Analyze 1-propanol**:
- Structure: CH3-CH2-CH2-OH
- Carbon atoms: The three carbon atoms in 1-propanol are:
- C1: CH3
- C2: CH2
- C3: CH2 (attached to OH)
- None of these carbons are bonded to four different groups.
- **Conclusion**: 1-propanol is **not optically active**.

5. **Analyze 2-methyl-1-propanol**:
- Structure: CH3-CH(CH3)-CH2-OH
- Carbon atoms: The relevant carbon atom is the second carbon (C2):
- C1: CH3
- C2: CH (attached to OH and CH3)
- C3: CH2
- C2 is bonded to two CH3 groups and does not have four different groups.
- **Conclusion**: 2-methyl-1-propanol is **not optically active**.

### Final Answer:
The compound which is optically active is **(b) 2-butanol**.

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