Which of the following alkanes is optically active ?

Options

A. 3-methyl hexane
B. propane
C. 2,3, 4-trimethyl pentane
D. 2-methyl butane

Explanation

To determine which of the given alkanes is optically active, we need to identify if any of them contain a chiral carbon atom. A chiral carbon is a carbon atom that is bonded to four different groups, leading to non-superimposable mirror images (enantiomers).

### Step-by-Step Solution:

1. **Identify the Alkanes**: The options given are:
- A) 3-methylhexane
- B) Propane
- C) 2,3,4-trimethylpentane
- D) 2-methylbutane

2. **Analyze Each Alkane for Chirality**:
- **Option A: 3-methylhexane**
- Structure: The main chain is hexane, and at the 3rd carbon, there is a methyl group.
- Check for chiral carbon: The carbon at position 3 is bonded to:
- A propyl group (from the hexane chain)
- A methyl group
- A hydrogen atom
- A butyl group (the rest of the hexane chain)
- Since all four groups attached to this carbon are different, this carbon is chiral. Therefore, 3-methylhexane is optically active.

- **Option B: Propane**
- Structure: Propane has three carbon atoms.
- Check for chiral carbon: All carbons in propane are either bonded to hydrogens or are part of the same alkyl group.
- There are no carbons with four different substituents, so propane is optically inactive.

- **Option C: 2,3,4-trimethylpentane**
- Structure: This alkane has a pentane chain with three methyl groups at positions 2, 3, and 4.
- Check for chiral carbon: The carbons at positions 2 and 3 are bonded to two identical methyl groups.
- Since there are no carbons with four different substituents, this compound is optically inactive.

- **Option D: 2-methylbutane**
- Structure: This alkane has a butane chain with a methyl group at position 2.
- Check for chiral carbon: The carbon at position 2 is bonded to:
- A methyl group
- A butyl group (the rest of the butane chain)
- Two hydrogen atoms
- Since there are two identical hydrogen atoms, this carbon is not chiral, making 2-methylbutane optically inactive.

3. **Conclusion**: The only alkane that is optically active is **3-methylhexane** (Option A).

### Final Answer:
**A) 3-methylhexane is the optically active alkane.**