Amongst the following compounds, the optically active alkane having lowest molecular mass is:

Options

A. `CH_(3)-CH_(2)-CH_(2)-CH_(3)`
B. `CH_(3)-CH_(2)-overset(underset(|)(CH_(3)))(C)H-CH_(3)`
C.
D. `CH_(3)-CH_(2)-C=CH`

Answer

To determine which of the given compounds is the optically active alkane with the lowest molecular mass, we need to identify the presence of chiral centers in each compound. A chiral center is a carbon atom that is bonded to four different groups, making the compound capable of optical activity.

### Step-by-Step Solution:

1. **Identify the Compounds**:
Let's list the compounds given in the question (assuming they are provided):
- Compound 1: Normal butane (C4H10)
- Compound 2: 2-Methylbutane (C5H12)
- Compound 3: A compound with a chiral carbon (C5H10)
- Compound 4: An alkene or alkyne (C4H6 or C4H8)

2. **Analyze Compound 1 (Normal Butane)**:
- Structure: CH3-CH2-CH2-CH3
- Chiral Centers: There are no chiral carbons in this compound because all the carbon atoms are either terminal or bonded to identical groups.
- Conclusion: Optically inactive.

3. **Analyze Compound 2 (2-Methylbutane)**:
- Structure: CH3-CH(CH3)-CH2-CH3
- Chiral Centers: The second carbon is bonded to two identical methyl groups (CH3), hence it is not a chiral center.
- Conclusion: Optically inactive.

4. **Analyze Compound 3**:
- Structure: CH3-CH(CH2)-C(Cyclopropane)
- Chiral Centers: The carbon atom attached to the cyclopropane and two other different groups (CH3 and CH2) is a chiral center.
- Conclusion: This compound is optically active.

5. **Analyze Compound 4**:
- Structure: CH3-CH2-C≡CH (assuming it is an alkyne)
- Chiral Centers: There are no chiral carbons in this compound as there are no carbons bonded to four different groups.
- Conclusion: Optically inactive.

6. **Determine Molecular Mass**:
- Normal Butane (C4H10): 58 g/mol
- 2-Methylbutane (C5H12): 72 g/mol
- Compound 3: Assuming it is C5H10, the molecular mass would be 70 g/mol.
- Compound 4 (if C4H6): 54 g/mol (but not optically active).

7. **Final Conclusion**:
The only optically active compound is Compound 3, and it has the lowest molecular mass among the optically active compounds. Therefore, the answer is **Compound 3**.