Which of the following explanation is correct ?

Options

A. Boiling point of alkane increases with decrease in molecular weight of alkane
B. Boiling point of alkane increases with increase in branching of alkane
C. Boiling point of alkane decreases with increase in branching of alkane
D. None of the above

Answer

To solve the question regarding the boiling points of alkanes and their relationship with molecular weight and branching, we can break it down step by step.

### Step-by-Step Solution:

1. **Understanding Boiling Points of Alkanes**:
- The boiling point of a substance is influenced by the strength of intermolecular forces. In the case of alkanes, these forces are primarily Van der Waals forces (dispersion forces).

2. **Effect of Molecular Weight**:
- Generally, as the molecular weight of alkanes increases, the boiling point also increases. This is because larger molecules have more electrons, leading to stronger Van der Waals forces due to increased polarizability.

3. **Effect of Branching**:
- When comparing straight-chain alkanes to branched alkanes, it is observed that branched alkanes have lower boiling points than their straight-chain counterparts. This is due to the shape of the molecules.
- Branched alkanes tend to adopt a more spherical shape, which reduces their surface area. A smaller surface area results in weaker Van der Waals forces, making it easier for the molecules to escape into the vapor phase at lower temperatures.

4. **Conclusion**:
- From the above analysis, we can conclude:
- The statement "boiling point of alkane increases with decrease in molecular weight" is incorrect.
- The statement "boiling point of alkane increases with increase in branching" is also incorrect.
- The statement "boiling point of alkane decreases with increase in branching" is correct.

5. **Final Answer**:
- Therefore, the correct explanation is that the boiling point of alkanes decreases with an increase in branching.