Boiling point is highest for which compound?

Options

A. Pentane
B. 2-Methylbutane
C. 2, 2-dimethylpropane
D. 2-Methylpropane

Answer

To determine which compound has the highest boiling point, we need to analyze the factors that affect boiling points, particularly focusing on van der Waals forces (also known as London dispersion forces) and how they relate to molecular structure.

### Step-by-Step Solution:

1. **Understand the Factors Affecting Boiling Point**:
- The boiling point of a compound is influenced by the strength of intermolecular forces. The stronger the forces, the higher the boiling point.
- The main types of intermolecular forces include hydrogen bonding, dipole-dipole interactions, and van der Waals forces. For non-polar compounds, van der Waals forces are the primary consideration.

2. **Identify the Role of Molar Mass**:
- Generally, larger molecules (higher molar mass) have stronger van der Waals forces due to increased surface area, leading to higher boiling points.
- If the molar mass is the same, the shape of the molecule and surface area become important.

3. **Consider Molecular Shape and Branching**:
- Linear molecules have a larger surface area compared to branched molecules. More surface area allows for stronger van der Waals interactions.
- As branching increases, the molecule becomes more compact, which decreases surface area and consequently weakens van der Waals forces.

4. **Analyze the Given Compounds**:
- If we have isomers (compounds with the same molecular formula but different structures), we can compare them based on their branching.
- For example, if we consider isomers like 2-methylbutane, 2,2-dimethylbutane, and n-pentane, we note that n-pentane is a straight-chain molecule with no branching, while the others are branched.

5. **Determine the Highest Boiling Point**:
- Among the isomers, the straight-chain (linear) structure will have the highest boiling point due to the maximum surface area and the strongest van der Waals forces.
- Therefore, the compound with the highest boiling point is the one with the least branching and the highest molar mass.

### Conclusion:
Based on the analysis, the compound with the highest boiling point is the one that is linear and has the highest molar mass among the given options.