Organic Chemistry MCAT Practice Exam

What characteristic of a molecule typically leads to a higher boiling point?

1. Less branching in the molecule's structure

2. Lower molecular weight

3. Higher volatility

4. More functional groups

The correct answer is: Less branching in the molecule's structure

A higher boiling point in a molecule is often influenced by its structural characteristics, particularly the degree of branching. In general, less branching in a molecule's structure tends to lead to a higher boiling point due to increased surface area and stronger van der Waals (dispersion) forces between the molecules. When a molecule has more branching, it tends to have a more compact shape, which reduces the overall surface area available for intermolecular interactions. This diminished contact can result in weaker dispersion forces. In contrast, a less branched structure allows for more effective packing and interaction between molecules, leading to stronger forces that require more energy (in the form of heat) to break apart during the phase transition from liquid to gas. While molecular weight and the presence of functional groups also play significant roles in determining boiling points, the degree of branching has a particularly pronounced effect. Higher molecular weight typically correlates with higher boiling points due to increased London dispersion forces, but this is not always exclusively the contributing factor. Volatility generally opposes boiling point, as more volatile substances have lower boiling points. Thus, the characteristic that usually leads to a higher boiling point is indeed associated with less branching in the molecule's structure, allowing for greater surface area and stronger intermolecular forces.