How does branching in alkanes affect their boiling points?

Branching in alkanes generally decreases their boiling points due to a reduction in surface area and intermolecular forces.

In more detail, alkanes are hydrocarbons, compounds made up of hydrogen and carbon atoms. They are categorised as saturated hydrocarbons because they contain only single bonds between the carbon atoms. The boiling point of an alkane is primarily determined by the strength of the intermolecular forces between the molecules. These forces are known as van der Waals forces, or London dispersion forces, which are temporary attractive forces that occur when electrons in the molecule are not evenly distributed.

In straight-chain alkanes, the molecules can pack closely together, maximising the contact between them and thus maximising the strength of the van der Waals forces. This results in a higher boiling point because more energy is required to overcome these forces and convert the substance from a liquid to a gas.

However, when alkanes are branched, the shape of the molecules changes. They become more spherical and the surface area available for contact with other molecules is reduced. This decreases the strength of the van der Waals forces because there is less opportunity for the molecules to interact. As a result, less energy is needed to overcome these forces, so the boiling point is lower.

For example, compare the boiling points of pentane (a straight-chain alkane with five carbon atoms) and 2,2-dimethylpropane (a branched alkane with the same number of carbon atoms). Pentane has a boiling point of 36°C, while 2,2-dimethylpropane has a boiling point of just 9.5°C. This clearly demonstrates the effect of branching on the boiling points of alkanes.