In fractional distillation the shorter hydrocarbons have lower boiling points and distil off first. Why?

Boiling points are determined by the interactions between the individual hydrocarbon molecules. The weaker the interactions, the less energy is required to boil the hydrocarbon and the lower the boiling point. Shorter hydrocarbons have weaker interactions whereas longer hydrocarbons have stronger interactions between each other. As a result they have different boiling points and can be separated by fractional distillation.