Alkanes experience inter-molecular Van der Waals forces. The stronger the force will be, the greater would be the boiling point of the alkane.

As branching increases, the surface area of the molecule decreases resulting in a small area of contact. As a result, the Van der Waals force also decreases which can be overcome at a relatively lower temperature. Hence, the boiling point of an alkane chain decreases

with an increase in branching.