The n -alkanes have the most extended structure and larger surface area in comparison to branched chain isomers having compact structure (as the shape approaches that of a sphere in the branched chain isomers). Thus, intermolecular forces are weaker in branched chain isomers, therefore, they have lower boiling points in comparison to straight chain isomers i.e., higher the branching, lower the boiling point, hence, the order is : I > II > III.
