London dispersion forces are the weak intermolecular attractive forces that occur when the molecules are spaced closely. If the spacing is compact, the interactions are stronger. This means that the temporary dipoles induced due to the dispersion of electrons are also larger. Such a compact cluster will require more energy to break, raising the physical properties of the melting and boiling point.
The long, linear hydrocarbon chain in organic molecules is more compactly packed than round, spherical ones. For example, n-pentane and neo-pentane are isomeric forms of pentane, where n-pentane is linear and unbranched, whereas neo-pentane is spherical and branched. The intermolecular force of attraction is more pronounced in n-pentane, raising its boiling point to 36 oC, while neo-pentane has a weaker force of attraction and a boiling point of 9°C.
Related Reading: London Dispersion Force