Van der Waals force
The melting and boiling points of unsaturated hydrocarbons compared to saturated ones are determined by two opposing factors. On the one hand, the strength of Van der Waals force depends on the number of electrons in a molecule. Unsaturated hydrocarbons have less electrons than saturated ones, so the boiling and melting point may decrease as intermolecular force decreases. On the other hand, the delocalized π electrons existing in the unsaturated hydrocarbons make the electron flow more easily within one molecule, so temporary dipoles are easier to form. Thus, the Van der Waals force may also increase due to delocalization of electrons. It turns out that alkynes are more affected by electron delocalization and usually have higher boiling points than alkanes with the same number of carbon. Alkenes are more affected by number of electrons and have lower boiling points than alkanes.