Question

Why are the melting points and boiling points of haloalkanes much higher than those of the parent alkanes?

Answer 1

The melting points and boiling points of haloalkanes are much higher than those of their parent alkanes due to the following reasons:

1. Polarity of C–X Bond: Haloalkanes have a polar carbon–halogen (C–X) bond due to the large difference in electronegativity between carbon and halogen atoms. This polarity induces permanent dipole moments in the molecules.
2. Dipole-Dipole Interactions: The polar C–X bonds in haloalkanes result in dipole-dipole intermolecular forces, which are significantly stronger than the weak London dispersion forces present in nonpolar alkanes.
3. Increased Molecular Mass and Polarizability: Halogen atoms are heavier and larger than hydrogen atoms. This increases the molecular mass and polarizability of haloalkanes, enhancing van der Waals (London dispersion) forces compared to alkanes.
4. Consequences on Melting and Boiling Points: Because haloalkanes possess both stronger dipole-dipole and enhanced van der Waals forces, more energy is required to overcome these intermolecular interactions during melting and boiling. Thus, haloalkanes have higher melting and boiling points than their parent alkanes.