[SidM]

Why does the Boiling Point of Haloalkanes increase as we go down the group? I understand that dispersion forces increase, but polarity of the C-X bond decreases down the group. I remember having learned that dipole-dipole interactions dominate dispersion forces, why is this not the case in this instance?

[Corribus]

You have to consider the polarity of the entire molecule, not just the electronegativity differences for the carbon-halogen bonds. If we use the dipole moment as a metric for "polarity" and, hence, strength of dipolar forces, consider these dipole moment values for the halomethanes:

Dipole moment (D), boiling point (°C)
Fluoromethane: 1.856, -78.4
Chloromethane: 1.8963, -24.1
Bromomethane: 1.8203, 3.4
Iodimethane: 1.6406, 42.4

Particularly with the fluoro, chloro, and bromo substituted methanes, there is very little difference in dipole moment. The dipole moment is a function of charge over difference. Although C-F certainly has a larger EN difference than C-Br, the latter is a much longer bond, and so the degree of effective polarity is moderated along the series.

Clearly the boiling points still change despite the dipole moments of the molecules being reasonably similar. The halogens differ substantially in molecular mass, and so their strength and number of dispersion forces also increases substantially going down the series. This is why dispersion forces seem to dominate the trend in boiling points in the haloalkane series.