Physical Properties of Haloalkanes and Haloarenes

Boiling points. Haloalkanes have distinctly higher boiling points than the alkane of the same carbon skeleton. This is because the polar $\text{C--X}$ bond adds a dipole-dipole attraction on top of the London (dispersion) forces already present between the hydrocarbon parts of neighbouring molecules, and because the halogen atom itself, especially the larger ones, is far more polarisable than a hydrogen atom, strengthening the dispersion contribution too. Among the methyl halides, boiling point rises steadily down the group: $\text{CH}_3\text{F}$ ($-78\,^\circ\text{C}$) $<$ $\text{CH}_3\text{Cl}$ ($-24\,^\circ\text{C}$) $<$ $\text{CH}_3\text{Br}$ ($3.6\,^\circ\text{C}$) $<$ $\text{CH}_3\text{I}$ ($42\,^\circ\text{C}$). This may look surprising given that the $\text{C--F}$ bond is the most polar of the four, but boiling point is governed by the total strength of intermolecular attraction, and the much greater polarisability and greater mass of the heavier halogens dominate over the shrinking bond-dipole contribution as the halogen gets larger.

Effect of chain branching. As with ordinary alkanes, increasing branching in a haloalkane lowers its boiling point for a given molecular formula, because a more branched, more nearly spherical molecule presents less surface area for van der Waals contact with its neighbours than a straight-chain isomer does. So, for isomers of the same molecular formula, the straight-chain haloalkane boils higher than its branched isomers, and the most branched isomer (e.g. a tertiary halide) boils lowest.

Density. Monohalogenated methanes, ethanes and propanes are typically less dense than water, but density rises with the number of halogen atoms and with the mass of the halogen; polyhalomethanes such as $\text{CHCl}_3$, $\text{CCl}_4$ and $\text{CH}_2\text{I}_2$ are all denser than water. …