Physical Properties

Physical State

Pure haloalkanes and haloarenes are colourless. Exposure to light, however, causes bromo- and iodo-compounds to develop colour over time as traces of the free halogen are liberated. Several of the lower, more volatile members carry a distinctly sweet smell.

Not every low-molecular-mass halide is a liquid. $\text{CH}_3\text{Cl}$, $\text{CH}_3\text{Br}$, $\text{CH}_3\text{CH}_2\text{Cl}$ and some chlorofluoromethanes are gases under normal room conditions — the halogen substitution alone isn't enough to hold these small, light molecules in the liquid state at everyday temperature. From there upward the homologous series is made up of liquids, and the very heavy members are solids.

Melting and Boiling Points

Carbon–halogen bonds are polar, so haloalkane and haloarene molecules carry a permanent dipole. Compared with the hydrocarbon they are derived from, a halide also has a distinctly higher molecular mass. Both effects combine — the added dipole-dipole attraction plus stronger van der Waals interaction from the heavier, more polarisable halogen — so a haloalkane's intermolecular pull is consistently greater than that of the parent hydrocarbon of similar size. This is why chloro-, bromo- and iodoalkanes boil well above hydrocarbons of comparable molecular mass.

Chain length. Within one homologous series, adding $-\text{CH}_2-$ units increases the electron count and the surface area available for van der Waals contact, so boiling point climbs steadily as the carbon chain gets longer.

Halogen identity. For a fixed alkyl group, boiling point rises down the halogen group:

A bigger, more easily polarised halogen atom sets up stronger van der Waals attraction between molecules, so the boiling point order tracks the halogen's size and mass rather than its electronegativity. (This comparison across the series $\text{CH}_3\text{X}$, $\text{CH}_3\text{CH}_2\text{X}$ and $\text{CH}_3\text{CH}_2\text{CH}_2\text{X}$ is summarised graphically in Fig. 6.1 in the textbook — chlorides sit lowest, bromides in the middle, iodides highest, at every chain length shown.)

Branching (isomer effect). Among isomeric haloalkanes, boiling point falls as branching increases. A straight-chain isomer presents more surface area to neighbouring molecules than a compact, branched one, so its van der Waals attractions are stronger and it boils higher; the most branched isomer, being closest to spherical, has the least surface contact and therefore the lowest boiling point. For the three isomeric bromobutanes this gives:

The fully branched tertiary halide, 2-bromo-2-methylpropane, has the lowest boiling point of the three, consistent with it having the smallest exposed surface area.

Dihalobenzenes. Isomeric dihalobenzenes ($ortho$-, $meta$- and $para$-) boil at nearly identical temperatures, since boiling point depends mainly on the strength of intermolecular attraction in the liquid, which changes little with the position of the two halogens on the ring. Melting point behaves differently: the $para$-isomer melts distinctly higher than its $ortho$- and $meta$-counterparts. Its symmetrical shape allows the molecules to pack more efficiently into a crystal lattice, so more energy (a higher temperature) is needed to break that ordered solid-state arrangement.

Density

Bromo- and iodo-alkanes, along with polychlorinated compounds, are all denser than water. Density rises as the number of carbon atoms increases, as the number of halogen atoms on the molecule increases, and as the atomic mass of the halogen present increases — heavier, more numerous halogen atoms add proportionally more mass than volume. This trend is why, for instance, moving from a mono- to a dichloro- to a trichloro- to a tetrachloro- derivative of methane raises the density step by step, and why the polyhalogen compounds among these are markedly denser than water.

Solubility …