Physical Properties of Haloalkanes

Pure haloalkanes are colourless. However, Bromoalkanes and Iodoalkanes develop colour when exposed to light and stored for longer periods.
    
1. Boiling points

For same alkyl group the boiling points of haloalkanes are in the order RCl < RBr < RL. It is because with the increase in size and mass of the halogen atom the magnitude of van der Waal forces of attraction increases.



Among isomeric alkyl halides, the boiling point increases with increase in branching in the alkyl group. This is due to the reason that with increase in branching, the molecule attains spherical shape with less surface area. As a result interparticle forces become weaker, resulting in lower boiling point. For example, among isomeric butyl chlorides, the straight chain isomer, butyl chloride has the highest boiling point whereas tert-butyl chloride has the lowest boiling point.



For same halogen, the boiling point increases with increase in molecular mass.

It may be mentioned here that methyl chloride, methyl bromide and some chlorofluoromethanes are gases at room temperature. Higher members of haloalkanes are liquids and solids.

Generally, the boiling points of various halogen compounds increase in number of halogen atoms. For example, consider the boiling points of chloros derivatives of methane.

Haloalkanes have higher boiling points as compared to those of corresponding alkanes. This is due to their polarity and strong dipole-dipole attractive interactions between haloalkanes molecules.

2. Solubility in water

Although polar in nature, alkyl halides are insoluble in water because they are not able to form hydrogen bonds with water molecules. As a result, the energy required overcoming the forces of attraction between the haloalkane molecules and that required to break the hydrogen bonds between water molecules is more than the energy released when new attractions are set up between haloalkane and water molecule.

However, they are soluble in organic solvents such as alcohols, ether and benzene.
    
3. Density

Alkyl chlorides are generally lighter than water whereas bromides and iodides are heavier. Their densities are in the order R-I > R-Br > R-CI. Polychlorides are however, heavier than water. In general, the density of halogen derivatives increases with increase in number and atomic mass of the halogen atoms.

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