Why are solid alkyl halides insoluble in water?

Sep 28, 2016

The majority of the molecule is non-polar.

Explanation:

Alkyl halides are basically aliphatic carbon chains with a halogen atom or atoms bonded to them.

The halogen atom is electronegative, so it will have some polarising effect on the molecule, however, unless the molecule is very small, most of it will consist of aliphatic carbon chain, which is non-polar.

If you take a small molecule such as chloroform ($C {H}_{3} C l$) then there is quite a substantial polarising effect, and chloroform is soluble in water at around 1 g per 100 ml. If you add another carbon, and take the example of chloroethane ($C {H}_{3} C {H}_{2} C l$) the solubility drops to 0.5 g per 100 ml (you still have a polarising chlorine atom there, but an extra methyl group which is not polar).
By the time we move to the n-butyl chloride (CH_3CH_2CH_2CH_3Cl) the effect of the 4 carbon chain has dropped polarity so much that water solubility reduces to around 0.05 g per 100 ml!

For an alkyl halide to be solid, you would expect the alkyl chain to be very long - even the C10 derivative 1-chlorodecane is liquid at room temperature. It means that the molecules will have very long chains of carbon-carbon bonds, which are non polar and insoluble in water.