Why are solid alkyl halides insoluble in water?

Question

Emilia Alvarez · Accepted Answer

The majority of the molecule is non-polar.

Alkyl halides are essentially chains of aliphatic carbon that have one or more halogen atoms bound to them. Because the halogen atom is electronegative, it will polarize the molecule to some extent; however, unless the molecule is extremely small, the majority of its structure will be made up of the non-polar aliphatic carbon chain. If you take a small molecule such as chloroform (#CH_3Cl#) 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 (#CH_3CH_2Cl#) 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! Since even the C10 derivative 1-chlorodecane is liquid at room temperature, you would expect an alkyl halide to be solid if its alkyl chain were very long. This would mean that the molecules would have very long chains of non-polar carbon-carbon bonds that are insoluble in water.

Ryan Andrews · Answer

undefined Solid alkyl halides are insoluble in water due to the large difference in polarity between the alkyl group and the water molecule. Alkyl halides consist of a nonpolar hydrocarbon chain (the alkyl group) bonded to a polar halogen atom (such as chlorine, bromine, or iodine). Water, on the other hand, is a polar molecule with a partial positive charge on the hydrogen atoms and a partial negative charge on the oxygen atom. This difference in polarity results in a lack of attractive forces between alkyl halides and water molecules, leading to their insolubility in water. Additionally, the relatively weak van der Waals forces between alkyl halide molecules further contribute to their inability to form stable interactions with water molecules.