Organic Chemistry - Solubility

As members of each homologous series have the same functional group, they're expected to have similar properties, but also to have some sort of trend in these properties with the increasing carbon number.

There are two factors to consider when determining the solubility of an organic compound in water:

• The length of the hydrocarbon chain

Since this part of the molecule is non-polar, it does not facilitate the solubility of the molecule in water and so solubility will decrease as the chain length increases.

As chain lengths increase, the hydrocarbon "tails" of the molecules start to get in the way. By forcing themselves between water molecules, they break the relatively strong hydrogen bonds between water molecules without replacing them with anything as good. This makes the process energetically less profitable, and so solubility decreases. 

• The nature of the functional group

Solubility is determined by the extent to which this part of the molecule is able to interact with water (for example by forming hydrogen bonds). 

Considering these two factors, the lower members of the following homologous series are quite soluble in water:

Alcohols, Aldehydes, Ketones and Carboxylic Acids

These homologous series can't form hydrogen bonds with themselves, but can form hydrogen bonds with water molecules. 

There will also be dispersion forces and dipole-dipole attractions between the organic molecules and water molecules. Forming these attractions releases energy which helps to supply the energy needed to seperate the water molecules and organic molecules from each other before they can mix together. 

Halogenoalkanes are not soluble in water as, despite their polarity, they're unable to form hydrogen bonds with water.