Physical properties of organic molecules depend both on the nature of the functional group and the size and shape of the molecule. Organic chemists often use the water solubility or insolubility as a means of separating an organic product from other organic or inorganic substances that may be present in a reaction mixture.
How can water solubility be correlated with structure?
For a substance to be water-soluble
Organic molecules containing moderately polar or polar functional groups and fewer than five carbons are to some extent water-soluble because
The water solubility of ALL organic compounds decreases as the number of carbons increases.
How can water solubility be correlated with structure?
For a substance to be water-soluble
the attractive forces between the particles of the substance and water must be at least as strong as
the attractive forces between the particles of the substance and the attractive forces between water molecules.
Hydrocarbons are insoluble in water becausethe attractive forces between the particles of the substance and the attractive forces between water molecules.
the water molecule-hydrocarbon molecule attractive forces are weaker than the hydrocarbon-hydrocarbon and water-water attractive forces
Organic molecules containing moderately polar or polar functional groups and fewer than five carbons are to some extent water-soluble because
the attractive forces between water and polar region of the organic molecule are stronger than for a hydrocarbon.
functional group | nonpolar hydrocarbon | low polarity ether | ||
 | < | CH3OCH3 | < | |
| water solubility | none | soluble |
| low polarity ether | moderate polarity ester | ||
| CH3OCH3 | < |  | < |
| soluble | soluble |
| very polar alcohol | very polar carboxylic acid | ionic carboxylate salt | ||
| CH3CH2OH | < |  | < |  |
| soluble | soluble | soluble |
The water solubility of ALL organic compounds decreases as the number of carbons increases.
Low polarity functional group example:
CH3OCH3 328 g L–1 CH3CH2OCH2CH3 69 g L–1
Very polar functional group example:
CH3CH2OH fully miscible (soluble in all proportions)
CH3CH2CH2CH2OH 77 g L–1
Compounds with more than six carbons have low (or no) water solubility EXCEPT if the functional group is ionic.CH3OCH3 328 g L–1 CH3CH2OCH2CH3 69 g L–1
Very polar functional group example:
CH3CH2OH fully miscible (soluble in all proportions)
CH3CH2CH2CH2OH 77 g L–1