how can you tell if a molecular compound will be soluble in water?
organic salts (RCO2-Na+ & NR4+Cl-)
monofunctional alcohols (ROH)
(all less than five carbons)
As a general rule, polar substances dissolve in polar solvents and nonpolar substances dissolve in nonpolar solvents. As a result, hydrocarbons don't dissolve in water. Molecules which have a hydrogen atom attached directly to an oxygen or a nitrogen are capable of hydrogen bonding: intermolecular bonds are formed between the water and the -OH or -NH functional group leading to increased solubility.
The soluble molecules in the list above have properties between the extremes of hydrocarbons and water. When the hydrocarbon chain is short, the molecule is soluble in water. For example, there is no limit on the amount of methanol (CH3OH) and ethanol (CH3CH2OH) that can dissolve in a given quantity of water.
As the hydrocarbon chain becomes longer, molecules with -OH or -NH functional groups become less soluble in water. The hydrophobic character of the molecule increases, and the solubility of the molecules in water gradually decreases until they becomes essentially insoluble in water.
Points to note are:
A compound is deemed soluble in water if it dissolves to the extent of 3.3 g / 100 mL of water.
In most homologous series of molecules with -OH or -NH functional groups, the longest chain with appreciable water solubility will be reached at about five carbons.
Introduction of a halogen will usually lower the solubility.
A benzene ring approximates 4 aliphatic carbons.
Position of functional group, branching, and the compactness of the molecule will affect solubility:
functional group toward centre of molecule - increased water solubility
increase in the number of branches - increased water solubility
more compact molecular shape - increased water solubility