Solubility Tests
Like dissolves like; a substance is most soluble in that solvent to which it is most
closely related in structure. This statement serves as a useful classification scheme
for all organic molecules. The solubility measurements are done at room temperature
with 1 drop of a liquid, or 5 mg of a solid (finely crushed), and 0.2 mL of
solvent. The mixture should be rubbed with a rounded stirring rod and shaken
vigorously. Lower members of a homologous series are easily classified; higher
members become more like the hydrocarbons from which they are derived.
If a very small amount of the sample fails to dissolve when added to some of
the solvent, it can be considered insoluble; and, conversely, if several portions dissolve
readily in a small amount of the solvent, the substance is obviously soluble.
If an unknown seems to be more soluble in dilute acid or base than in
water, the observation can be confirmed by neutralization of the solution; the
original material will precipitate if it is less soluble in a neutral medium.
If both acidic and basic groups are present, the substance may be amphoteric
and therefore soluble in both acid and base. Aromatic aminocarboxylic
acids are amphoteric, like aliphatic ones, but they do not exist as zwitterions.
They are soluble in both dilute hydrochloric acid and sodium hydroxide, but not
in bicarbonate solution. Aminosulfonic acids exist as zwitterions; they are soluble
in alkali but not in acid.
The solubility tests are not infallible and many borderline cases are known.
Carry out the tests according to the scheme of Fig. 70.2 and the following “Notes
to Solubility Tests,” and tentatively assign the unknown to one of the groups I–X.