Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

CHOICE OF SOLVENT 5
The choice of the right solvent is therefore of great importance
for the process of recrystallisation. The most commonly used
solvents are the following : water, ethyl alcohol, methyl alcohol, ether,
acetone, glacial acetic acid, ethyl acetate, benzene, petrol ether, chloroform,
carbon bisulphide.
For quite sparingly soluble substances, formic acid, pyridine,
bromobenzene, nitrobenzene, and occasionally also phenol, ethyl
benzoate, aniline, and dioxan are used. A distinct relation exists
between the constitution of solute and solvent, and is expressed by
the old rule : similia similibus solvuntur. Thus, as is well known,
substances containing hydroxyl (e.g. sugars, carboxylic acids) are
soluble in water, whereas hydrocarbons are more soluble in benzene
and petrol ether than, for example, in alcohols.
The above statements, however, generally hold with some degree
of certainty for simple organic compounds only. With complicated
substances the conditions are more involved, and unless the worker
has long experience he is obliged to test the available solvents
seriatim. Alcohol is used most, and with this one usually begins;
then perhaps water, benzene, and petrol ether. It may be said that,
on the whole, of the more usual solvents, benzene, chloroform, and
ether have a very great, petrol ether and water a moderate solvent
power for organic substances. Although the validity of this rule is
contravened by many substances, it nevertheless gives some indication
for testing purposes. Thus if the sample is too sparingly
soluble in alcohol a solvent from the first group is chosen; if it is too
soluble, one from the second. In the case of sparingly soluble substances
a higher boiling homologue of the same class is often chosen
—in place of the lower alcohol, propyl or amyl alcohol, instead of
benzene, toluene or xylene—because the higher boiling point brings
about increased solvent power.
It very often happens that the preparation of a substance leads
to an amorphous crude product, resinous or flocculent, which becomes
crystalline on digestion with a suitable solvent or else by
direct recrystallisation. It must be remembered that the solubilities
of the amorphous and crystalline forms of the same substance are
altogether different, and that the amorphous preparation is always
much the more soluble.
Salts dissolve quite generally with ease in water, and often also
in the alcohols, acetone, and chloroform, but they are not dissolved
by ether, benzene, or petrol ether. Consequently organic acids can