A Trick for Using Acetone as a Crystallizing Solvent.

Acetone is a good solvent for many organic compounds. It would be advantageous if for crystallizations it could be diluted with a less volatile anti-solvent and then warmed at a low temperature to remove predominantly acetone to decrease the substrate’s overall solubility in the residual fluid so that it could be crystallized in good yield.

If this anti-solvent is higher boiling, however, the substrate is increasingly likely to oil out as the pot temperature rises. Preferably, therefore, the anti-solvent should form a lower boiling azeotrope with the acetone so that the acetone can be completely substituted at a temperature below acetone’s own boiling point of 56.2 ℃.

Fortunately, there are quite a few potential anti-solvents that form such azeotropes.

Low Boiling Binary Azeotropes of Acetone with Other Low-

Boiling, Organic Solvents

Azeotropes           v/v ratio b.p. ℃

Acetone/ 1-chloropropane    15/85         45.8

Acetone/ cyclohexane            67/33         53.0

Acetone/ cyclopentane            36/64 41.0

Acetone/ hexane           59/41          49.8

Acetone/ isobutyl chloride        73/27         55.8

Acetone/ diisopropyl ether        56.5/43.5 53.3

Acetone/ methanol          88/12         55.7

Note that the first five in the list lead to a less-polar mixture as acetone is removed. Diisopropyl ether leads to something of similar polarity while removing the azeotrope with methanol eventually gives a more polar medium as the acetone is driven off.

Using diisopropyl ether is to be discouraged since it can have such strict safety requirements.