Racemic Mixture - Crystallization

Crystallization

There are four ways in which a racemate can crystallize, three of which H. W. B. Roozeboom had distinguished by 1899:

• Conglomerate (sometimes racemic mixture or racemic conglomerate)

A mechanical mixture of enantiomerically pure crystals of one enantiomer and its opposite. Molecules in the crystal structure have a greater affinity for the same enantiomer than for the opposite enantiomer. The melting point of the racemic conglomerate is always lower than that of the pure enantiomer. Addition of a small amount of one enantiomer to the conglomerate increases the melting point.

• Racemic compound (sometimes true racemate)

Molecules have a greater affinity for the opposite enantiomer than for the same enantiomer; the substance forms a single crystalline phase in which the two enantiomers are present in an ordered 1:1 ratio in the elementary cell. Adding a small amount of one enantiomer to the racemic compound decreases the melting point. But the pure enantiomer can have a higher or lower melting point than the compound.

• Pseudoracemate (sometimes racemic solid solution)

In contrast to the racemic compound or conglomerate, there is no big difference in affinity between the same and opposite enantiomers. Overall, both enantiomers occur in equal proportions in the crystal, but they coexist in an unordered manner in the crystal lattice. Addition of a small amount of one enantiomer changes the melting point just little bit or not at all.

• Quasiracemate

A quasiracemate is a mixture of two similar but distinct compounds, one of which is left-handed and the other right-handed. Although chemically different, they are sterically similar (isosteric) and are still able to form a racemic crystalline phase. One of the first such racemates studied, by Pasteur in 1853, forms from a 1:2 mixture of the bis ammonium salt of (+)-tartaric acid and the bis ammonium salt of (−)-malic acid in water. Re-investigated in 2008, the crystals formed are dumbbell-shape with the central part consisting of ammonium (+)-bitartrate, whereas the outer parts are a quasiracemic mixture of ammonium (+)-bitartrate and ammonium (−)-bimalate.