Quasiracemate – Racemic mixture

There are four ways to crystallize a racemate; three of which H. W. B. Roozeboom had distinguished by 1899 and this is one of them.

QuasiracemateA quasiracemate is a co-crystal 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.

• Wheeler, Kraig A.; Grove, Rebecca C.; Davis, Raymond E.; Kassel, W. Scott (January 2008). “Rediscovering Pasteur’s Quasiracemates”. Angewandte Chemie International Edition. 47 (1): 78–81. doi:10.1002/anie.200704007. PMID 18022885.