The definition of racemate, an equimolar mixture of enantiomers, is old hat to most of us having learned it in our introductory organic chemistry courses. And so it was for me until recently.
Being a synthetic chemist, I don’t have much time or energy to gain a deep understanding of physicochemical properties. Therefore, I was not surprised to find that I was ignorant of the fact that in the solid state racemates come in two flavors and that the IUPAC has developed specefic nomenclature to define each. There is a “racemic compound” where the two enantiomers are co-crystallized homogenously into a single crystal. However, racemates can also exist as “racemic conglomerates” which describes the situation where the two enantiomers crystallize separately but still might be occluded in a single particle.
Tangentially related to this topic is Wallach’s rule which states that racemic crystals (ie. racemic compounds) are more dense than the crystals of the pure enantiomers. Which makes me think…since increased crystalline density is generally associated with increased stability and increased crystalline stability can be correlated with reduced aqueous solubility, one would expect that racemates would be less soluble than their pure enantiomers. I did a search of the literature see if this was the case and come up with the examples below..there appears to be a slight trend.