Formation of racemic compound crystals by mixing of two enantiomeric crystals in the solid state. Liquid transport of molecules from crystal to crystal

Abstract

Mixing of powdered (-)- and (+)-enantiomer crystals in the solid state gives crystals of the racemic compound. This racemic crystal formation was followed by IR spectral measurement of a 1∶1 mixture of (-)- and (+)-enantiomer crystals as a Nujol mull. As the formation of racemic crystals proceeds, the OH absorptions of the enantiomer disappear gradually and new OH absorptions due to the racemic compound appear. The formation of racemic crystals from enantiomer crystals has been studied for various kinds of chiral compounds: 2,2′-dihydroxy-1,1′-binaphthyl (1) and its derivatives, 10,10′-dihydroxy-9,9′-biphenanthryl (4), 2,2′-dihydroxy-4,4′,6,6′-tetramethylbiphenyl (5) and its derivatives, 4,4′-dihydroxy-2,2′,3,3′,6,6′-hexamethylbiphenyl (8), 1,6-di(o-chlorophenyl)-1,6-diphenylhexa-2,4-diyne-1,6-diol (11) and its derivatives, trans-4,5-bis[hydroxy(diphenyl)methyl]-2,2-dimethyl-1,3- dioxacyclopentane (17) and its derivatives, tartaric acid (20) dimethyl tartrate (21), malic acid (22), mandelic acid (23), and norephedrine (24). These molecular movements and blending occur rapidly in the presence of liquids such as liquid paraffin (Nujol), seed oils such as olive, coconut, rapeseed and soybean oil, artificial oil such as silicone oil and water, although the same movement also occurs in the absence of the liquid. For example, keeping a mixture of powdered (-)-1 (1a) and (+)-1 (1b) at room temperature for 48 h gives racemic crystals (1c). However, molecular aggregation sometimes occurs in solution but not in the solid state. For example, recrystallization of (-)-16 (16a) and (+)-16 (16b) from solvent gives racemic crystals of 16c, although mixing of these two components as powders in the presence of liquid does not give 16c. In order to determine the mechanism of the molecular movement in the solid state, X-ray crystal structures of optically active and racemic compounds and also the molecular movements from optically active crystal to racemic crystal have been studied.