FORMATION OF RACEMIC COMPOUND-CRYSTALS BY MIXING OF 2 ENANTIOMERIC CRYSTALS IN THE SOLID-STATE - LIQUID TRANSPORT OF MOLECULES FROM CRYSTALTO CRYSTAL

Mixing of powdered (-)- and (+)-enantiomer crystals in the solid state gives crystals of the racemic compound. This racemic crystal formatio n 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 grad ually and new OH absorptions due to the racemic compound appear. The f ormation 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), -chlorophenyl)-1,6-d iphenylhexa-2,4-diyne-1,6-diol (11) and its derivatives, iphenyl)methy l]-2,2-dimethyl-1,3-dioxacyclopentane (17) and its derivatives, tartar ic acid (20) dimethyl tartrate (21), malic acid (22), mandelic acid (2 3), and norephedrine (24). These molecular movements and blending occu r 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 o ccurs 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, recrystallizatio n of (-)-16 (16a) and (+)-16 (16b) from solvent gives racemic crystals of 16c, although mixing of these two components as powders in the pre sence of liquid does not give 16c. In order to determine the mechanism of the molecular movement in the solid state, X-ray crystal structure s of optically active and racemic compounds and also the molecular mov ements from optically active crystal to racemic crystal have been stud ied.