Active dimethyl tartrate melts at 43.3°; racemic dimethyl tartrate at 89.4°. Active mandelic acid melts at 132.8°; the racemic acid at 118.0°. In the one case, therefore, the racemic compound has a higher, in the other a lower melting point than the active forms.
In the case of partially racemic compounds (i.e. the compound of a racemate with an optically active substance) the type of curve will be the same, but the figure will no longer be symmetrical. Such a curve has been found in the case of the l-menthyl esters of d- and l-mandelic acid (Fig. 72).[[300]] The freezing point of l-menthyl d-mandelate is 97.2°, of l-menthyl l-mandelate 77.6°, and of l-menthyl r-mandelate 83.7.° It will be observed that the summit of the curve for the partially racemic mandelate is very flat, indicating that the compound is largely dissociated into its components at the temperature of fusion.
III. The inactive substance is a pseudo-racemic mixed crystal.
In cases where the active components can form mixed crystals, the freezing-point curve will exhibit one of the forms given in Fig. 65. The inactive mixed crystal containing 50 per cent. of the dextro and laevo compound, is known as a pseudo-racemic mixed crystal.[[301]] So far, only curves of the types I. and II. have been obtained.
Examples.—The two active camphor oximes are of interest from the fact that they form a continuous series of mixed crystals, all of which have the same melting point. The curve which is obtained in this case is, therefore, a straight line joining the melting points of the pure active components; the melting point of the active isomerides and of the whole series of mixed crystals being 118.8°.
In the case of the carvoximes mixed crystals are also formed, but the equilibrium curve in this case exhibits a maximum (Fig. 73). At this maximum point the composition of the solid and of the liquid solution is the same. Since the curve must be symmetrical, this maximum point must occur in the case of the solution containing 50 per cent.
of each component, which will therefore be inactive. Further, this inactive mixed crystal will melt and solidify at the same temperature, and behave, therefore, like a chemical compound (p. [187]). The melting point of the active compounds is 72°; that of the inactive pseudo-racemic mixed crystal is 91.4°·