Optically Active and Racemic Glycerides. I. The Thermodynamic and Structural Relationship between Optically Active and the Corresponding Racemic α-Monostearins in Their Crystalline, Molten, and Dissolved States

The thermodynamic and spectrographic properties, such as the melting temperature, the latent heat of fusion, the solubility, the heat of solution, and the infrared and NMR spectra, were studied for the stable β-forms of the optically active (S)- and the corresponding racemic (RS)-α-monostearin crystals. The phase diagram for the binary mixture and the IR spectra of (S)- and (RS)-crystals indicate that the (RS)- form is a racemic molecular compound consisting of (R)- and (S)-α-monostearin molecules. The latent heats of fusion for the (S)- and (RS)-α-monostearin crystals were 82.4 kJ mol−1 and 75.4 kJ mol−1 respectively. The heats of solution in benzene for the (S)- and the (RS)-crystals were 86.3 and 99.9 kJ mol−1 respectively. The difference in the heat of solution between the (S)- and the (RS)-crystals, Δ(ΔHs), was 13.6 kJ mol−1, which may be attributable to the difference in the extent of hydrogen bonding. These results suggest that (R)- and (S)-molecules in the (RS)-form strongly interact to form a racemic pair not only in the crystalline state, but also in the molten state. Such a racemic pair dissociates into the free molecules when it is dissolved in a nonpolar solvent. The conclusion is supported by the IR and NMR spectra for both the glycerides in dissolved states.