Effect of crystallization conditions on polymorphic diversity of malic acid RS — Racemate
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Enantiomers (R(+) and S(−)), RS-racemate (double compound) and (R+S)-conglomerate (mechanical mixture of enantiomers) of malic acid C4H6O5 have been investigated by means of X-ray diffractometry and high temperature X-ray diffraction method. The RS-racemate was found to be able to form three polymorphic modifications, which we denominated as M1 (monoclinic, space group P21/c), M2 (monoclinic, space group Cc), and Tc (triclinic, space group P-1), the latter modification having been unknown before. Modification Tc was also described, and its Xray diffraction characteristics, including interplanar spacings d, hkl indices, unit cell parameters, were defined. In addition, X-ray diffraction characteristics for both reported earlier M1 and M2 monoclinic polymorphic modifications were measured with higher accuracy. The ability of RS-racemate to form one of the above three modifications (M1, M2, and Tc) or their mixtures containing various proportions and combinations of the components (M1+M2, M1+Tc, or M2 +Tc) was found to depend on the type of crystallization medium (a melt, aqueous medium, ethanol or acetone solution), crystallization rate (from 2–3 minutes to 4 months), and crystallization temperature. Heating S-enantiomer and M1 RS-racemate up to their respective melting points (100 °C and 124 °C, correspondingly) only made them undergo thermal deformations, while heating (R+S)-conglomerate in the temperature range of 96–110 °C resulted in its homogenization to form M2 RS-racemate, which, near the melting point (118 °C), namely, in the range of 112–116 °C, was transformed into M1 RS-racemate.
Keywordspolymorphism racemic chiral
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