Insights into the crystal structure, polymorphism and thermal behavior of menthol optical isomers and racemates

The physico-chemical properties of the levo- and dextrorotatory menthol isomers as well as the corresponding racemic compound were studied using X-ray single-crystal or powder diffraction and differential scanning calorimetry experiments. As a result, the not yet determined crystal structure of DL-menthol was solved. Moreover, the stable and metastable experimental temperature-composition phase diagrams of the L-menthol/D-menthol binary system were determined. The thermodynamic relative stability of the different menthol polymorphs was also established. The present paper provides new physical, chemical and thermodynamic data of L-, D- and DL-menthol and offers new insight into their polymorphism as well as into the levorotatory–dextrorotatory menthol interactions. Both the thermodynamic and crystallographic approaches demonstrate unambiguously that racemic menthol is a racemate.