Racemic Cis-π-Camphanic Acid. Unusual Solid State Transformation of a Conglomerate into a Plastic Phase

Abstract The phase diagram between the enantiomers of cis-π-camphanic acid has been reexamined by differential scanning calorimetry (DSC). The phase diagram is that of a conglomerate (a mechanical mixture of the crystals of the enantiomers) which transforms into a plastic phase before melting. Despite anomalous thermograms observed on first heating, the existence of a solid solution intermediate between the conglomerate and the plastic phase is not confirmed. At the transition, both an orientational disorder and a molecular diffusion (leading to a chiral disorder) take place. These features are in agreement with the X-ray structure (space group P61) of the low-temperature form of the enantiomer. This structure suggests an hexagonal compact stacking for the plastic phase.