On the Melting of Binary Organic Compounds

The melting behaviors of three binary organic compounds, a racemate, a cocrystal, and a salt, are compared with their individual components and with each other. The three compounds are the racemic compound of mandelic acid, the benzophenone–diphenylamine cocrystal, and ephedrine pimelate. Similarities and differences are accounted for by changes in entropy, hydrogen bonding, molecular conformation, and charged state on melting. An unusual combination of thermodynamic, structural, and spectroscopic data gives insight into the nature and extent of association in these melts. The three binary compounds show surprisingly similar melting thermodynamics. The main differences are in the salt system, driven by ionization and access to a 2:1 salt. The implications for melt and solution eutectics are discussed.