Diastereomeric salts of lactic acid and 1-phenylethylamine, their structures and relative stabilities

Crystal structures have been determined for the two diastereomeric salts formed between S-lactic acid (S-LA) and 1-phenylethylamine (PEA). The relative stabilities of the salts have been investigated by differential scanning calorimetry and solubility measurements in acetonitrile. The less soluble salt obtained from water, (R-PEA)(S-LA), is the less dense. It belongs to the orthorhombic space group P2(1)2(1)2(1), Z = 4. The more soluble salt, (S-PEA)(S-LA) x H(2)O, crystallized from ethanol is monoclinic, space group P2(1), Z = 2. The crystal structure showed that the water molecule is well integrated into the hydrogen-bond network in the more soluble salt, which explains the fruitless attempts made to obtain the corresponding unhydrated salt. The lactate ion adopts different conformations in the two salts. The relative energies were investigated by Hartree-Fock calculations, showing that the lactate ion is in a conformation with higher energy in the more soluble salt. The difference in solubility between the two salts can be attributed to an interplay of enthalpy and entropy effects.