ISOLATION OF DIFFERENT ENANTIOMERS CAUSED BY VARIATION IN THE STOICHIOMETRIC RATIO OF RACEMATE AND RESOLVING AGENT - THE CRYSTAL-STRUCTURE OF (R)-1-PHENYLETHYLAMMONIUM (S)-MANDELATE.DIMANDELIC ACID

From an aqueous solution of racemic 1-phenylethylamine and (S)-mandelic acid in the molar ratio 1:3, the less soluble compound isolated is (R)-l-phenylethylammonium (S)-mandelate cocrystallized with two molecules of mandelic acid. This compound, C8H12N+.C8H7O3.(C8H8O3)2, has been characterized by X-ray diffraction at -163-degrees-C measured with Cu-Kalpha radiation. It crystallizes in the orthorhombic space group P2(1)2(1)2(1) with a = 9.5497(5), b = 9.8186(9), c = 31.001 (3) angstrom, v = 2906.8(7) angstrom3, Z = 4, refinement using 5177 observed reflections gives R = 0.037. An extensive system of hydrogen bonds stabilizes the structure. Comparisons are made to the product formed by the reaction of equimolar amounts of racemic 1-phenylethylamine and (S)-mandelic acid in water, where (S)-l-phenylethylammonium (S)-mandelate precipitates as the less soluble salt. The solubility in water of the 1 :3 compound has been determined and the observation that different enantiomers of 1-phenylethylamine can be precipitated with (S)-mandelic acid depending on the ratio between the base and the acid is rationalized from the solubilities in water.