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

Abstract

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)-1-phenylethylammonium (S)-mandelate co-crystallized with two molecules of mandelic acid. This compound, C8H12N+·C8H7O3–·(C8H8O3)2, has been characterized by X-ray diffraction at –163°C measured with Cu-Kα radiation. It crystallizes in the orthorhombic space group P212121 with a = 9.5497(5), b= 9.8186(9), c = 31.001 (3)Å, V= 2906.8(7)Å3, 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)-1-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.