OPTICAL RESOLUTION THROUGH DIASTEREOMERIC SALT FORMATION - THE CRYSTAL-STRUCTURES OF CINCHONINIUM (R)-MANDELATE AND CINCHONINIUM (S)-MANDELATE AT LOW-TEMPERATURE

The crystal structures of the diastereomeric salts formed between cinchonine and mandelic acid have been analysed. The more soluble of the two salts is cinchoninium (S)-mandelate, C19H23N2O+.C8H7O3-, M(r) = 446.55, monoclinic, P2(1), T = 110(1) K, a = 11.2478(10), b = 7.0880(6), c = 14.684(2) angstrom, beta = 99.068 (7)-degrees, V = 1156.0 (4) angstrom3, Z = 2, D(x) = 1.283 g cm-3, lambda(Cu Kalpha) = 1.54184 angstrom, mu = 6.57 cm-1, F(000) = 476, R = 0.029 for 2338 observed reflections. The less soluble salt is cinchoninium (R)-mandelate, C19H23N2O+.C8H7O3-, M(r) = 446.55, orthorhombic, P2(1)2(1)2(1), T = 122.0 (5) K, a = 12.078 (4), b = 17.423 (2), c = 22.136 (4) angstrom, V = 4658 (3) angstrom3, Z = 8, D(x) = 1.273 g cm-3, lambda(Cu Kalpha) = 1.54184 angstrom, mu = 6.52 cm-1, F(000) = 1904, R = 0.055 for 4125 observed reflections. In the (R)-mandelate salt one of the two independent cinchoninium cations displays disorder of the -CH=CH2 moiety. Apart from variations in the conformation of the vinyl group the cinchoninium cations are alike. The difference in conformation of the (R)- and (S)mandelate ions can be related to differences in the hydrogen-bonding systems. Interionic interactions in the S salt stabilize it relative to the R salt. The differences in the melting enthalpies and entropies of the two diastereomeric salts are related to their structural differences.