Discrimination in Resolving Systems: Pseudoephedrine - Mandelic Acid. E. J. Valente, P. M. Knight, M. C. Moore, Department of Chemistry, Mississippi College, Clinton, MS 39058.

(-)- (1S,2S)-Pseudoephedrine and racemic mandelic acid form three distinct diastereomeric salts from solutions in 95% ethanol. The least-soluble phase, a hemihydrate, contains the 2R-mandelate. A salt phase of intermediate solubility is the unsolvated pseudoracemate, containing both the 2R-and the 2S-mandelate. The most-soluble salt phase contains the 2S-mandelate. Mandelate configuration and order of solubility (based on the heats of fusion) is inverted from that found in the same system synthesized from chiral base and acid, and then crystallized from benzene. The 2R-mandelate hemihydrate (-H₂O at 349.5K, mp 389.2K), monoclinic, P2₁, a = 6.788(5), b = 29.415(35), c = 9.488(10)Å, be = 108.91(8)deg, Z = 4 (2 ion-pairs/asymmetric unit). Intermediate pseudoracemate 2S- and 2R-mandelate, mp 377.6K, anorthic, P1, a = 7.758(4), b = 9.966(5), c = 13.366(6)Å, al = 72.99(4), be = 79.98(4), ga = 70.51(4)deg, Z = 1 (2 ion-pairs/asymmetric unit). The 2S-mandelate (mp 386.2K), orthorhombic, P2₁2₁2₁, a = 7.079(6), b = 13.443(10), c = 18.820(14)Å, Z = 4. While differing from ephedrine mandelates in configuration at one center, solubilities of pseudoephedrine mandelates in 95% ethanol are much larger. A comparison of molecular structures (packing and H-bonding) of pseudoephedrine and ephedrine mandelates shows they are unrelated. This study reemphasizes the necessity for consistency in solvent use in resolution and in phase identification and comparison because of the frequency of polymorphism.