Reactions of Crystalline (R)-(-)- and (S)-(+)-Mandelic Acid with Amines. Crystal Structure and Dipole Moment of (S)-Mandelic Acid. A Method of Determining Absolute Configuration of Chiral Crystals

Article abstract of DOI:10.1021/ja00239a037

The reactions of (+)-1-phenylethylamine vapor occurs more rapidly with single crystals of (+)-mandelic acid to form the (+)(+) or"p" salt than with (-)-mandelic acid to give the (+)(-) or "n" salt, known to be the less soluble on crystallization from solution.The reaction fails to show the anisotropy characteristic of some other reactions of crystalline acids with chiral gases.The crystal structure of (S)-(+)-mandelic acid has been determined.The structure is monoclinic, P2%, with a=8.629 (1) A, b=5.861 (1) A, c=15.185 (2) A, β=102.76 (1) deg, and Z=4.The structure was determined by direct methods and refined to R=0.035 for the 1347 reflections recorded with a single-crystal diffractometer.The dipole moments of the mandelic acid molecules in the conformations found in the crystal structure determination were calculated by an ab initio SCF method.These results, together with the crystal morphology and known absolute configuration of the crystal, can be used to determine the direction of the electric dipole of Single crystals of (S)-mandelic acid.The result is in agreement with the direction determined by spraying heated crystals with the powder mixture of Burker in the classic test for absence of a crystal center of symmetry.When the argument is inverted it provides a method for determining the absolute configuration of chiral, electrically polar molecules from the X-ray crystal structure, crystal morphology, and direction of the electric dipole of the crystal.The reaction of mandelic acid crystals with phenylethylamine vapor is discussed in terms of the crystal structure.A survey has been made of the solid-gas reactions of mandelic acid with ammonia, 2-butylamine, and tert-butylamine.