Structural and Thermodynamic Relationships Between Optically Active and Racemic Compounds. The Crystal Structures of Optically Active Chloro‐ and Bromo‐Substituted 3‐Hydroxy‐3‐phenylpropionic Acids

The crystal structures of the isostructural optically active R-(+)-meta-chloro- and meta-bromo-substituted 3-hydroxy-3-phenylpropionic acids have been determined from low-temperature [122.0(5)K] X-ray [λ(Cu Kα) = 1.54184 A] diffraction data. (R)-(+)-3-Hydroxy-3-(3'-chlorophenyl)propionic acid (C 9 H 9 ClO 3 , M r = 200.61), orthorhombic, P2 1 2 1 2 1 , a = 5.6938 (7), b = 5.7913 (7), c = 27.052 (4) A, Z= 4, m.p. = 365.8 (5) K. 3-Hydroxy-3-(3'-bromophenyl)propionic acid (C 9 H 9 BrO 3 , M r = 245.07), orthorhombic, P2 1 2 1 2 1 , a = 5.8020(8), b = 5.8638 (8), c = 27.345 (4) A, Z = 4, m.p. = 350.3 (5) K. The O-H...O hydrogen-bond pattern found in these acids is identical to those found in the equivalent racemic compounds with hydrogen-bonded chains parallel to the a axis. The enantiomeric and racemic crystals differ only in the orientation of these chains. Short contact distances are found between the halogen atoms and one of the H atoms of the phenyl group. Statistical data show that these interactions are of repulsive character. The structural results are related to the thermodynamic measurements, which show that the conglomerate and racemic compound are in equilibrium for the chloro-substituted acid. The formation of a racemic compound for the bromo-substituted acid appears to be favoured by repulsive H...Br interactions in the enantiomeric crystals, giving the latter a higher enthalpy.