Structures of racemic halogen-substituted 3-hydroxy-3-phenylpropionic acids; relations between spontaneously resolved and racemic compounds

Low-temperature [122.0(5)K] X-ray diffraction data [λ(CuKα)=1.54184A] showed that racemic 3-hydroxy-3-(3'-chlorophenyl)propionic acid and its bromo-substituted analogue are isostructural. 3-Hydroxy-3-(3'-chlorophenyl)propionic acid : C 9 H 9 ClO 3 , M r = 200.61, monoclinic, P2 1 /c, a = 15.301 (2), b = 5.7117 (8), c = 11.2462 (13) A, β = 110.873 (10)°, V = 918.4 (2) A 3 , Z = 4, D x = 1.451 gcm -3 , μ = 35.15 cm -1 , F(000) = 416, wR 2 = 0.0775 for 1880 contributing reflections [R = 0.0274 for F > 4σ(F)], m.p. 342.7 (5) K. 3-Hydroxy-3-(3'-bromophenyl)propionic acid : C 9 H 9 BrO 3 , M r = 245.07, monoclinic, P2 1 /c, a = 15.498 (3), b = 5.7513 (12), c = 11.298 (2) A, β = 111.280 (14)°, V = 938.4 (3)A 3 , Z = 4, D x = l.735 gcm -3 , μ = 58.07cm -1 , F(000) = 488, wR 2 = 0.0769 for 1926 contributing reflections [R = 0.0284 for F > 4σ(F)], m.p. 348.7(5)K. The results from these structure determinations were combined with results from crystal structure determinations of other halogen-substituted 3-hydroxy-3-phenylpropionic acids in an analysis performed to obtain insight into their conglomerate versus racemic compound formation. In all the structures studied, the O-H...O hydrogen bonds are formed between identical donor and acceptor atoms leading to two distinct motifs. The crystallization of the racemates as either racemic compounds or mechanical mixtures of enantiomers can only be rationalized if weaker intermolecular interactions are taken into account.