Structures of the Optically Active Monofluoro-Substituted Mandelic Acids: Relation to Their Racemic Counterparts and Thermochemical Properties

The crystal structures of the three optically active monofluoro-substituted mandelic acids (C₈H₇FO₃, M_r = 170.14) have been determined from low-temperature X- ray diffraction data. (R)-(−)-ortho-Fluoromandelic acid, monoclinic, P2₁, a = 8.356 (2), b = 10.842 (2), c = 8.544 (2) Å, β = 94.13(2)°, Z = 4, m.p. 361.8 (5) K. (R)-(−)-meta-Fluoromandelic acid, monoclinic, P21, a = 8.493 (3), b = 5.8426 (7), c = 15.628 (3) Å, β = 104.10(2)°, Z = 4, m.p. 394.2 (5) K. (R)-(−)-para-Fluoromandelic acid, monoclinic, C2, a = 8.464 (2), b = 5.8518 (13), c = 15.868 (2) Å, β = 107.56 (2)°, Z = 4, m.p. 425.8 (5) K. The hydrogen-bonding schemes in these structures have been analysed and compared with the hydrogen-bond patterns in the equivalent racemic acids. The structural data from six other structures were included in the analysis, which led to a general description of the great variety of hydrogen-bond motifs observed in α-hydroxycarboxylic acids. Differences between crystal structures of the racemic and enantiomerically pure fluoromandelic acids have been related to their melting enthalpies. The observed differences in the binary phase diagrams of the three acids can be rationalized in terms of their structural differences.

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