Structures of Racemic Monofluoro-Substituted Mandelic Acids, Their Relation to the Thermochemical Properties and an Analysis of Short Intermolecular Fluorine-Carbon Contacts

Article abstract of DOI:10.1107/S0108768193010766

The structures of the three monofluoro-substituted mandelic acids (C8H7FO3, M_r=170.14) have been determined from low-temperature <122 (1) K> X-ray diffraction data <λ(Cu Kα)=1.54184 Angstroem>. o-Fluoromandelic acid, monoclinic, P2₁/c, a=8.4238 (12), b=5.4766 (7), c=15.959 (2) Angstroem, β=95.962 (11)^o, V=732.3 (3) Angstroem³, Z=4, D_x=1.543 g cm^-3, μ=11.25 cm^-1, F(000)=352, R=0.040 for 1357 contributing reflections, m.p. 388.3 (5) K. m-Fluoromandelic acid (metastable modification), monoclinic, P2₁/a, a=10.8657 (14), b=9.2663 (10), c=15.722 (2) Angstroem, β=107.474 (10)^o, V=1509.9 (6) Angstroem³, Z=8, D_x=1.497 g cm^-3, μ=10.92 cm^-1, F(000)=704, R=0.048 for 2977 contributing reflections, m.p. 368.4 (5) K. p-Fluoromandelic acid, orthorhombic, Pbca, a=9.4685 (9), b=16.497 (2), c=9.7677 (8) Angstroem, V=1525.7 (5) Angstroem³, Z=8, D_x=1.481 g cm^-3, μ=10.80 cm^-1, F(000)=704, R=0.042 for 1489 contributing reflections, m.p. 408.9 (5) K.The results obtained from the structure determination are related to their physico-chemical properties examined by differential scanning calorimetry (DSC).The DSC measurements showed that m-fluoromandelic acid is prepared as a metastable modification.The distortions of the angles of the benzene rings observed in the three acids are identical, within experimental error, to those predicted from the sum of the substituent effects.Hydrogen bonds are important for the crystal packing in all these structures, and in addition very short distances are observed in the ortho- and para-substituted acids between the F atom and the C atom of the carboxylic acid group.Semi-empirical calculations indicated that these short distances correspond to attractive electrostatic interactions.The carbon-fluorine interactions were also elucidated by a search in the Cambridge Structural Database. m-Fluoromandelic acid has the lowest melting enthalpy and entropy.This is consistent with the lack of C...F interactions and the presence of two molecules per asymmetric unit in this compound.The ortho and para acids which have similar intermolecular interactions have similar melting enthalpies and entropies, which are higher than those observed in m-fluoromandelic acid.