Structural studies of analgesics and their interactions. VIII. Rotational isomerism and disorder in the crystal structure of meclofenamic acid

Abstract

Meclofenamic acid, C_I4H_I1CI₂NO₂, probably the most potent among analgesic fenamates, crystallizes in the triclinic space group P1̅, with a = 8.569 (5), b = 8.954(8), c = 9.371 (4) Å, α = 103.0 (2), β = 103.5 (2), γ = 92.4 (2)°, Z = 2, D_m = 1.43 (4), D_c = 1.41 Mg m^-3. The structure was solved by direct methods and refined to R = 0.135 for 1062 observed reflections. The anthranilic acid moiety in the molecule is nearly planar and is nearly perpendicular to the 2,6-dichloro-3-methylphenyl group. The molecules, which exist as hydrogen-bonded dimers, have an internal hydrogen bond involving the imino and the carboxyl groups. The methyl group is disordered and occupies two positions with unequal occupancies. The disorder can be satisfactorily explained in terms of the rotational isomerism of the 2,6-dichloro-3-methylphenyl group about the bond which connects it to the anthranilic acid moiety and the observed occupancies on the basis of packing considerations.