Crystal structures of four isomeric hydrogen-bonded co-crystals of 6-methyl-quinoline with 2-chloro-4-nitro-benzoic acid, 2-chloro-5-nitro-benzoic acid, 3-chloro-2-nitro-benzoic acid and 4-chloro-2-nitro-benzoic acid.

The structures of the four isomeric compounds of 6-methyl-quinoline with chloro- and nitro-substituted benzoic acids, C(7)H(4)ClNO(4)·C(10)H(9)N, namely, 2-chloro-4-nitro-benzoic acid-6-methyl-quinoline (1/1), (I), 2-chloro-5-nitro-benzoic acid-6-methyl-quinoline (1/1), (II), 3-chloro-2-nitro-benzoic acid-6-methyl-quinoline (1/1), (III), and 4-chloro-2-nitro-benzoic acid-6-methyl-quinoline (1/1), (IV), have been determined at 185-190†K. In each compound, the acid and base mol-ecules are linked by a short hydrogen bond between a carboxyl O atom and an N atom of the base. The O⋯N distances are 2.5452†(12), 2.6569†(13), 2.5640†(17) and 2.514†(2)†à , respectively, for compounds (I)-(IV). In the hydrogen-bonded acid-base units of (I), (III) and (IV), the H atoms are each disordered over two positions with O site:N site occupancies of 0.65†(3):0.35†(3), 0.59†(4):0.41†(4) and 0.48†(5):0.52†(5), respectively, for (I), (III) and (IV). The H atom in the hydrogen-bonded unit of (II) is located at the O-atom site. In all of the crystals of (I)-(IV), π-π inter-actions between the quinoline ring system and the benzene ring of the acid mol-ecule are observed. In addition, a π-π inter-action between the benzene rings of adjacent acid mol-ecules and a C-H⋯O hydrogen bond are observed in the crystal of (I), and C-H⋯O hydrogen bonds and O⋯Cl contacts occur in the crystals of (III) and (IV). These inter-molecular inter-actions connect the acid and base mol-ecules, forming a layer structure parallel to the bc plane in (I), a column along the a-axis direction in (II), a layer parallel to the ab plane in (III) and a three-dimensional network in (IV). Hirshfeld surfaces for the title compounds mapped over d (norm) and shape index were generated to visualize the weak inter-molecular inter-actions.