Crystal structures of four isomeric hydrogen-bonded co-crystals of 6-methyl­quinoline with 2-chloro-4-nitro­benzoic acid, 2-chloro-5-nitro­benzoic acid, 3-chloro-2-nitro­benzoic acid and 4-chloro-2-nitro­benzoic acid

The structures of the four isomeric compounds of 6-methyl­quinoline with chloro- and nitro-substituted benzoic acids, C7H4ClNO4·C10H9N, namely, 2-chloro-4-nitro­benzoic acid–6-methyl­quinoline (1/1), (I), 2-chloro-5-nitro­benzoic acid–6-methyl­quinoline (1/1), (II), 3-chloro-2-nitro­benzoic acid–6-methyl­quinoline (1/1), (III), and 4-chloro-2-nitro­benzoic acid–6-methyl­quinoline (1/1), (IV), have been determined at 185–190 K. In each compound, the acid and base mol­ecules 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) Å, 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 inter­actions between the quinoline ring system and the benzene ring of the acid mol­ecule are observed. In addition, a π–π inter­action between the benzene rings of adjacent acid mol­ecules 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 inter­molecular inter­actions connect the acid and base mol­ecules, 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 dnorm and shape index were generated to visualize the weak inter­molecular inter­actions.