A range of crystallisation experiments seeking to investigate the chloro/methyl structural similarity of o-toluic acid (o-methylbenzoic acid) and o-chlorobenzoic acid, including heterogeneous seeding experiments, has produced single crystals of both acids, and a 1 : 1 co-crystal (1) in which the chlorine atom and methyl group are disordered. All structures are characterized by the same hydrogen bonded ribbons, but they are not isostructural because of significant differences in the packing of the ribbons. Additional polymorphs of o-toluic acid were detected: microcrystalline form II, and form III with a large unit cell. The observed structures were compared with computationally generated isomorphs and ordered models, and alternative low energy structures generated in computational searches. These calculations show that the structures generated by interchanging the chloro and methyl groups are slightly less stable than the experimentally observed forms. The small energy differences associated with interchanging the two functional groups and between alternative packing arrangements of the hydrogen-bonded ribbons is consistent with the polymorphism of o-toluic acid and the observed disorder in the co-crystal.
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