We report the formation of cocrystals of p-chloro- and p-methyl-benzamide with oxalic and sebacic acid. The expected trimer unit of base–acid–base is observed in all 4 cocrystals, linked by hydrogen bonds to form infinite tapes. Our purpose in synthesising these cocrystals was to engineer isostructural replacement of Cl by a methyl group, by establishing a robust hydrogen bonded framework, and providing a larger molecular environment in the crystal. No isostructures were found in these cocrystals. We studied the close contact environment of the Cl and methyl groups in these cocrystals, and relevant cases in the CSD, and find a tendency for isostructure to occur when there are no Cl
Cl contacts less than 4.0 Å.
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