2-carboxy- and 2,7-dicarboxy-4,5-benzotropone: Hydrogen-bonding patterns of a β-keto acid and a related β-keto 1,5-diacid

The two title compounds exhibit strikingly different hydrogen-bonding modes. The monoacid C₁₂H₈O₃, (I) (alternative systematic name: 7-oxo-7H-benzocycloheptene-6-carboxylic acid), adopts a nearly planar conformation with the carboxyl internally hydrogen-bonded to the ketone; the molecules pack in stacks at an interplanar distance of 3.397 (4) Å with six lateral C=O⋯H-C close contacts. In the diacid C₁₃H₈O₅, (II) (alternative systematic name: 7-oxo-7H-benzocycloheptene-6,8-dicarboxylic acid), the ketone carbonyl, which is not involved in hydrogen bonding, lies out of the general molecular plane, while the carboxylic acids are paired by hydrogen bonding with those of neighboring molecules, forming flat zigzag chains. Both (I) and (II) adopt slightly asymmetric conformations.