Phenyl- and mesitylglyoxylic acids: catemeric hydrogen bonding in two α-keto acids

alpha-Oxobenzeneacetic (phenylglyoxylic) acid, C8H6O3, adopts a transoid dicarbonyl conformation in the solid state, with the carboxyl group rotated 44.4 (1)degrees from the nearly planar benzoyl moiety. The heterochiral acid-to-ketone catemers [O ... O = 2.686 (3) and H ... O = 1.78 (4) Angstrom] have a second, longer, intermolecular O-H ... O contact to a carboxyl sp(3) O atom [O ... O = 3.274 (2) and H ... O = 2.72 (4) Angstrom], with each flat ribbon-like chain lying in the bc plane and extending in the c direction. In alpha-oxo-2,4,6-trimethylbenzeneacetic (mesitylglyoxylic) acid, C11H12O3, the ketone is rotated 49.1 (7)degrees from planarity with the aryl ring and the carboxyl group is rotated a further 31.2 (7)degrees from the ketone plane. The solid consists of chiral conformers of a single handedness, aggregating in hydrogen-bonding chains whose units are related by a 3(1) screw axis, producing hydrogen-bonding helices that extend in the c direction. The hydrogen bonding is of the acid-to-acid type [O ... O = 2.709 (6) and H ... O = 1.87 (5) Angstrom] and does not formally involve the ketone; however, the ketone O atom in the acceptor molecule has a close polar contact with the same donor carboxyl group [O ... O = 3.005 (6) and H ... O = 2.50 (5) Angstrom]. This secondary hydrogen bond is probably a major factor in stabilizing the observed cisoid dicarbonyl conformation. Several intermolecular C-H ... O close contacts were found for the latter compound.