3-Acetylbenzoic acid

In the crystal structure of the title compound, C9H8O3, essentially planar molecules [the carboxyl group makes a dihedral angle of 4.53 (7)° with the plane of the ring, while the acid group forms a dihedral angle of 3.45 (8)° to the ring] aggregate by centrosymmetric hydrogen-bond pairing of ordered carboxyl groups. This yields dimers which have two orientations in a unit cell, creating a herringbone pattern. In addition, two close C—H⋯O intermolecular contacts exist: one is between a methyl H atom and the ketone of a symmetry-related molecule and the other involves a benzene H atom and the carboxyl group O atom of another molecule. The crystal studied was a non-merohedral twin with twin law [100, 00, 0] and a domain ratio of 0.8104(14): 0.1896(14).

In the crystal structure of the title compound, C 9 H 8 O 3 , essentially planar molecules [the carboxyl group makes a dihedral angle of 4.53 (7) with the plane of the ring, while the acid group forms a dihedral angle of 3.45 (8) to the ring] aggregate by centrosymmetric hydrogen-bond pairing of ordered carboxyl groups. This yields dimers which have two orientations in a unit cell, creating a herringbone pattern. In addition, two close C-HÁ Á ÁO intermolecular contacts exist: one is between a methyl H atom and the ketone of a symmetry-related molecule and the other involves a benzene H atom and the carboxyl group O atom of another molecule. The crystal studied was a non-merohedral twin with twin law [100, 010, 101] and a domain ratio of 0.8104(14): 0.1896(14).

Comment
Since keto carboxylic acids can crystallize in five different hydrogen-bonding modes, these types of compounds always present a challenge to predict a priori what the H-bonding mode will be. Three of these modes are either relatively rare or have geometries which preclude some structures. Racemates crystallize with a center of symmetry and generally produce an H-bonded dimer between the acid groups. Complete or partial averaging of C-O bond lengths and C-C-O angles due to disorder in carboxyl dimers was not found in (I), where these lengths and angles (Table 1) are similar to those in other highly ordered carboxyl situations (Borthwick, 1980). This meta-acetylbenzoic acid has centrosymmetric H-bonded dimer pairs across two different cell edges in the chosen cell, at (0,0,1/2) & at (0,1/2,0). The parallel planes making up the dimer pair are offset from each other by 0.36 Å. Two sets of these dimers are screw-related and form a herringbone angle of 46.15 (3)° between them in the chosen cell (see Fig 2).
Two close C-H···O intermolecular contacts exist: one is between a methyl H atom and the ketone of the adjacent molecule, and the 2nd one is from a phenyl H atom to the carboxyl O atom of another molecule.
Unlike both the 3-acetyl and the 4-acetylbenzoic acids, 2-acetylbenzoic acid crystallizes in the phthalide form with a single H-bond betwen the hydroxyl of one molecule and the ketone of the adjacent molecule (Dobson & Gerkin, 1996).

Experimental
Compound (I) was purchased from Acros Organics, Geel, Belgium. X-ray quality crystals were obtained by evaporation from formic acid at room temperature. The solid-state (KBr) infrared spectrum of (I) features a single broad asymmetric peak supplementary materials sup-2 at 1686 cm -1 for both C=O functions, typical of unstrained carboxyl-paired keto acids. In CHCl 3 solution, this combined absorption is seen at the same wavenumber.

Refinement
All H atoms for (I) were found in electron density difference maps. The fractional coordinates of the acid H was allowed to refine and its U iso (H) was set at 1.5U eq (O). The methyl H atoms were put in ideally staggered positions with C-H distances of 0.98 Å and U iso (H) = 1.5U eq (C). The phenyl Hs were constrained to ride on their parent C atoms with C-H distances of 0.95 Å and U iso (H) = 1.2U eq (C). Fig. 1. A view of the asymmetric unit of (I) with its numbering scheme. Displacement ellipsoids are drawn at the 40% probability level for non-H atoms.

Special details
Experimental. 'crystal mounted on a Cryoloop using Paratone-N' Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.