4-(2-Benzoylethyl)benzoic acid

The title compound, C16H14O3, adopts a conformation in which each functional group is almost coplanar with its adjacent ring, while the two aromatic rings are twisted with respect to one another with a dihedral angle of 78.51 (3)°. The compound dimerizes by standard centrosymmetric hydrogen-bonded carboxyl pairing [O⋯O = 2.6218 (11) Å and O—H⋯O = 176 (2)°]. The packing includes two intermolecular C—H⋯O close contacts with the ketone group.


Related literature
For related literature, see: Borthwick (1980);Steiner (1997 H atoms treated by a mixture of independent and constrained refinement Á max = 0.24 e Å À3 Á min = À0.15 e Å À3 Table 1 Hydrogen-bond geometry (Å , ). Comment Our X-ray study of ketocarboxylic acids seeks to uncover the structural features governing the choice among the five known keto-acid hydrogen-bonding modes. One major determinant is the availability of centrosymmetry, such that carboxyl dimerization, overall the commonest type of aggregation, is rare among single enantiomers. A second important influence is molecular flexibility, as reflected by the number of fully rotatable bonds in the molecule, with dimerization becoming less common as structural or conformational rigidity increases. In this context, the title molecule (I) is relatively flexible, containing four fully rotatable bonds in addition to the carboxyl. Fig. 1 shows the asymmetric unit for (I) with its numbering. A principal feature is the near coplanarity of the two carbonyl-bearing functional groups with their respective benzene rings, permitting strong conjugation. The phenone's dihedral angle is 7.83 (8)° (C8-C9-C10-O1 versus C10-C11-C12-C13-C14-C15) and that for the carboxylic acid is 19.92 (6)° (C1-C16-O2-O3 versus C1-C2-C3-C4-C5-C6). The connecting alkyl chain, however, is not maximally staggered, the C4-C7-C8-C9 torsion being -160.20 (10)° rather than 180°. The two separate aromatic rings lie at a mutual dihedral angle of 78.51 (3)°.
Carboxyl dimers often display complete or partial averaging of C-O bond lengths and C-C-O angles due to disorder; however, no significant averaging is observed in (I), where these lengths and angles are similar to those in other highly ordered carboxyl situations (Borthwick, 1980). Fig. 2 shows the packing arrangement for (I), typical for acids that are either racemic or, as with (I), achiral but capable of forming conformational racemates. Centrosymmetric dimers with two different orientations are centered at 1/2,1/2,1/2 and 1/2,0,0 in the chosen cell. The eight-membered carboxyl dimer of one orientational type lies close to the phenone aromatic ring in a dimer of the second type and nearly parallel to it [dihedral angle = 6.38 (7)°]. The normal distance from the centroid of this ring to the carboxyl-dimer plane = 3.472 Å and the intermolecular acid-to-ketone O═C···C═O distance = 3.3508 (15) Å.
Two intermolecular C-H···O═C close contacts were found in the packing, linking the ketone (O1) to H8A and to H5 in separate neighboring molecules (Table 1). These contacts lie within the 2.6 Å range we routinely survey for non-bonded dipolar packing interactions (Steiner, 1997).

Experimental
Methyl 4-(3-oxo-3-phenyl-1-propenyl)benzoate, purchased from Acros Organics/Fisher Scientific, Springfield, NJ, USA, was hydrogenated in ethyl acetate at atmospheric pressure and room temperature over a 5% Pd/C catalyst. The resulting reduced methyl ester, m.p. ca 365 K, was saponified by refluxing with aqueous KOH to yield (I). Crystals of X-ray quality were obtained from Et 2 O, m.p. 431 K. Typically for carboxyl-paired keto acids, the solid-state (KBr) and the solution infrared supplementary materials sup-2 spectra of (I) display only slight differences in the C═O region. The former features intense absorption at 1682 cm -1 for both C═O functions; in CHCl 3 solution this combined peak is seen at 1688 cm -1 .

Refinement
All H atoms for (I) were found in electron density difference maps. The positional parameters and the isotropic thermal parameter of the O-H were allowed to refine fully. The methylene and the phenyl Hs were placed in geometrically idealized positions and constrained to ride on their parent C atoms with C-H distances of 0.99 Å for the methylene Hs and 0.95 Å for the phenyl Hs, and U iso (H) = 1.2U eq (C). Fig. 1. The asymmetric unit of (I), with its numbering. Displacement ellipsoids are drawn at the 30% probability level. Fig. 2. A partial packing diagram for (I), illustrating the dihedral relationships among the aromatic rings of the packed centrosymmetric dimers located at 1/2,1/2,1/2 and 1/2,0,0 in the unit cell. Displacement ellipsoids are drawn at the 30% probability level.