2-(Trifluoromethyl)benzoic acid

In the title compound, C8H5F3O2, a halogenated derivative of benzoic acid, the carboxyl group is tilted by 16.8 (3)° with respect to the plane of the aromatic ring. In the crystal, O—H⋯O hydrogen bonding gives rise to carboxylic acid dimers, which are further connected into double chains along [1,1/4,1] by C—H⋯O contacts. C—H⋯F and C—F⋯π contacts are also observed.


R. Betz and T. Gerber
Comment Benzoic acid has found widespread use as a ligand in coordination chemistry for a variety of transition metals and elements from the s-and p-block of the periodic system of the elements. It can act as a neutral or -upon deprotonation -an anionic ligand and serve as mono-or bidentate ligand. By varying the substituents on the phenyl moiety, the acidity of the carboxylic acid group can be fine-tuned. Particular interest rests in benzoic acid derivatives showing an asymmetric pattern of substituents on the aromatic moiety due to different possible orientations of the ligand in coordination compounds and the possible formation of stereoisomeric products. At the beginning of a comprehensive study aimed at rationalizing the coordination behaviour of various benzoic acid derivatives towards a number of transition metals in dependence of the pH value of the reaction batches it seemed interesting to determine the crystal structure of the title compound to enable comparative studies.
The crystal structure of unsubstituted benzoic acid (Bruno & Randaccio (1980); Wilson et al. (1996)) as well as the crystal structures of benzoic acid derivatives bearing a halogen atom in an ortho-position to the carboxylic acid group (Krausse & Dunken (1966); Ferguson & Sim (1961); Polito et al. (2008)) are apparent in the literature.
C-C-C angles within the phenyl ring span a range of 118 ° to 121 ° with the biggest as well as the smallest angle found on both C-atoms in the ortho-position to the C-atom bearing the carboxylic acid group. The latter one is found on the carbon atom bearing the trifluoromethyl group (Fig. 1).
The carboxylic acid group is slightly tilted with respect to the plane of the aromatic moiety. The least-squares planes defined by their respective atoms enclose an angle of 16.81 (26) °.
In the crystal structure, hydrogen bonds between the carboxylic acid groups of two molecules give rise to centrosymmetric dimers. These are further connected into double chains along [1 1/4 1] by C-H···O contacts whose range falls slightly below the sum of van-der-Waals radii of the corresponding atoms (Fig. 2). The latter contacts can be observed between the hydrogen atoms bonded to the carbon atoms in the ortho-as well as the meta-position to the carboxylic acid group and have the alcoholic as well as the carbonylic O-atom as acceptor. Additionally, the H-atom in the meta-position to the carboxylic acid group forms a contact to one of the fluorine atoms of the trifluoromethyl group. In terms of graph-set analysis (Etter et al. (1990); Bernstein et al. (1995).), the descriptor for the classical hydrogen bonds building the centrosymmetric dimers is R 2 2 (8) on the unitary level while the C-H···O-contacts necessitate a C 1 1 (6)R 2 2 (10) descriptor on the same level. No π-stacking is obvious in the compound, however, a C-F···C g interaction (F···C g : 3.4803 (17) Å) can be observed.
The packing of the compound is shown in Figure 3.

Experimental
The compound was obtained commercially (fluorochem). Crystals suitable for the X-ray diffraction study were grown from an aqueous solution of the compound.