2,4,6-Trifluorobenzoic acid

In the title compound, C7H3F3O2, the C—C—C angles in the ring are greater than 120° for F-bonded C atoms [123.69 (13), 123.88 (12) and 123.66 (12)°]. In the crystal, intermolecular O—H⋯O hydrogen bonds between carboxyl groups give rise to the formation of a centrosymmetric dimer, while dispersive F⋯O contacts [2.8849 (16) Å] connect the dimers into infinite strands along the a axis.

In the title compound, C 7 H 3 F 3 O 2 , the C-C-C angles in the ring are greater than 120 for F-bonded C atoms [123.69 (13), 123.88 (12) and 123.66 (12) ]. In the crystal, intermolecular O-HÁ Á ÁO hydrogen bonds between carboxyl groups give rise to the formation of a centrosymmetric dimer, while dispersive FÁ Á ÁO contacts [2.8849 (16) Å ] connect the dimers into infinite strands along the a axis.

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 carboxyl group can be fine-tuned. 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 C-C-C angles in the phenyl ring are found to be invariably larger than 120° for C-atoms bonded to F-atoms while the remaining C-C-C angles are measured at values smaller than 120°. The biggest deviation is found for the C-atom bearing the carboxyl group where a value of only about 115° is detected. The least-squares plane defined by the atoms of the carboxyl group encloses an angle of 38.17 (7)° with the plane of the aromatic system (Fig. 1).
In the crystal structure, intermolecular hydrogen bonds connect two molecules to centrosymmtric dimeric units. These dimers are joined by dispersive F···O contacts to infinite strands along the crystallographic a axis. In terms of graph-set analysis, the unitary descriptor for the hydrogen bonds is R 2 2 (8) while the F···O contacts are described by a R 2 2 (10) descriptor on the unitary level (Fig. 2).
The aromatic rings of the title compound show π-stacking with the COOH group rotated by about 90° with respect to the carboxyl groups of two neighbouring molecules (Fig. 3). The distance between two centers of gravity was determined to be 3.7501 (8) Å, the distance between the perpendicularily-projected centers of gravity of two neighbouring aromatic moieties with respect to the carbocycles was found to be 3.5507 (5) Å and 3.4651 (5) Å, respectively. The molecular packing of the compound is shown in Figure 4.

Experimental
The compound was obtained commercially from Fluorochem. Crystals suitable for X-ray diffraction were obtained upon slow evaporation of an aqueous solution of the compound at room temperature.