2-Amino-3,4,5,6-tetrafluorobenzoic acid

The asymmetric unit of the title compound, C7H3F4NO2, obtained as an intermediate in the synthesis of a coupling reagent, contains four independent and conformationally similar molecules. The amine H atoms form both intramolecular and intermolecular N—H⋯Ocarboxyl hydrogen bonds which, together with intermolecular O—H⋯Ocarboxyl hydrogen bonds and N—H⋯F associations form ribbon structures along the a axis.

The asymmetric unit of the title compound, C 7 H 3 F 4 NO 2 , obtained as an intermediate in the synthesis of a coupling reagent, contains four independent and conformationally similar molecules. The amine H atoms form both intramolecular and intermolecular N-HÁ Á ÁO carboxyl hydrogen bonds which, together with intermolecular O-HÁ Á ÁO carboxyl hydrogen bonds and N-HÁ Á ÁF associations form ribbon structures along the a axis.

Experimental
To a stirred solution of 39.2 g of KOH in 356 mL of distilled water, 11.3 g of bromine was added and after 30 min, 70 mmol of 2-carboxy-3,4,5,6-tetrafluorobenzamide was added. After allowing the reaction to proceed for 30 min at 293 K, the mixture was heated to 363K-368K and maintained at that temperature for 4 h, after which the mixture was cooled to room temperature and allowed to stand for 48 h. To the mixture was then added 100 mL of water, the pH adjusted to 1 at ice-water temperature, stirred and filtered, giving a yellow solid (16.2 g, yield 94%). Pale yellow crystals of (I) suitable for X-ray analysis grew over a period of a week from a solution of the solid in methanol at room temperature.

Refinement
The carboxylic acid H atoms were positioned geometrically and were included in the refinement in the riding-model approximation with O-H = 0.82 Å and U iso (H) = 1.5U eq (O) The amine H atoms were located in difference Fourier maps and the positional parameters were refined but with the displacement parameters riding with U iso (H) = 1.5U eq (N).

Special details
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.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.