rac-3,3,3-Trifluorolactic acid

The title compound (systematic name: rac-3,3,3-trifluoro-2-hydroxypropanoic acid), C3H3F3O3, is a fluorinated derivative of lactic acid. The O=C—C—O(H) torsion angle is 13.26 (15)°. In the crystal, O—H⋯O hydrogen bonds and C—H⋯O contacts connect the molecules into sheets perpendicular to the c axis.


rac-3,3,3-Trifluorolactic acid Thomas Gerber and Richard Betz Comment
Chelate ligands have found widespread use in coordination chemistry due to the increased stability of coordination compounds they can form in comparison to monodentate ligands (Gade, 1998). Hydroxycarboxylic acids are particularily interesting in this aspect as they offer two hydroxyl groups of markedly different acidity as potential bonding partners.
Upon varying the substitution pattern on the hydrocarbon backbone, the acidity of the respective hydroxyl groups can be finetuned over a wide range and they may, thus, serve as probes for establishing the rules in which pK a range coordination to various central atoms can be observed. To allow for comparisons of metrical parameters of the carboxylic-acid-derived ligand in envisioned coordination compounds, the crystal and molecular structure of 3,3,3-trifluorolactic acid as the free ligand was determined. The crystal structure of a related compound, 2-hydroxy-2-(trifluoromethyl)proprionic acid, is apparent in the literature (Soloshonok et al., 2007).
The carboxyl group is nearly in plane with the C-OH moiety. The respective O═C-C-O(H) dihedral angle was found at 13.26 (15) ° only (Fig. 1).
In the crystal, intra-as well as intermolecular hydrogen bonds are apparent. The former ones appear between the alcoholic hydroxyl group as donor and the ketonic oxygen atom as acceptor and, therefore, might be the cause for the small dihedral angle discussed above. The intermolecular hydrogen bonds are supported by the carboxyl group as donor and the alcoholic hydroxyl group as acceptor. In addition, C-H···O contacts whose range falls by more than 0.1 Å below the sum of van-der-Waals radii of the corresponding atoms can be observed. These stem from the hydrogen atom of the methine group and apply the oxygen atom of the carboxylic hydroxyl group as acceptor. Metrical parameters as well as information about the symmetry of these hydrogen bonds is summarized in Table 1. In terms of graph-set analysis (Etter et al., 1990;Bernstein et al., 1995), the descriptors for the hydrogen bonds are C 1 1 (5) and R 2 2 (10) on the unary level while the C-H···O contacts necessitate a R 2 2 (8) descriptor on the same level. In total, the intermolecular interactions connect the molecules to planes perpendicular to the crystallographic c axis (Fig. 2).
The packing of the title compound in the crystal structure is shown in Figure 3.

Experimental
The compound was obtained from Alfa Aesar. Crystals suitable for the diffraction study were taken directly from the provided product.

Refinement
The carbon-bound H atom of the methine group was placed in a calculated position (C-H 1.00 Å) and was included in the refinement in the riding model approximation, with U(H) set to 1.2U eq (C). The H atoms of the hydroxyl groups were allowed to rotate with a fixed angle around the C-O bond to best fit the experimental electron density (HFIX 147 in the SHELX program suite (Sheldrick, 2008)), with U(H) set to 1.5U eq (O).