Crystal structure of (S)-sec-butylammonium l-tartrate monohydrate

The title hydrated molecular salt was prepared by deprotonation of enantiopure l-tartaric acid with racemic sec-butylamine in water. Only one enantiomer was observed crystallographically, resulting from the combination of (S)-sec-butylamine with l-tartaric acid.


Chemical context
Given that the two enantiomers of chiral compounds can display significantly different reactivity in the presence of other chiral compounds (e.g., enzymatic reactions), the separation of racemic mixtures is an important process in chemical synthesis. Since enantiomers have identical physical properties, they cannot be separated by standard physical means such as distillation, crystallization, or chromatography. One common method to overcome this issue is to convert the racemic compound into a mixture of diastereomers through reaction with an enantiopure component (Fogassy et al., 2006). This method has been used for the resolution of amine enantiomers by protonation with chiral tartaric acid to produce diastereomeric salts. Examples include resolution of -phenylethylamine (Ault 1965;Kokila et al., 2002), N-methylamphetamines (Kmecz et al., 2004), 2-(benzylamino)-4-oxo-4-phenylbutanoate (Berkeš et al., 2003), 3-aminobutanol (Yatcherla et al., 2015), aminonaphthols (Periasamy et al., 2009), and serotonin and dopamine antagonists (Campiani et al., 2002).

Figure 2
A view of the crystal packing of the title hydrated molecular salt, viewed along the c axis (major component of the disorder only). Red dashed lines indicate the intermolecular hydrogen-bonding network (see Table 1). Displacement ellipsoids are drawn at the 50% probability level.

Figure 1
The molecular structure of the title hydrated molecular salt, showing (a) the major and (b) the minor components of the disordered secbutylammonium moiety. Displacement ellipsoids are drawn at the 50% probability level. Red lines indicate the hydrogen bonds present within the asymmetric unit (see Table 1).

Database survey
The

Synthesis and crystallization
The title compound was prepared via a modification to a previously published procedure (Helmkamp & Johnson, 1983). Racemic sec-butylamine (23.7 g, 17.2 ml, 324.0 mmol) was added to 40 ml of water and stirred to ensure homogeneity. While stirring, l-tartaric acid (50.0 g, 333.1 mmol) was slowly added. The solution was covered and allowed to stand at ambient temperature. After 24 h, crystal formation was evident. The crystallization process was allowed to continue undisturbed for one week, at which point a crystal for diffraction analysis was selected directly from the reaction mixture without further purification or isolation. The crystals can be isolated by vacuum filtration to yield a white crystalline solid (33.5 g, 42%).

Refinement
Crystal data, data collection, and structure refinement details are summarized in Table 2. The H atoms on the N and O atoms were located in a difference-Fourier map and freely refined.
The alkyl H atoms were included at geometrically idealized positions (C-H = 0.98-1.00 Å ) and treated as riding with U iso (H) = 1.5U eq (C-methyl) and 1.2U eq (C) for other H atoms.
The sec-butyl ammonium moiety displays a twofold disorder arising from two different rotamers being present that is best described as a 0.68 (1):0.32 (1) ratio of the two possible conformations. In the final cycles of refinement SAME restraints were applied to the two components of the disordered sec-butyl ammonium moiety and DFIX restraints were applied to the N-H bonds [N-H = 0.91 (2)    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.