DL-alaninium semi-oxalate monohydrate.

The structure of the title compound, C(3)H(8)NO(2)(+)·C(2)HO(4)(-)·H(2)O, is formed by two chiral counterparts (L- and D-alaninium cations), semi-oxalate anions and water molecules, with a 1:1:1 cation-anion-water ratio. The structure is compared with that of the previously known anhydrous DL-alaninium semi-oxalate [Subha Nandhini, Krishnakumar & Natarajan (2001). Acta Cryst. E57, o666-o668] in order to investigate the role of water molecules in the crystal packing. The structure of the hydrate resembles that of anhydrous alaninium semi-oxalate, with the water molecule incorporated into the general three-dimensional network of hydrogen bonds where it forms four hydrogen bonds with neighbours disposed tetrahedrally about it. Although the main structural motifs in the hydrate and in the anhydrous form are topologically similar, the incorporation of water molecules in the network results in significant geometric distortion. There are several types of hydrogen bond in the crystal structure of the hydrate, two of which (O-H···O bonds between the semi-oxalate anions and O-H···O hydrogen bonds between water and alaninium cations) are very short. Such hydrogen bonds between semi-oxalate anions are also present in the anhydrous form of this compound. Short distances between semi-oxalate anions in neighbouring chains in the hydrate alternate with longer ones, whereas in the anhydrous structure they are equidistant. Despite the similarity of these compounds, dehydration of the hydrate on storage is not of a single-crystal to single-crystal type, but gives a polycrystalline pseudomorph, preserving the crystal habit. This transformation proceeds through the formation of an intermediate compound, presumably a hemihydrate.