Crystal structure of 5-hydroxy-5-propylbarbituric acid

Molecules of the title compound are linked via N—H⋯O(carbonyl), N—H⋯O(hydroxy) and O—H⋯O(carbonyl) bonds into a 5-connected framework.


Chemical context
As part of a systematic investigation of solid-state properties of derivatives of barbituric acid Zencirci et al., 2014;Rossi et al., 2012), we are studying the polymorphism of a group of 5-monosubstituted barbituric acids. The title compound is an oxidation product of 5-propylbarbituric acid, formed during a crystallization experiment and the structure is reported herein. The analogous oxidation product of 5-ethylbarbituric acid was previously reported by Gatehouse & Craven (1971).

Supramolecular features
One NH group and one carbonyl group of the molecule are engaged in a centrosymmetric two-point interaction, N3-H3Á Á ÁO4 ii (Table 1), resulting in an R 2 2 (8) ring (Etter et al., 1990;Bernstein et al., 1995). This kind of ring is a ubiquitous feature in crystal structures of barbiturates (Gelbrich et al., 2011). The other NH group is bonded to the hydroxy group of a second molecule via a 2 1 operation, N1-H1Á Á ÁO7 i , and this interaction is accompanied by a short O6Á Á ÁC4 i contact [2.8654 (18) Å ]. Additionally, the hydroxy group donates a hydrogen bond to the C2 carbonyl group of another molecule related by glide symmetry (O7-H7Á Á ÁO2 iii ). Altogether, six hydrogen bonds connect each molecule to five other molecules. In addition to the aforementioned R 2 2 (8) rings, the resulting hydrogen-bonded framework structure also displays rings composed of four and six molecules (Fig. 2). This 5connected framework has the topology of the nov structure (Blatov et al., 2004). Fig. 3 shows a graph of the hydrogenbonded structure (HBS) according to the methodology proposed by Hursthouse et al. (2015). The short descriptor according to Hursthouse et al. (2015) for this HBS is F6 5 [4 4 .6 6nov].

Figure 2
Layer fragment of the H-bonded framework which contains rings connecting four and six molecules in addition to R 2 2 (8) rings. Hydrogen bonds are drawn as dashed lines. H and O atoms engaged in hydrogen bonding are drawn as balls and all the other H atoms are omitted for clarity.

Figure 1
Asymmetric unit with displacement ellipsoids drawn at the 50% probability level and hydrogen atoms drawn as spheres of arbitrary size.
connections based on N-HÁ Á ÁO C bonds which result in characteristic R 2 2 (8) rings are found in each of these compounds.
The title structure displays just one such interaction which involves the carbonyl group at ring position 4 ( Fig. 4). One such connection, albeit via the C2 carbonyl group, also exists in the 5,5-dihydroxybarbituric acid structure. Here it forms part of the C-4 ladder motif which is known from 5,5-disubstituted derivatives of barbituric acid (Gelbrich et al., 2011).
The monohydrate and 1,4-dioxane hemisolvate each contain two two-point N-HÁ Á ÁO C connections per molecule, in the first case via the topologically equivalent C4 and C6 carbonyl groups and in the second via the C4 and C2 carbonyl groups, resulting in the looped chain motifs C-2 and C-1 (Gelbrich et al., 2011), respectively, which are frequently encountered in barbiturates. C-2 chains are also found in the structure of the trihydrate. The molecular conformation of 5hydroxy-5-ethylbarbituric acid (Gatehouse & Craven, 1971; HEBARB) is similar to that of the title structure with respect to the pseudo-torsion angle of 124.3 , which is structurally analogous to the C2Á Á ÁC5-C8-C9 angle discussed above. A comparison with the program XPac (for details, see below) indicated that these two compounds are indeed isostructural. Geometrical differences between the two molecular packing arrangements are small (Fig. 4), which is reflected in a calculated XPac dissimilarity index of just 5.4. This close packing similarity is remarkable insofar as the substitution of a propyl with an ethyl group alters the molecular shape considerably and leads to an 11% decrease in the volume of the unit cell. The unit-cell parameters of the two isostructures correspond directly with one another. The a and b axes of the ethyl analogue (determined at room temperature) are 6.1% and 6.5% shorter than those of the title compound. Simultaneously, the c axis of the ethyl analogue is 1.5% longer and the angle is enlarged by 1.0 .

Synthesis and crystallization
A glass slide with a sample of 5-propylbarbituric acid embedded in paraffin oil was placed on a hot bench. The sample was melted and left to crystallize. Within a few days, the original crystals had partially converted and cube-shaped single crystals of the title compound had formed. An illustration of the similar packing of molecules in the title compound (left) and its ethyl analogue (right). Each structure is viewed along its [010] direction. H atoms in alkyl groups are omitted for clarity.

Refinement
Crystal data, data collection and structure refinement details are summarised in Table 2. The data collection was carried out in the manner described by Coles & Gale (2012). All H atoms were identified in difference maps. Methyl H atoms were idealized and included as rigid groups allowed to rotate but not tip (C-H = 0.98 Å ). H atoms bonded to secondary CH 2 carbon atoms were positioned geometrically (C-H = 0.99 Å ). Hydrogen atoms bonded to N atoms were refined with restrained distances [N-H = 0.86 (1) Å ]. The hydrogen atom of the hydroxy group was refined freely and the U iso parameters of all hydrogen atoms were also refined freely.