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Crystals of 5-hydroxy-6-methyl-2-pyridone, (I), grown from a variety of solvents, are invariably trigonal (space group R3); these are 5-hydroxy-6-methyl-2-pyridone acetone 0.1667-solvate, C(6)H(7)NO(2)·0.1667C(3)H(6)O, (Ia), and 6-methyl-5-hydroxy-2-pyridone propan-2-ol 0.1667-solvate, C(6)H(7)NO(2)·0.1667C(3)H(8)O, (Ib), and the forms from methanol, (Ic), water, (Id), benzonitrile, (Ie), and benzyl alcohol, (If). They incorporate channels running the length of the c axis that contain extensively disordered solvent molecules. A solvent-free sublimed powder of 5-hydroxy-6-methyl-2-pyridone microcrystals is essentially isostructural. Inversion-related host molecules interact via pairs of N-H...O hydrogen bonds to form R(2)(2)(8) dimers. Six of these dimers form large R(12)(6)(42) puckered rings, in which the O atom of each N-H...O hydrogen bond is also the acceptor in an O-H...O hydrogen bond that involves the 5-hydroxy group. The large R(12)(6)(42) rings straddle the -3 axes and form stacked columns via π-π interactions between inversion-related molecules of (I) [mean interplanar spacing = 3.254 Å and ring centroid-centroid distance = 3.688 (2) Å]. The channels are lined by methyl groups, which all point inwards to the centre of the channels.

On crystallization from CHCl3, CCl4, CH2ClCH2Cl and CHCl2CHCl2, 6-chloro-5-hydroxy-2-pyridone, C5H4ClNO2, (I), undergoes a tautomeric rearrangement to 6-chloro-2,5-dihydroxypyridine, (II). The resulting crystals, viz. 6-chloro-2,5-dihydroxypyridine chloroform 0.125-solvate, C5H4ClNO(2).0.125CHCl3, (IIa), 6-chloro-2,5-dihydroxypyridine carbon tetrachloride 0.125-solvate, C5H4ClNO(2)..0.125CCl4, (IIb), 6-chloro-2,5-dihydroxypyridine 1,2-dichloroethane solvate, C5H4ClNO2.C2H4Cl2, (IIc), and 6-chloro-2,5-dihydroxypyridine 1,1,2,2-tetrachloroethane solvate, C5H4ClNO2.C2H2Cl4, (IId), have I4(1)/a symmetry, and incorporate extensively disordered solvent in channels that run the length of the c axis. Upon gentle heating to 378 K in vacuo, these crystals sublime to form solvent-free crystals with P2(1)/n symmetry that are exclusively the pyridone tautomer, (I). In these sublimed pyridone crystals, inversion-related molecules form R(2)(2)(8) dimers via pairs of N-H...O hydrogen bonds. The dimers are linked by O-H...O hydrogen bonds into R(4)(6)(28) motifs, which join to form pleated sheets that stack along the a axis. In the channel-containing pyridine solvate crystals, viz. (IIa)-(IId), two independent host molecules form an R(2)(2)(8) dimer via a pair of O-H...N hydrogen bonds. One molecule is further linked by O-H...O hydrogen bonds to two 4(1) screw-related equivalents to form a helical motif parallel to the c axis. The other independent molecule is O-H...O hydrogen bonded to two 4 related equivalents to form tetrameric R(4)(4)(28) rings. The dimers are pi-pi stacked with inversion-related dimers, which in turn stack the R(4)(4)(28) rings along c to form continuous solvent-accessible channels. CHCl3, CCl4, CH2ClCH2Cl and CHCl2CHCl2 solvent molecules are able to occupy these channels but are disordered by virtue of the 4 site symmetry within the channels.