A simple compound with an unexpectedly complex structure: 4-pyridone 6/5-hydrate.

The crystal structure of the title compound, C5H5NO*6/5H2O, contains five independent molecules of pyridone and six independent water molecules. The space group is P2(1), but four of the pyridones and four waters correspond closely to P2(1)/n. The packing involves two layers; one consists of head-to-tail chains of pyridone molecules 1-4 linked by N--H...O hydrogen bonds, and a second layer involves all the waters and the fifth pyridone. The layers are linked by hydrogen bonds from water to pyridone oxygen. The four water O atoms that accept only one classical hydrogen bond have their environment completed by C--H...O interactions.