Allyl­ammonium hydrogen oxalate hemihydrate

In the title hydrated mol­ecular salt, C₃H₈N⁺·C₂HO₄⁻·0.5H₂O, the water O atom lies on a crystallographic twofold axis. The C=C—C—N torsion angle in the cation is 2.8 (3)° and the dihedral angle between the CO₂ and CO₂H planes in the anion is 1.0 (4)°. In the crystal, the hydrogen oxalate ions are linked by O—H...O hydrogen bonds, generating [010] chains. The allyl­ammonium cations bond to the chains through N—H...O and N—H...(O,O) hydrogen bonds. The water mol­ecule accepts two N—H...O hydrogen bonds and makes two O—H...O hydrogen bonds. Together, the hydrogen bonds generate (100) sheets.