4-Carbamoylpiperidinium phenyl-acetate hemihydrate.

The asymmetric unit of the title compound, C(6)H(13)N(2)O(+)·C(8)H(7)O(2) (-)·0.5H(2)O, comprises two isonipecotamide cations, two phenyl-acetate anions and a water mol-ecule of solvation. The hydrogen-bonding environments for both sets of ion pairs are essentially identical with the piperidinium and amide 'ends' of each cation involved in lateral heteromolecular hydrogen-bonded cyclic N-H⋯O associations [graph set R(2) (2)(11)] which incorporate a single carboxyl O-atom acceptor. These cyclic motifs enclose larger R(5) (5)(21) cyclic systems, forming sheet substructures which lie parallel to (101) and are linked across b by the single water mol-ecule via water O-H⋯O(c) (c = carboxylate) associations, giving a duplex-sheet structure.