Anilinium mono­hydrogen dl‐malate

Crystals of the title salt, (C₆H₅NH₃)]⁺·(HOOC(CH₂)CH(OH)COO)]^? or C₆H₈N⁺·C₄H₅O₅^?, are built up from protonated anilinium residues and monodissociated dl ‐malate ions. The NH₃⁺ group of the anilinium cation is ordered at room temperature. Rotation of the NH₃⁺ group along the C(aromatic)—Nsp³ bond (often observed at room temperature in other anilinium salts) is prevented by N—H?O hydrogen bonds between the NH₃⁺ group and the malate anions. The anions are connected by four O—H?O hydrogen bonds into two‐dimensional sheets parallel to the (001) plane. The charged moieties, i.e. the anilinium cations and the sheets of hydrogen‐bonded malate anions, form two‐dimensional layers in which the phenyl rings of the anilinium residues lie perpendicular to the malate‐ion sheets. The conformation of the monodissociated malate ion in the crystal is compared with that obtained from ab initio molecular‐orbital calculations.