Hydrogen‐bonding motifs in 4‐carboxy­phenyl­ammonium nitrate and perchlorate monohydrate,and in bis(4‐carboxy­phenyl­ammonium) sulfate

In the title compounds, C₇H₈NO₂⁺·NO₃^?, (I), C₇H₈NO₂⁺·ClO₄^?·H₂O, (II), and 2C₇H₈NO₂⁺·SO₄^2?, (III), the carboxyl planes of the 4‐carboxy­phenyl­ammonium cations are twisted from the aromatic plane. A homonuclear C(8) hydrogen‐bonding motif of 4‐carboxy­phenyl­ammonium cations is observed in both (I) and (II), leading to `head‐to‐tail' layers. The cations in (III) form carboxyl group dimers, making a graph‐set motif of R₂²(8). In all the structures, anions connect the cationic layers and an infinite chain running along the c axis is observed, having the C₂²(6) graph‐set motif. Inter­estingly, in (II), the anions are connected through weak hydrogen bonds involving the water mol­ecules, leading to a graph‐set motif of R₄⁴(12). Alternate hydro­phobic and hydro­philic layers are observed in all three compounds as a result of the column‐like arrangement of the aromatic rings of the cations and the anions. Furthermore, in (I), head‐to‐tail N—H?O inter­actions and inter­actions linking the cations and anions form an R₆⁴(16) hydrogen‐bonding motif, resulting in a pseudo‐inversion centre at (, , 0).