Two-dimensional hydrogen-bonded polymers in the crystal structures of the ammonium salts of phen­oxy­acetic acid, (4-fluoro­phen­oxy)acetic acid and (4-chloro-2-methyl­phen­oxy)acetic acid

The structures of the ammonium salts of phen­oxy­acetic acid, NH(4) (+)·C(8)H(6)O(3) (−), (I), (4-fluoro­phen­oxy)acetic acid, NH(4) (+)·C(8)H(5)FO(3) (−), (II), and the herbicidally active (4-chloro-2-methyl­phen­oxy)acetic acid (MCPA), NH(4) (+)·C(9)H(8)ClO(3) (−)·0.5H(2)O, (III) have been determined. All have two-dimensional layered structures based on inter-species ammonium N—H⋯O hydrogen-bonding associations, which give core substructures consisting primarily of conjoined cyclic motifs. The crystals of (I) and (II) are isomorphous with the core comprising R (1) (2)(5), R (1) (2)(4) and centrosymmetric R (4) (2)(8) ring motifs, giving two-dimensional layers lying parallel to (100). In (III), the water mol­ecule of solvation lies on a crystallographic twofold rotation axis and bridges two carboxyl O atoms in an R (4) (4)(12) hydrogen-bonded motif, creating two R (4) (3)(10) rings, which together with a conjoined centrosymmetric R (4) (2)(8) ring incorporating both ammonium cations, generate two-dimensional layers lying parallel to (100). No π–π ring associations are present in any of the structures.