Two-dimensional hydrogen-bonded polymers in the crystal structures of the ammonium salts of phen-oxy-acetic acid, (4-fluoro-phen-oxy)acetic acid and (4-chloro-2-methyl-phen-oxy)acetic acid.

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The structures of the ammonium salts of phen-oxy-acetic acid, NH4 (+)·C8H6O3 (-), (I), (4-fluoro-phen-oxy)acetic acid, NH4 (+)·C8H5FO3 (-), (II), and the herbicidally active (4-chloro-2-methyl-phen-oxy)acetic acid (MCPA), NH4 (+)·C9H8ClO3 (-)·0.5H2O, (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 mol-ecule 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.