True symmetry or pseudosymmetry: 5-amino-1-(4-methylphenylsulfonyl)-4-pyrazolin-3-one and a comparison with its 1-phenylsulfonyl analogue.

The title compound, C(10)H(11)N(3)O(3)S, (I), crystallizes as the NH tautomer. The two rings subtend an interplanar angle of 72.54 (4)°. An intramolecular hydrogen bond is formed from the NH(2) group to a sulfonyl O atom. The molecular packing involves layers of molecules parallel to the bc plane at x ≃ 0, 1 etc., with two classical linear hydrogen bonds (amino-sulfonyl and pyrazoline-carbonyl N-H...O) and a further interaction (amino-sulfonyl N-H...O) completing a three-centre system with the intramolecular contact. The analogous phenyl derivative, (II) [Elgemeie, Hanfy, Hopf & Jones (1998). Acta Cryst. C54, 136-138], crystallizes with essentially the same unit cell and packing pattern, but with two independent molecules that differ significantly in the orientation of the phenyl groups. The space group is P2(1)/c for (I) but P2(1) for (II), which is thus a pseudosymmetric counterpart of (I).