Different molecular conformations co-exist in each of three 2-aryl- N-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1 H-pyrazol-4-yl)acetamides: hydrogen bonding in zero, one and two dimensions.

4-Antipyrine [4-amino-1,5-dimethyl-2-phenyl-1 H-pyrazol-3(2 H)-one] and its derivatives exhibit a range of biological activities, including analgesic, antibacterial and anti-inflammatory, and new examples are always of potential interest and value. 2-(4-Chlorophenyl)- N-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1 H-pyrazol-4-yl)acetamide, C19H18ClN3O2, (I), crystallizes with Z′ = 2 in the space group P, whereas its positional isomer 2-(2-chlorophenyl)- N-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1 H-pyrazol-4-yl)acetamide, (II), crystallizes with Z′ = 1 in the space group C2/ c; the molecules of (II) are disordered over two sets of atomic sites having occupancies of 0.6020 (18) and 0.3980 (18). The two independent molecules of (I) adopt different molecular conformations, as do the two disorder components in (II), where the 2-chlorophenyl substituents adopt different orientations. The molecules of (I) are linked by a combination of N-H...O and C-H...O hydrogen bonds to form centrosymmetric four-molecule aggregates, while those of (II) are linked by the same types of hydrogen bonds forming sheets. The related compound N-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1 H-pyrazol-4-yl)-2-(3-methoxyphenyl)acetamide, C20H21N3O3, (III), is isomorphous with (I) but not strictly isostructural; again the two independent molecules adopt different molecular conformations, and the molecules are linked by N-H...O and C-H...O hydrogen bonds to form ribbons. Comparisons are made with some related structures, indicating that a hydrogen-bonded R22(10) ring is the common structural motif. [ABSTRACT FROM AUTHOR]