Two different products from the reaction of 1-aryl-5-chloro-3-methyl-1H-pyrazole-4-carbaldehyde with cyclohexylamine when the aryl substituent is phenyl or pyridin-2-yl: hydrogen-bonded sheets versus dimers.

Cyclohexylamine reacts with 5-chloro-3-methyl-1-(pyridin-2-yl)-1H-pyrazole-4-carbaldehyde to give 5-cyclohexylamino-3-methyl-1-(pyridin-2-yl)-1H-pyrazole-4-carbaldehyde, C16H20N4O, (I), formed by nucleophilic substitution, but with 5-chloro-3-methyl-1-phenyl-1H-pyrazole-4-carbaldehyde the product is (Z)-4-[(cyclohexylamino)methylidene]-3-methyl-1-phenyl-1H-pyrazol-5(4H)-one, C17H21N3O, (II), formed by condensation followed by hydrolysis. Compound (II) crystallizes with Z' = 2, and in one of the two independent molecular types the cyclohexylamine unit is disordered over two sets of atomic sites having occupancies of 0.65 (3) and 0.35 (3). The vinylogous amide portion in each compound shows evidence of electronic polarization, such that in each the O atom carries a partial negative charge and the N atom of the cyclohexylamine portion carries a partial positive charge. The molecules of (I) contain an intramolecular N-H...N hydrogen bond, and they are linked by C-H...O hydrogen bonds to form sheets. Each of the two independent molecules of (II) contains an intramolecular N-H...O hydrogen bond and each molecular type forms a centrosymmetric dimer containing one R2(2)(4) ring and two inversion-related S(6) rings.