1-(4-Chlorophenyl)-1H-pyrazol-3-ol

In the title compound, C9H7ClN2O, the dihedral angle between the aromatic ring planes is 11.0 (2)°. In the crystal, inversion dimers linked by pairs of O—H⋯N hydrogen bonds generate R 2 2(8) loops.

In the title compound, C 9 H 7 ClN 2 O, the dihedral angle between the aromatic ring planes is 11.0 (2) . In the crystal, inversion dimers linked by pairs of O-HÁ Á ÁN hydrogen bonds generate R 2 2 (8) loops.

Related literature
For a related structure, see: Jian et al. (2005). For background to herbicides and plant-growth promoters related to the title compound, see: Shi et al. (1995); Xu et al. (2002).

S2. Experimental
A mixture of p-Chlorophenylhydrazing hydrochloride (0.02 mol) and methyl acrylate (0.02 mol) was stirred in ethanol (30 ml) at 353 K for 2 h to afford the title compound (yield 50%). Colourless bars of (I) were obtained by recrystallization from acetone at room temperature.

S3. Refinement
The O-bound H atom was located in a difference map and freely refined. The C-bound H atoms were positioned geometrically and allowed to ride on their parent atoms, with C-H = 0.93-0.96 Å and with U iso (H) = 1.2U eq of the parent atoms.  The molecular structure of (I) with displacement ellipsoids drawn at the 30% probability level. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.23 e Å −3 Δρ min = −0.32 e Å −3 Extinction correction: SHELXL97 (Sheldrick, 2008), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.049 (5) Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Cl1