(2E)-1-(2,6-Dichloro-3-fluorophenyl)-3-(4-methoxyphenyl)prop-2-en-1-one

There are two independent molecules in the asymmetric unit of the title compound, C16H11Cl2FO2. The F atom equally populates both meta positions of the 6-dichloro-3-fluorophenyl ring in each molecule, resulting in 0.5 occupancy for both the F and H atoms in these positions. The dihedral angle between the mean planes of the benzene rings are 77.5 (2) and 89.8 (8)°in the two molecules. In the crystal, weak C—H⋯F and C—H⋯O interactions involving the half-occupied H and F atoms are observed. Weak π–π stacking interactions [centroid—centroid distance = 3.150 (2) Å] also contribute to the crystal stability.

There are two independent molecules in the asymmetric unit of the title compound, C 16 H 11 Cl 2 FO 2 . The F atom equally populates both meta positions of the 6-dichloro-3-fluorophenyl ring in each molecule, resulting in 0.5 occupancy for both the F and H atoms in these positions. The dihedral angle between the mean planes of the benzene rings are 77.5 (2) and 89.8 (8) in the two molecules. In the crystal, weak C-HÁ Á ÁF and C-HÁ Á ÁO interactions involving the half-occupied H and F atoms are observed. Weakstacking interactions [centroid-centroid distance = 3.150 (2) Å ] also contribute to the crystal stability.   Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx þ 1; y À 1 2 ; Àz þ 1 2 ; (ii) Àx þ 1; y þ 1 2 ; Àz þ 1 2 .
In (I) two molecules crystallize in the asymmetric unit (  (Allen et al., 1987). Crystal packing is enhanced by weak C-H···F and C-H···O intermolecular interactions (Table 1) from both half-occupied H and F atoms supporting parallel chains along the b axis ( Fig. 2) aa well as weak π-π stacking interactions (Table 2).

Experimental
To a stirred solution of 1-(2,6-dichloro-3-fluorophenyl)ethanone (1 g, 4.8 mmol) and 4-methoxybenzaldehyde (0.65 g, 4.8 mmol) in ethanol (10 ml), powdered KOH (0.40 g, 7.2 mmol) was added at 273 K. The reaction mixture was stirred at room temperature for 3 h. After completion of the reaction, the reaction mixture was poured to ice cold water and acidified with 1.5 N HCl (pH 3 ). The solid precipitated was filtered and dried to afford 1.4 g of the title compound, (I,) in 89% yield. X-ray quality crystals were obtained by slow evaporation of a tetrahydrofuran solution (m.p.: 361-362 K).

Refinement
All of the H atoms were placed in their calculated positions and refined using the riding model with C-H lengths of 0.95 Å (CH) or 0.98 Å (CH 3 ). The isotropic displacement parameters for these atoms were set from 1.19 to 1.20 (CH), or 1.49 (CH 3 ) times U eq of the parent atom. Overlapping of the F atoms in the meta position of the phenyl ring in each molecule resulted in H2A, H4A, H18A, H20A and F1, F1A, F2, F2A being refined at 0.50 occupancy. C2-F1 and C4-F1A bond distances were fixed at 1.33 (2) Å, C18-F2 and C20-F2A bond distances were fixed at 1.33 (06) Å.   Packing diagram of the title compound viewed along the a axis.  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.67 e Å −3 Δρ min = −0.57 e Å −3

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 Occ.