rac-4-(2-Methoxyphenyl)-2,6-dimethylcyclohex-3-enecarboxylic acid

The title compound, C16H20O3, was synthesized to study the hydrogen-bonding interactions of the two enantiomers in the solid state. Intermolecular O—H⋯O hydrogen bonds produce centrosymmetric R 2 2(8) rings which dimerize the two chiral enantiomers together through their carboxyl groups.

The title compound, C 16 H 20 O 3 , was synthesized to study the hydrogen-bonding interactions of the two enantiomers in the solid state. Intermolecular O-HÁ Á ÁO hydrogen bonds produce centrosymmetric R 2 2 (8) rings which dimerize the two chiral enantiomers together through their carboxyl groups.

Comment
The title carboxylic acid was prepared to study the interaction of the two enantiomers in the solid state. We have previously reported the structure of its precursor, which is achiral and forms hydrogen-bonded dimers (Xie et al., 2007b). The chirality of the title compound is solely generated by the presence of the double bond in the cyclohexene ring (Xie et al., 2004).
The resultant racemate is made up of carboxylic acid RS dimers (Xie et al., 2002(Xie et al., , 2007a(Xie et al., , 2008a. The structure and atom numbering are shown in Fig. 1, which illustrates the half-chair conformation of the cyclohexene ring. The torsion angles involving atoms C4, C5, C6, C1, and C2 are near 0°. The carboxyl group is almost perpendicular to the cyclohexene ring with an angle of 82.2° between the O1-C14-O2-C3 plane and the C1-C6 ring. The double bond between C5-C6 is not fully conjugated with the aromatic ring as shown by the C1-C6-C5 plane to benzene ring angle of 52.6°. Unlike other previously reported para substituted analogs and like other previously reported meta substituted analogs (Xie et al., 2008b), the molecule also has a chiral axis due to the ortho methoxy substituent on the aromatic ring. Fig. 2 shows the hydrogen bonding scheme. Atom O2 acts as a donor in an intermolecular hydrogen bond to atom O1, producing an R22(8) ring (Bernstein et al., 1995), thus creating a hydrogen-bonded dimer. There is no evidence to suggest that weak directional interactions interconnect the dimers. Hydrogen bond geometry is given in Table 1.

Experimental
The title carboxylic acid was synthesized following asimilar method reported by Xie et al., 2002. Purified compound was recrystallized from hexane-dichloromethane as colorless plates (m.p. 417-418 K).

Refinement
All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms not involved in hydrogen bonding were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 Rfactors(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.