Collect. Czech. Chem. Commun. 2000, 65, 216-226

Steric Effects and Steric Inhibition of Resonance in Benzene Derivatives: 2,3-Dimethylbenzoic Acid

Ivana Císařováa, Jaroslav Podlahaa, Stanislav Böhmb and Otto Exnerc,*

a Department of Inorganic Chemistry, Charles University, 128 40 Prague 2, Czech Republic
b Department of Organic Chemistry, Prague Institute of Chemical Technology, 166 28 Prague 6, Czech Republic
c Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, 166 10 Prague 6, Czech Republic


Structure of 2,3-dimethylbenzoic acid was determined both by single-crystal X-ray diffraction and by ab initio calculation at an RHF/6-31+G** level. Comparing with a previous X-ray analysis of another crystal modification of the same compound, it was possible to estimate the effect of crystal forces on the conformation. The isolated molecule is not planar as deduced previously, mainly from IR spectra, but the carboxyl group is twisted out of the ring plane by a torsion angle ϕ = 12°. In the crystal, the molecule is more flat and ϕ is reduced to 7 and 1° in the two modifications, respectively. Further significant differences between the modifications were not detected. The flattening in the crystal structure is accompanied by additional small changes in the geometry, connected with greater steric crowding in the less twisted molecule: the example shows well the limits in which molecular structure can be deduced from the solid-state structure. Previous division of methylated benzoic acids into two subgroups, planar and nonplanar, is to be formulated with more precision: the former group includes even acids with nearly planar conformation (ϕ up to 15°).

Keywords: Steric effects; Substituent effects; Conformation; Inhibition of resonance; X-Ray diffraction; Crystal structure; Carboxylic acids.

References: 23 live references.