X-Ray crystallographic study of the preferred conformations of 2,6- and 3,5-dimethyl-1,4-dihydrobenzoic acids

Abstract

A single-crystal X-ray diffraction study of 2,6-dimethyl-1,4-dihydrobenzoic acid (2b) reveals that the cyclohexadiene ring is somewhat puckered with an angle of pucker, α= 171.6°. Crystals are monoclinic, space group P2₁/n, with Z= 4, in a cell with lattice parameters a= 8.922(1), b= 7.890(1), c= 12.251(2)Å,β= 86.57(1)°. 853 Independent reflections gave a final R of 0.044. In contrast, 3,5-dimethyl-1,4-dihydrobenzoic acid (3c) adopts a planar conformation with α= 179.3°. Crystals of (3c) are also monoclinic, space group P2₁/n, with Z= 4, in a cell with lattice parameters a= 4.262(1), b= 13.154(1), c= 14.980(2)Å, β= 83.57(1)°. 1 005 Independent reflections gave a final R of 0.047. The conformations observed in the solid state are in close agreement with those deduced from solution ¹H n.m.r. data.