Little change but great effect: varying supramolecular interactions in 2,5‐dimethoxyterephthalic acid and 2,5‐diethoxyterephthalic acid

The title terephthalic acid derivatives, namely 2,5‐dimethoxyterephthalic acid, C₁₀H₁₀O₆, (I), and 2,5‐diethoxyterephthalic acid, C₁₂H₁₄O₆, (II), exhibit nearly planar molecular structures, with maximum deviations from the least‐squares planes calculated for all non‐H atoms of 0.0418 (6) and 0.0902 (10) Å for (I) and (II), respectively. The molecules of both title compounds contain an inversion centre and thus the asymmetric unit of both crystal structures consists of only half a molecule. It is a remarkable fact that a comparatively small change in the substitution of the terephthalic acid [dimethoxy in (I) versus diethoxy in (II)] causes major differences in the dominating supramolecular interactions. While in (II) the packing structure is stabilized by typical intermolecular hydrogen‐bonded carboxylic acid dimer interactions, the carboxyl group in (I) forms an unusual intramolecular hydrogen bond with the O atom of the neighbouring methoxy group.