The dissociation constants of the E- and Z-αβ-diarylacrylic acids

Abstract

The pK values of 36 αβ-diarylacrylic acids, in both E- and Z-configurations, have been measured in 80% aqueous 2-methoxyethanol at 25 °C. The Z-isomers are stronger acids than the E-isomers because of the deconjugation of the carboxy-group which is out of the plane of the aromatic rings. In agreement with these conformational factors, the transmission of electrical effects (Hammett ρ-values) is more effective in E- than in Z-isomers. The heterocycle-containing acrylic acids (2-furyl, 2-thienyl, selenophen-2-yl) are weaker than the corresponding benzene derivatives. This has been ascribed to a combination of polar and steric effects, as shown by correlation with a two-parameter equation. The experimental pK values, corrected for the contribution due to the different size of β-aryl ring systems, follow the order of polar effects.