Acidity and Dynamic Structure of the Potent Uncoupler SF6847 (2,6-Di-t-butyl-4-(2,2-dicyanovinyl)phenol) and Its Derivatives

Abstract The acid dissociation constant (Ka) of a series of 2,6-di-t-butyl-4-(disubstituted vinyl)phenols was measured in water spectrophotometrically at 25 °C. The electronic substituent effects of the substituted vinyl moiety on the pKa value were examined by dual-electronic-parameter analyses. The electronic effects of the two substituents at the end of the vinyl moiety were found to be not equal in the acid-base dissociation equilibrium. The inductive and resonance effects of the X-substituents trans to the benzene ring and the inductive effect of the Y-substituents cis to the benzene ring were significant factors governing the pKa value. These results and the molecular orbital calculation studies suggest that although the π-bonded X-substituents are coplanar to the vinyl plane, the π-bonded Y-substituent are twisted by a steric repulsion with the benzene ring. The variations in the pKa value of variously substituted analogs were related with the ease of intramolecular rotational motion around the bond connecting the vinyl group and the benzene ring, based on 1H NMR studies.