Structural effects on the acidity of E ‐α‐phenyl‐β‐arylacrylic acids

The dissociation equilibrium constants of some is‐α‐phenyl‐β‐arylacrylic acids (Ar = 2‐pyridyl, 3‐pyridyl, 4‐pyridyl, 1‐naphthyl, 2‐naphthyl, anthracen‐9‐yl) have been measured in 80% aqueous 2‐methoxyethanol at 25°. The p K a values of these acids, together with those of p ‐substituted phenyl, 2‐furyl, 2‐thienyl and selenophen‐2‐yl derivatives, have been rationalized by an equation involving separate contributions of polar, conjugative and steric effects of heterocycles. The p K a values of some E ‐α‐phenyl‐β‐alkylacrylic acids (alk = methyl; ethyl; n ‐propyl; i ‐propyl; n ‐butyl; i ‐butyl) are also reported.