Linear free energy ortho-correlations in the thiophen series. Part II. Acid dissociation of some 3-substituted thiophen-2-carboxylic acids in water

Abstract

The acid dissociation constants of some 3-substituted thiophen-2-carboxylic acids (I; X = OMe, Me, SMe, H, I, Br, SOMe, or SO₂Me) in water, at 25 °, have been measured. The data obtained furnish a good linear free energy relationship of the Yukawa–Tsuno type (ρ+1.34, r⁺ 0.57, R 0.996) emphasizing the large difference, probably linked to the geometry of the five-membered ring, from the situation observed for ortho-substituted benzoic acids. A ρ_2,3/ρ_2,5 ratio higher than unity (1.37) has been observed.