Protonation of some 3-, 4-, and 5-substituted thiophen-2-carboxamides

Abstract

The protonation behaviour at 25 °C in aqueous sulphuric acid solution of some 3-, 4-, and 5-substituted thiophen-2-carboxamides has been investigated by u.v. spectrophotometry. pK_BH⁺ Values were calculated by H_A and Bunnett–Olsen methods using the multivariate analysis. A positive σ_α–s value was obtained indicating that the sulphur atom acts as an electron-withdrawing substituent. Plots of pK_BH⁺ of 4- and 5-substituted thiophen-2-carboxamides and 3-substituted derivatives against σ were linear with the slope +1.10 and +1.28, respectively. A ρ_o/ρ_m,p ratio higher than unity (1.16) has been observed indicating a larger transmission of electronic effects between the 2- and 3-positions of the thiophen ring than between the 2- and 4- or 5-positions.