The protonation of some 5-substituted thiophen-2-carboxamides

Abstract

The protonation behaviour at 25 °C in aqueous sulphuric acid solution of some 5-X-thiophen-2-carboxamides (X = CH₃O, CH₃, H, Cl, Br, I, and NO₂) has been investigated by u.v. spectrophotometry. pK_BH⁺ Values were calculated by H_A, Bunnett–Olsen, and Cox–Yates methods from experimental absorption curves and by the method of multivariate analysis to separate the effect of protonation from that of the medium. Hammett treatment of the data show a value of σ_{α– s} of 0.63 ± 0.13 indicating that the sulphur atom acts as an electron withdrawing substituent. A plot of pK_BH⁺ against σ is linear and the slope (ρ– 1.16) is greater than that of the corresponding benzamides (ρ– 0.956) indicating that the thiophen nucleus is more efficient than the benzene ring in relaying the electronic effects of substituents to the reaction centre.