1H nuclear magnetic resonance spectra of some 3- and 5-substituted thiophen-2-carboxamides

Abstract

The ¹H n.m.r. spectra of some 3- and 5-substituted thiophen-2-carboxamides in CDCl₃ are reported. 3-Substituted thiophen-2-carboxamides show intramolecular hydrogen bonding between the N–H proton and the 3-substituent and also an intermolecular hydrogen bond, except for the 3-methyl derivative, which exhibits only intermolecular hydrogen bonding. 5-Substituted thiophen-2-carboxamides exhibit intermolecular hydrogen bonding. The chemical shifts for the 4-protons of 3-substituted thiophen-2-carboxamides correlate with σ_p(r= 0.962; ρ= 1.39). Correlation for the 4-protons of the 5-substituted amides is poor.

The activation energies of rotation of the 3-substituted amides are higher than those of the corresponding 5-substituted derivatives. For 5-substituted amides the activation energies of rotation depend on the electronic nature of the substituents, and ΔG(T_c)/σ_p⁺ shows a linear relationship (r= 0.954; ρ= 1.77).