A simple relationship between the hammett acidity function,Ho, and the thermodynamic pKa values of unsubstituted aromatic carboxamides

The well-known failure of the Hammett H_o function to describe the prototropic reaction isotherms of aromatic carboxamides can be corrected by taking account of the hydration requirements of the prototropic reactions of the carboxamides relative to the hydration requirements of the dissociations of the primary amines used as indicators to establish the H_o scale. This approach has been successfully applied to benzamide and the naphthamides. For up to a 4:1 molar ratio of water to sulfuric acid, the amide acidity scale, H_A and the H_o scale are simply related by H_A = H_o — 2 log a_w , where a_w is the activity of water.