Thermodynamics of protonation of some five-membered heteroaryl-carboxylates, -alkanoates and -trans-propenoates

Abstract

Thermodynamic parameters for the proton complex formation of some five-membered heteroaryl-carboxylates, -alkanoates and -trans-propenoates have been determined in aqueous solution at 25 °C and l= 0.10 mol dm^–3(KNO₃) by means of potentiometric and calorimetric measurements. The acidity of heteroarylcarboxylic acids follows the order 2-furoic > 2-thenoic > selenophene-2-carboxylic > 3-furoic > 3-thenoic > pyrrole-2-carboxylic > N-methylpyrrole-2-carboxylic. Such a sequence agrees with the different effects of the heteroaryl groups on the carboxylic side chain. A similar trend is observed for the corresponding furyl- and thienyl-alkanoic acids, even though polar effects of heteroatoms on the acidity are less pronounced owing to the presence of the methylenic spacers. The pK values of trans-heteroarylpropenoic acids turn out to be independent of both the heteroatom and the carboxylic group position. Enthalpic and entropic changes are typical of a ‘hard-hard’ interaction.