Comparative Study of Enthalpies of Hydrogen Bond Formation of Enamino and Amino Ketones

Abstract

AbstractThe complexation enthalpies ‐ΔHab and the stability constants Kab of complexes between 3‐dimethylamino‐1‐phenyl‐2‐propene‐1‐one and some nine pnenols of increasing acidic strengths were determined in benzene at 298.16 K and compared with the values characteristic of the corresponding base without double CC bond (β‐dimethylaminopropiophenone). The complexation enthalpy ‐ΔHab with a given phenol is systematically lower for the unsaturated base, the differences being of the order of 6 to 10 kJmole−1. The complexation enthalpy of the saturated base with a given phenol is of the order of magnitude of that found for the imidazoles and for the tertiary alkylamines. For the unsaturated base however, the values ressemble these found for the ketones and for the amides. This suggests that contrarily to the saturated base where the complexation involves the nitrogen atom, hydrogen bond formation by the unsaturated compound mainly takes place on the carbonyl group. The complexation enthalpy ‐ΔHab increases for both bases with the pKa of the phenol. From a comparison with similar relations for the amides a value of ‐1.9 for the pKa in water of the unsaturated base is found. The experimental value in acetic acid anhydride is ‐1.0. Applying the same method a value of ‐4.0 is estimated for the pKa of acetone and of ‐5.8 for acetophenone. The standard entropy change −ΔS0ab varies linearly with ‐ΔHab for both compounds but the straight line is shifted downwards from about 10 JK−1mole−1 for the unsaturated base. This is partly due to the presence of two lone pairs of electrons on the carbonyl group.