Studies on amphiprotic compounds. 3. Hydrogen-bonding basicity of oxygen and sulfur compounds (original) (raw)

1988, The Journal of Organic Chemistry

The hydrogen bonding (HB) basicity parameters, &, for various "monomeric" alcohols and thiols, as well as for a variety of carbonyl and thiocarbonyl amphiprotic bases, have been determined. Also determined were the HB basicity parameters, @, for a series of aprotic carbonyl and thiocarbonyl bases (@, = B for these materials). The analysis of the data for a set of 62 compounds provides the basis for comparison of structural effects on the HB basicity of oxygen and sulfur bases. In general, the HB basicity of sulfur bases is lower than that of their oxygen homologues. Within families, structural effects on the basicity of oxygen and sulfur bases are proportional. Our results show the contributions from polarizability and Aeld/inductive and resonance effects to the basicity of these compounds. Also some differential contributions from steric hindrance and hybridization changes have been singled out. The relative stability of cyclic 1:l complexes involving amphiprotic thioamides and 4-chlorophenol has been evaluated. I. Introduction Recent studies have shown the usefulness of the empirical parameters ** (dipolarity-polarizability), a (hydrogen-bonding acidity), and j3 (hydrogen-bonding basicity) for the quantitative analysis of solvent effects on chemical reactivity: solubility? and spectral (UV-vis, IR, EPR) and biological proper tie^.^ We have pointed out' that the determination of j3 values for "monomeric" self-associating compounds, p, , requires the use of special techniques. This is so because the use of solvatochromic indicators'& dissolved in the bulk liquids provides a measure of the "averagen basicity of "monomers" and "oligomers". Consider equilibrium 1, describing the hydrogen-bonding (HB) association between a nonamphiprotic HB base, B, and a proton donor, H-A, in dilute solution in an inert solvent, S. K, and K, respectively stand for the equilib-(1) rium constants for this reaction expressed in liters/mole and in mole fraction units. I t is found'Job that (2) where a, b, and c are constants and j3 and p respectively stand for the HB basicity parameter and the molecular dipole moment of the base. For nonamphiprotic bases, j3,