Partition coefficients and intramolecular hydrogen bonding. 1. The hydrogen-bond basicity of intramolecular hydrogen-bonded heteroatoms

Measurements were made in CCl4 of the formation constant KHB of the 1:1 hydrogen-bonded complexes between the reference donor 4-fluorophenol and the intramolecular hydrogen-bonded systems I (one lone pair on heteroatom Y, one intramolecular hydrogen bond: 8-hydroxyquinaldine and 2-(2-hydroxyphenyl)benzoxazole); II: (two lone pairs, two intramolecular hydrogen bonds: 2,2′-dihydroxybenzophenone and 1,8-dihydroxyanthrone) and III (two lone pairs, one intramolecular hydrogen bond: tropolone, salicylic acid derivatives and guaiacol). The pKHB values and the structural vibrational studies show that system I has a non-zero hydrogen-bond basicity which is due to the oxygen atom. In system II the non-zero basicity is explained by the two oxygens and the breaking of one intramolecular hydrogen bond. In the push-pull system III (e.g. tropolone), in spite of the great decrease of the basicity of the free lone pair by the intramolecular hydrogen bond (e.g. compared with tropone), Y remains the major site for intermolecular association. However in guaiacol, a non push-pull system III, the cooperativity effect makes the phenolic oxygen the major site.