Abstract
Association in derivatives of 3-hydroxypyridine has been examined via the chemical shifts δ for OH in PMR for equimolar mixtures of the derivatives of pyridine and phenol in CCl4 and CH2Cl2. It is found that spatial screening of the pyridine N and phenol OH by methyl groups has an anomalous effect on the strength of the complex. A possible explanation is that the ionic form NH . . . O is stabilized on account of increased basicity of the N and acidity of the H in the methyl derivatives of pyridine and phenol. Domination by the intermolecular hydrogen bond between OH and N is the distinctive feature of association in 3-hydroxypyridines relative to association in phenol. The temperature dependence of δOH gives the enthalpy and entropy of association for 2, 4, 6-trimethyl-3-hydroxypyridine and of complex formation for various model systems. IR measurements confirm the conclusions.
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