Inter- and Intramolecular Association in Differently Substituted 2-(X-Pyridyl-)Ethan-1-ols

The homoassociative behaviour in molecules carrying dominant hydrogen bond donor and strong hydrogen bond acceptor functionalities has been investigated on the example of differently substituted 2-(X-pyridyl-)ethan-1-ols with X equals to 4, 3, and 2, abbreviated in this paper as I, II, and III, respectively. The associative behaviour is rather similar for the different substances. Only monomers and cyclic dimers have been found in the solutions of I and II. Compound III behaves differently due to the occurrence of intramolecular hydrogen bonds. The assumption of the existence of open dimers possessing one free OH group per associate does not fit the experimental results. The ring sizes of the cyclic associates decrease from 16 to 14 to 12 members, in compounds I, II, III, respectively, depending on the site of substitution in the pyridyl ring. The values of the association enthalpy ΔH of intermolecular hydrogen bonding have been calculated as ca. −14 kJ/mol per two hydrogen bonds for all substances in CH₂Cl₂ solutions, but the intramolecular hydrogen bond is characterized by a ΔH of −4.5 kJ/mol. The equilibrium constants of association K and entropy changes −ΔS differ characteristically depending on the substance. Experiments and results are described and discussed.