Spectroscopic and quantum-chemical study of 5-alkoxypyridine-2- and 6-alkoxypyridine-3-carboxylic acids

Abstract

The peculiarities of the structures of 5-alkoxypyridine-2- and 6-alkoxypyridine-3-carboxylic acids were studyed by experimental (IR, UV, and PMR spectroscopy) and quantum-chemical methods. It was established that both types of compounds exist in the form of dimers in the crystalline state, whereas in CC1₄, the first type exist in the form of monomers, while the second type exist in the form of dimers and monomers in dynamic equilibrium. The ability to form an intramolecular hydrogen bond and the interaction of the unshared pairs of electrons of the ring nitrogen atom and the oxygen atom of the C=0 group are among the reasons for the absence of liquid-crystal properties in 5-alkoxypyridine-2-carboxylic acids as compared with 6-alkoxypyridine-3-carboxylic acids, which have such properties. From the point of view of the electronic structures, 6-alkoxypyridine-3-carboxylic acids differ from 5-alkoxypyridine-2-carboxylic acids in that in the former the ring nitrogen atom and the COOH and OAlk groups have an identical effect on the sign of the π-electron charges of the ring carbon atoms, and their π-dipole moments are directed virtually along the longitudinal axis of the molecule.