SELF-ASSOCIATION OF PYRIDINE AND SOME OF ITS DERIVATIVES IN AQUEOUS-SOLUTION - INFLUENCE OF ALKYL SUBSTITUENTS

The self-association of pyridine and some of its alkyl derivatives (2-, 3- and 4-picoline, 2,4- and 2,6-lutidine, 4-tert-butyl- and 2,6-di-tert-butylpyridine) was studied in aqueous solution, at different pH values, by UV spectroscopy. The variation in molar absorptivity with concentration was measured not only for the main maximum, but also for the different component bands of the absorption band in the mid-UV region (above 200 nm) of these compounds. From the experimental curves of hypochromic effects, self-association constants for dimerization (K2) and polymerization (K(n)) were calculated. The results obtained are discussed in detail with relation to the position and nature of alkyl substituents on the pyridine ring. The most relevant result is the influence of alkyl substituents on self-association, particularly the special role of the methyl substituents in ortho positions with respect to the nitrogen atom in 2-picoline, 2,4- and 2,6-lutidine. In 2-picoline and 2,6-lutidine, polymerization can be studied separately from dimerization by measuring the band at longest wavelength, which suggests a possible relationship between the mechanism of formation of polymers and the pi* <-- n transitions which give rise to these bands.