Solvent effects on the tautomerism of t-butyl 2-picolyl and t-butyl quinaldyl ketones

The tautomeric equilibria (constant Kτ) of tbutyl 2-picolyl ketone (ketone ⇋ enol) and of tbutyl quinaldyl ketone (ketone ⇋ enamine) in semi-dilute solution (0.3 mole.l−1) were studied by 1H-nmr and uv spectroscopy as a function of solvent polarity, for a set of twenty-five different solvents including apolar, dipolar aprotic as well as amphiprotic ones. The contribution of electrostatic field effects (π*), of solute-solvent hydrogen bonding interactions (α) and of solvent cavitation effects (δh) were estimated separately and critically discussed, using multiparametric regression analysis and empirical polarity parameters, according to the “Solvatochromic Comparison Method” developed by Taft, et al. For both heterocyclic ketones, the hydrogen bond donor power of the solvent (α) is the major factor of the Kτ variations, as a result of preferential solvation of the ketonic tautomer.