Effect of ring size on the tautomerization and ionization reaction of cyclic 2-nitroalkanones: an experimental and theoretical study

The keto-enol tautomerism of some cyclic 2-nitroalkanones was studied in cyclohexane. Keto-enol equilibrium constants, K(T), at 25 °C were obtained from (1)H NMR spectra. The relative enol content for the investigated ketones as a function of ring size decreases in the order 6 > 7 > 11 > 12 > 15. This trend apparently is different from that observed in water. Density functional theory (DFT) calculations have been performed to rationalize the effects of ring size and of the solvent on tautomerism. The acidity constants, K(a)(KH), for the different keto tautomers were measured spectrophotometrically at 25 °C in buffered aqueous solutions. No simple correlations between K(a)(KH) and ring size was observed, and this is in agreement with a DFT analysis performed on the same compounds.