Free energy relationships via tautomerism in para-acetophenone derivatives

Quantitative studies on free energy relationships have been the subject of interest by scientists during recent years. In present work, the equilibrium and rate constants of keto–enol tautomerism in acetophenone derivatives were calculated using density functional theory. From these calculations, molecular parameters, natural atomic charges and equilibrium and rate constants for tautomeric interconversions were calculated. The results show linear relations between log(Keq/KeqH) (logarithmic ratio of equilibrium constants) and σp, σR or R values. Moreover, the same relations between log(k/kH) (logarithmic ratio of rate constants) and σp, σR or R values were found in the gas phase and five different solvents. The best correlation was observed when σp values were used as substituent constants. The other diagrams using σm, σ+, σ−, σI, σ m + or F as substituent constants were not shown acceptable regressions or appropriate correlation coefficient. The diagrams of log(Keq/KeqH) (versus substituent constants) showed positive slopes (ρ) while diagrams of log(k/kH) showed negative slopes. In other words, by the increase of electron-withdrawing property of para-substituent, the equilibrium constant increases while the rate constant decreases.