Acidities and pKEnol values of stable simple enols in the gas phase

Abstract

The equilibrium constants for the ionization of nine stable simple enols and of eight ketones were determined by the ion cyclotron resonance (ICR) method in the gas phase. From these pK_a values, seven ketone–enol equilibrium constants K_Enol were calculated. The most acidic enol in the series Mes₂CC(OH)R (3) is when R = p-CF₃C₆H₄ (ΔG^°_acid = 323.4 kcal mol⁻¹) and the least acidic one is when R = t-Bu (ΔG^°_acid = 334.7 kcal mol⁻¹). There is a good correlation between the ΔG^°_acid values for the ketones and the enols. For 4 α-aryl-substituted enols (R = Ar) and their keto isomers, there is a rough correlation with Hammett’s σ values. The ΔG^°_acid’s for the enols where R = Ar correlate linearly with their ΔG^°_acid’s in hexane, but other enols deviate from the relationship. The pK_a’s and pK_Enol values were calculated by RHF/3-21G* and some values were calculated by B3LYP/3-21+G*. The observed ΔG^°_acid’s for the enols give an approximately linear correlation with the calculated ΔE^°_acid values. However, the ΔG ^°_eq values for the keto–enol equilibrium in the gas phase or in hexane do not correlate linearly with ΔE_eq for all the enols. The calculated Ar–CC dihedral angles in the enols 3 change only slightly to modestly on ionization, presumably due to a relatively rigid geometry caused by steric hindrance. The substituent effects on the acidities and the K_Enol values are discussed.