Protolytic equilibria of several 4‐acyl‐substituted 1‐phenyl‐3‐methylpyrazol‐5‐ones in dioxane–water mixtures

Abstract

AbstractThermodynamic proton ionization constants, TpKa, of several 4‐acyl‐substituted pyrazol‐5‐ones [acyl = trifluoroacetyl (HPMTFP), acetyl (HPMAP), hexanoyl (HPMHP)] were determined in various dioxane–water mixtures at 25 and 35 ± 0.1°C. The TpKa values were determined by glass‐electrode potentiometry and refined by using the extensive weighted least‐squares FORTRAN program TPKA. Both extrapolation and leastsquares methods were used to obtain TpKa values in pure water (0%). All three acyl derivatives are weak monoprotic acids with TpKa values between 2.5 and 3.9, following the order HPMTFP < HPMAP ≤ HPMHP. The TpKa values do not vary linearly with the reciprocal of the dielectic constant of the medium; however, a plot of TpKa versus the mole fraction of dioxane, n2, is linear at a given temperature. Values of standard free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) changes associated with their protolytic equilibria were also calculated. Temperature, medium and substituent effects are briefly discussed.