ISSN:
0894-3230
Keywords:
Organic Chemistry
;
Physical Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
Thermodynamic 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.
Additional Material:
2 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/poc.610041104
