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Equilibrium Constants for Ionisation and Enolisation of 3‐Nitrobutan‐2‐one
Author(s) -
Fontana Antonella,
De Maria Paolo,
Siani Gabriella,
Pierini Marco,
Cerritelli Simona,
Ballini Roberto
Publication year - 2000
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/(sici)1099-0690(200004)2000:8<1641::aid-ejoc1641>3.0.co;2-5
Subject(s) - tautomer , chemistry , enol , ketone , keto–enol tautomerism , acid dissociation constant , reaction rate constant , acetone , computational chemistry , equilibrium constant , ionization , organic chemistry , kinetics , catalysis , aqueous solution , ion , quantum mechanics , physics
The equilibrium constant for the keto‐enol tautomerism of 3‐nitrobutan‐2‐one K T = [enol]/[ketone] has been measured in water as 4.57 × 10 −3 (p K T = 2.34) by combining the rate constants for ketonisation of the enolate form and p K a of the ketone at 25 °C. The rates of ketonisation were measured by a rapid kinetic technique and the p K a was determined spectrophotometrically and potentiometrically as 5.15. A comparison with 2‐butanone and acetone shows a strong influence of the nitro group in enhancing the acidity of the substrate and in stabilizing the enol relative to the keto tautomer. By means of semiempirical AM1 calculations, good correlations were found between the atomic charge on the acidic hydrogens and the p K a (in water at 25 °C) of both tautomeric forms for a number of simple ketones whose p K a s and p K T s are available in the literature. The agreement of experimental acidity constants of the enol, p K a EH , the ketone, p K a KH , and the tautomeric constant, p K T , with predicted values is satisfactory.