Keto–enol equilibrium constants

Toullec, Jean

doi:10.1002/9780470772294.ch6

Cited by 43 publications

(60 citation statements)

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“…As the H 3 O ϩ catalysed reaction of the enolate E -is kinetically indistinguishable from the ''water'' reaction of the enol EH, rate constants for the ''water'' ketonisation of the enolate, k o E Ϫ ϭ 0.046 s -1 , and of the enol, k o EH ϭ 0.140 s -1 , and a value of the acidity constant of EH, pK a EH ϭ 2.81±0.10, could be obtained from the best fit of experimental data to the sigmoid curve of Figure 1 by means of Equation (2).…”

Section: Scheme 2 H 3 Omentioning

confidence: 99%

“…[8] However, kinetic studies on the tautomerism of nitro ketones are scarce. [2,9] The enol content for simple derivatives is believed [10] to be very low but ''the paucity of the data do not make it possible to examine structural effects on the enol content of α-nitro ketones in detail.'' [2] α-Nitro ketones are, in principle, an equilibrium mixture of the tautomeric forms shown in Scheme 1: Scheme 1.…”

Section: Introductionmentioning

confidence: 99%

“…[2,9] The enol content for simple derivatives is believed [10] to be very low but ''the paucity of the data do not make it possible to examine structural effects on the enol content of α-nitro ketones in detail.'' [2] α-Nitro ketones are, in principle, an equilibrium mixture of the tautomeric forms shown in Scheme 1: Scheme 1. Tautomeric forms of 3-nitrobutan-2-one It is considered [2,11] that c can generally be neglected for practical purposes, probably due to the high acidity of the aci form in aqueous solutions.…”

Section: Introductionmentioning

confidence: 99%

“…There have been several recent investigations [1][2][3][4][5][6] on ketoenol interconversion and determinations of the tautomerisation equilibrium constants, K T , by combining the rate constants for enolisation and ketonisation in basic aqueous solutions. The substrates studied so far were generally acidic enough to be (about 10-20%) ionised in relatively dilute (0.5 mol dm -3 ) NaOH in order to follow the ketonisation reaction of the enol form accurately by stopped-flow techniques.…”

Section: Introductionmentioning

confidence: 99%

“…[2] α-Nitro ketones are, in principle, an equilibrium mixture of the tautomeric forms shown in Scheme 1: Scheme 1. Tautomeric forms of 3-nitrobutan-2-one It is considered [2,11] that c can generally be neglected for practical purposes, probably due to the high acidity of the aci form in aqueous solutions.…”

Section: Introductionmentioning

confidence: 99%

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Equilibrium Constants for Ionisation and Enolisation of 3-Nitrobutan-2-one

Fontana¹,

Maria

Siani

et al. 2000

Eur. J. Org. Chem.

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The equilibrium constant for the keto‐enol tautomerism of 3‐nitrobutan‐2‐one KT = [enol]/[ketone] has been measured in water as 4.57 × 10−3 (pKT = 2.34) by combining the rate constants for ketonisation of the enolate form and pKa of the ketone at 25 °C. The rates of ketonisation were measured by a rapid kinetic technique and the pKa 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 pKa (in water at 25 °C) of both tautomeric forms for a number of simple ketones whose pKas and pKTs are available in the literature. The agreement of experimental acidity constants of the enol, pKaEH, the ketone, pKaKH, and the tautomeric constant, pKT, with predicted values is satisfactory.

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Section: Scheme 2 H 3 Omentioning

confidence: 99%