NMR Determination of Keto-Enol Equilibrium Constants

In this experiment proton NMR spectroscopy is nsed in evalnating the equilibrium composition of various keto-enol mixtures. Chemical shifts and spin-spin splitting patterns are employed to assign the spectral features to specific protons, and the integrated intensities are used to yield a quantitative measure of the relative amounts of the keto and enol forms. Solvent effects on the chemical shifts and on the equilibrium constant are investigated for one or more j8-diketones and j8-ketoesters. 

Chemical Shifts. In Exp. 32, the Zeeman energy levels of a nucleus in an external applied field were given as 

Here the definition is based on the resonant frequencies for a fixed external induction (field) B. A second (nearly equivalent) relation is based on the alternative experimental case, where B is varied to achieve resonance at a fixed instrumental frequency v. In this case Bjyl - a = 5/1 - o- ) and 

Long-range deshlelding can occur in aromatic and other molecules with delocalized TT electrons. For example, when the plane of the benzene molecule is oriented perpendicular to B, circulation of the tt electrons produces a ring current (see Fig. lb). This ring current induces a secondary field at the protons that is aligned parallel to B and thus increases 

Shielding and deshielding of protons (a) shielding of proton due to induced diamagnetic electron circulation b) deshielding of protons in benzene due to aromatic ring currents. 

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Because the system exists essentially completely as the thiol isomer, a carbon-acid acidity constant for ionization starting with the thio-keto form as the initial state, QJ, could not be measured, and a keto-enol equilibrium constant, ATe, could not be determined. A lower limit for can nevertheless be estimated on the assumption that 5% of the keto isomer would have produced a detectable signal in the H NMR spectrum of the enol form. Because no such signal was seen, must be greater than 20, which makes pK less than —1.3. The relationship = KeQJ then leads to > 1.1 x 10 M, pQ <2.1. 

The keto-enol tautomeric equilibrium of acetylacetone is an intramolecular hydrogen exchange process. High-pressure NMR was used to study changes in this equilibrium over a pressure range to 2.5 kbar and temperatures to 145 C (51). With an increase in temperature at constant pressure, the equilibrium distribution shifted to the keto tautomer. An increase in pressure did not change the keto-enol distribution at any temperature. From the high-pressure experiments as a function of temperature the reaction enthalpy, A/J, and entropy, AS, were determined to be 2.80 0.02 kcal/mol and 7.2 0.3 cal/K mol, respectively. 

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