NMR determination of Keto-Enol Equilibrium constant Introduction NMR has been very effective in organic chemistry. Indetermining Keto-Enol equilibrium constant, Proton NMR spectroscopy is used to evaluating a number of keto-mixtures. The spin-spin splitting and the chemical shift pattern are employed. The equilibrium constant is affected by both temperature and solvent.
Material and methods
The NMR spectrometer is used. Several milliliters of acetylacetone (CH3OCH2COCH3) and ethyl acetoacetate (CH3CH2OCOCH2COCH3) together with two solvents are used in the experiment. The two solvents represent a non-polar and polar solvent because both are very important in determining the effect of solvent on the equilibrium constant. Hydrogen bonding stabilizes the enol form in polar solvents contrary to the non-polar solvents that lack hydrogen bonding. Stabilization of the enol form is attained by intramolecular hydrogen bonding.
Results/ Data Analysis
The HNMR spectrum for compound CH3OH display two peaks with a chemical shift of 4.9287 and 3.3721 and integration of 1 and 3 respectively. The computed equilibrium constant for acetylacetone in CCl4 is higher than the equilibrium constant for acetylacetone in CH3OH.
The equilibrium constant for acetylacetone in CCl4 is higher than that in CH3OH because it is a non-polar solvent. The equilibrium constant is a ratio of the quantity of enol form to the quantity of keto form. Therefore, the quantity of keto form is inversely proportional to the equilibrium constant. The keto form increases with increase in solvent polarity because keto is favored by hydrogen-bonding solvents. The CH3OH compound used in this experiment display only two peaks in the NMR spectrum. The spectrum lacks the OH peak thus gives a slightly lower keto form than expected.
The main factor in stabilizing any form is the intermolecular hydrogen bond. From research, the polar solvent has the capability of establishing two intermolecular bonds in one molecule and the non-polar solvent establishes only one intermolecular hydrogen bond. (Malcolm). However, in this case, the polar solvent only establishes one intermolecular hydrogen bond. The temperature affecting the equilibrium constant is related to the equilibrium change in enthalpy to that of entropy.
Work cited
Malcolm, Levitt. Spin Dynamics: Basics of Nuclear Magnetic Resonance. Chichester: John Wiley &amp. Sons, 2001.