An experimental and computational investigation of the acetone keto-enol tautomerism with acid catalysts
LE3 .A278 2022
Bachelor of Science
Organic compounds with an a-hydrogen can form enols that are structural isomers or tautomers of the carbonyls. Enols are nucleophilic and reactive. Reactions with enols are important in organic synthesis as carbon-carbon bonds can form larger molecules from smaller ones. This work considered the keto-enol tautomerism of acetone to prop-1-en-2-ol. The adol addition intermediate was diacetone alcohol and the products of condensation were mesityl oxide and water. The gas-phase keto-enol tautomerism was studied using gas chromatography and mass spectrometry and theoretically with the Gaussian-16 program suite. Theoretical results indicate the enthalpy change of acetone tautomerization is 49.9 kJ mol-1 and the Gibbs energy change is 43.9 kJ mol-1. For the overall reaction from the tautomerization step to condensation, the enthalpy change is 39.8 kJ mol-1 and the overall change of Gibbs energy is 27.3 kJ mol-1. Acid catalysts reduce the energy barrier associated with keto-enol tautomerism. The activation energies of transition states with catalysts (acetic acid, water and sulfuric acid) were compared to the energy of the transition state without a catalyst. The transition state for the sulfuric acid catalyst gives the lowest activation energy barrier, 230.0 kJ mol-1 , which is 40.0 kJ mol-1 lower than the transition state for the uncatalyzed tautomerization. The computational work supports the continued use of sulfuric acid as a catalyst. In the lab, numerous experiments were conducted with gas chromatography-mass spectrometry at different temperatures and catalysts. Slight reductions in acetone peak area over a 3-h period were observed at 120°C with a sulfuric acid catalyst, supporting continued work with gas chromatography-mass spectrometry with automated injections. Experiments with other catalysts did not show noticeable changes in acetone concentration over time. Quantitation methods will be developed to determine chemical kinetics in the future.
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