Sherri Young
Albright College
Subject Listing - Chemistry
Advisor: Dr. Christian Hamann
Thursday, Poster Session 2, Presentation Kiosk 14 A, Health & Fitness Center
CORRELATION OF KETO-ENOL EQUILIBRIUM AND ACID DISSOCIATION CONSTANTS
The primary objective of this project was to investigate the correlation between acid dissociation constants and keto-enol equilibrium constants of a group of keto-enol tautomers with varying steric bulk. We also planned to analyze these compounds under a variety of solvent and temperature conditions and to investigate if the same trends could be applied to their nitrogen analogues.
The compounds that we investigated thus far include: 2,4-pentanedione, dimethyl malonate, 3,5-heptanedione, diethyl malonate, 2,6-dimethyl-3,5-heptanedione, diisopropyl malonate, 2,2,6,6-tetramethyl-3,5-heptanedione, and di-tert-butyl malonate. This series of compounds is listed in order of increasing steric bulk, which forces nonplanarity in a molecule.
We used proton NMR spectroscopy to determine keto-enol equilibrium constant values for these compounds using the solvent deuterated chloroform. We calculated the keto-enol equilibrium constant values by integrating the NMR peaks for the keto and enol hydrogens for each compound and dividing the amount of enol hydrogens by the amount of keto hydrogens.
At the conclusion of our summer research, we found that for the investigated diketones the acid dissociation constant values increase with increasing steric bulk. In other words, forcing nonplanarity causes the keto-enol equilibrium to favor the enol form and thus increases the keto-enol equilibrium constant. Literature data indicate that acidity decreases with increasing steric bulk for these compounds, suggesting that there is some inverse correlation between keto-enol equilibrium and acidity. The data we collected were less clear for the diesters and further investigation using more sensitive NMR techniques may be necessary. Also, the literature dataset is incomplete concerning acid dissociation constants so our future work must include the determination of these values. This provides us with the opportunity to determine acid dissociation constants for all of the compounds using one technique.
We plan to use this preliminary dataset to propose a model for the stabilization of enols and enolates due to structural constraints that force planarity or nonplanarity, and determine if a similar model can be applied to the nitrogen analogues of these compounds.
Advisor: Dr. Christian Hamann, Assistant Professor, Chemistry and Biochemistry, Albright College, Reading, PA