Substituent Effects on Keto–Enol Equilibria Using NMR Spectroscopy

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Abstract

In this extension to a classic physical chemistry experiment, students record the proton nuclear magnetic resonance spectra of the β-diketones 2,4-pentanedione, 3-methyl-2,4-pentanedione, and 3-chloro-2,4-pentanedione to investigate the effect of substituents on keto–enol tautomerization equilibria. From the integrated intensities of keto and enol methyl proton peaks, students calculate the equilibrium constant for each 2,4-pentanedione. The students then use the effects of electron-donating and electron-withdrawing substituents to elucidate the nature of keto–enol equilibria.

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