COMMUNICATION pubs.acs.org/jchemeduc
Substituent Effects on KetoEnol Equilibria Using NMR Spectroscopy Kimberly A. Manbeck, Nicholas C. Boaz, Nathaniel C. Bair, Allix M. S. Sanders, and Anderson L. Marsh* Department of Chemistry, Lebanon Valley College, Annville, Pennsylvania 17003, United States
bS Supporting Information 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 ketoenol 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 ketoenol equilibria. KEYWORDS: Upper-Division Undergraduate, Laboratory Instruction, Physical Chemistry, Hands-On Learning/Manipulatives, Aldehydes/Ketones, Equilibrium, NMR Spectroscopy
T
automerization, a rapid equilibrium in which molecular rearrangement occurs through migration of double bonds and atoms or functional groups, is essential to several types of chemical systems and plays an important role in fields such as biochemistry, coordination chemistry, and synthetic chemistry.1 A widely used experiment in the physical chemistry laboratory involves the use of proton nuclear magnetic resonance (1H NMR) spectroscopy to determine the percentage of each tautomer in a ketoenol equilibrium, as separate resonance signals are observed for protons in each tautomer.2 Students then go on to calculate equilibrium constants using these percentages. Variations of this experiment examine the effect of concentration,3,4 solvent,35 and temperature3,4,6 on the equilibrium distribution. Adaptations involving kinetic measurements have also been published.6,7 In addition, a series of β-diketones and β-ketoesters may be utilized to investigate the effect of chemical structure.2,3,5 In the background section of the original experiment, it is suggested that substituents on the R-carbon of 2,4-pentanediones, as shown in Scheme 1, may affect the ketoenol equilibria.2 This communication presents the student results from implementing this variation in a upper-level physical chemistry laboratory course.
Scheme 1. The Tautomeric Equilibrium of a Generalized 2,4-Pentanedione Derivative, Where R Represents an Electron-Donating or Electron-Withdrawing Substituent
’ RESULTS AND DISCUSSION Using peak integrations for the keto and enol end methyl protons, students determine equilibrium constants by Kc ¼
ð1Þ
where [enol] and [keto] represent the corresponding peak integrations. Results from student NMR data are summarized in Table 2 and are based on an average of four student groups. These equilibrium constants obtained for the compounds dissolved in CDCl3 agree fairly well with those reported in the literature.8,11,10 Comparison of results between student groups reveals a small degree of variance between experimental measurements. Reasons for any deviation could include not correctly calculating the volume of ketone needed to prepare a 0.2 mol fraction solution or not integrating the peaks properly. As seen from the data in the table, the chloro substituent produces an equilibrium that favors the enol tautomer to the greatest extent. Students rationalize that the electron-withdrawing nature of the chloro group leads to the enol form being more favored. This result likely occurs due to the stabilization of the conjugate base enolate anion by the presence of the chloro substituent.11 On the contrary, the electron-donating nature of the methyl group leads to the keto form being more preferred.
’ EXPERIMENTAL PROCEDURE To carry out the experiment, deuterated chloroform containing tetramethylsilane (TMS), 2,4-pentanedione, 3-chloro-2,4pentanedione, and 3-methyl-2,4-pentanedione were obtained commercially from Sigma-Aldrich and used without further purification. Working in groups of three to four, students prepare solutions of a 0.2 mol fraction of each 3-substituted-2,4-pentanedione in deuterated chloroform and acquire a standard 1H NMR spectrum of each solution using a Bruker Avance 300 MHz FT-NMR spectrometer. The program uses a 30° excitation pulse, a 5.3 s acquisition time, and a 1 s relaxation delay over 16 scans. After recording 1H NMR spectra for each of the compounds, students integrate peak intensities and assign chemical shifts in their spectra. These assignments are summarized in Table 1 and agree well with those found in the literature.810 Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
½enol ½keto
Published: August 01, 2011 1444
dx.doi.org/10.1021/ed1010932 | J. Chem. Educ. 2011, 88, 1444–1445
Journal of Chemical Education
COMMUNICATION
Table 1. Assigned Chemical Shifts for the 1H NMR Spectra Chemical Shift/ppm (with TMS as a reference)a Compound
Proton
2,4-Pentanedione
Keto Form
Enol Form
R-H
3.5
5.4
-CH3
2.1
1.9
-OH 3-Methyl-2,4-pentanedione
3-chloro-2,4-pentanedione
15.3
R-H
3.5
R-CH3
1.1
-CH3
2.0
-OH R-H
4.7
-CH3
2.3
1.7 2.1 16.3 2.2
-OH
15.3
a
A 0.2 mol fraction of each 3-substituted-2,4-pentanedione in deuterated chloroform at room temperature.
Table 2. Student Results for Experimentally Determined Values of Kc Compound
Kc ((SD)
2,4-Pentanedione
5.11 ( 0.69
3-Methyl-2,4-pentanedione
0.654 ( 0.122
3-Chloro-2,4-pentanedione
13 ( 2
’ REFERENCES (1) Raczy nska, E. D.; Kosi nska, W.; Osmiazowski, B.; Gawinecki, R. Chem. Rev. 2005, 105, 3561–3612. (2) Garland, C. W.; Nibler, J. W.; Shoemaker, D. P. Experiment 42: NMR Determination of Keto-Enol Equilibrium Constants. In Experiments in Physical Chemistry, 8th ed.; McGraw-Hill: New York, 2009; pp 466 474. (3) Drexler, E. J.; Field, K. W. J. Chem. Educ. 1976, 53, 392–393. (4) Grushow, A.; Zielinski, T. J. J. Chem. Educ. 2002, 79, 707–714. (5) Cook, A. G.; Feltman, P. M. J. Chem. Educ. 2007, 84, 1827–1829. 2010, 87, 678–679. (6) Koudriavtsev, A. B.; Linert, W. J. Chem. Educ. 2009, 86, 1234– 1237. (7) Nichols, M. A.; Waner, M. J. J. Chem. Educ. 2010, 87, 952–955. (8) Rogers, M. T.; Burdett, J. L. J. Am. Chem. Soc. 1964, 86, 2105– 2109. (9) Tanaka, M.; Shono, T.; Shinra, K. Bull. Chem. Soc. Jpn. 1969, 42, 3190–3194. (10) Yoshida, Z.; Ogoshi, H.; Tokumitsu, T. Tetrahedron 1970, 26, 5691–5697. (11) Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry Part A: Structure and Mechanism, 5th ed.; Springer: New York, 2007. (12) Sigma-Aldrich Home Page; http://www.sigmaaldrich.com/ (accessed Jul 2011). (13) Alfa Aesar Home Page; http://www.alfa.com/ (accessed Jul 2011). (14) TCI America home page; http://www.tciamerica.com/ (accessed Jul 2011). (15) Silvernail, C. M.; Yap, G.; Sommer, R. D.; Rheingold, A. L.; Day, V. W.; Belot, J. A. Polyhedron 2001, 20, 3113–3117.
’ ADDITIONAL LABORATORY EXTENSIONS This extension presents a means of incorporating topics from organic chemistry into the physical chemistry laboratory course in which students must focus on the effect of molecular structure on chemical properties. Possible modifications could include having students carry out experiments using other commercially available 3-substituted-2,4-pentanediones and pooling the data because these pentanediones are relatively expensive.12,13,14 Additionally, students in an integrated laboratory setting could synthesize other 3-substituted-2,4pentanediones that are not commercially and record NMR spectra to determine a series of equilibrium constants. For example, as an independent project, one of the student coauthors successfully prepared 3-cyano-2,4-pentanedione15 and recorded the 1H NMR spectrum under similar conditions. It was observed that the cyano substituent leads to only the enol form being present in solution in CDCl3. When the NMR spectrum was acquired in deuterated dimethyl sulfoxide, a small amount of the keto form was present in solution. This finding is consistent with the observation of a decrease in the equilibrium constant for 2,4-pentanedione in going from CDCl3 to DMSO-d6.4 ’ ASSOCIATED CONTENT
bS
Supporting Information A student handout. This material is available via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] 1445
dx.doi.org/10.1021/ed1010932 |J. Chem. Educ. 2011, 88, 1444–1445
