LABORATORY EXPERIMENT pubs.acs.org/jchemeduc
Investigating the Mechanism of Heteroaromatic Decarboxylation Using Solvent Kinetic Isotope Effects and Eyring Transition-State Theory Scott O. C. Mundle, Liliana Guevara Opi~nska, Ronald Kluger, and Andrew P. Dicks* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada, M5S 3H6
bS Supporting Information ABSTRACT: An upper-level mechanistic organic experiment is outlined where undergraduates measure kinetic rate constants for decarboxylation of pyrrole-2-carboxylic acid by the initial-rates method. UV spectroscopy is used to monitor reactant disappearance in both hydrochloric acid and deuterium chloride at different temperatures. Individual data are pooled and utilized by the class to calculate solvent kinetic isotope effects and reaction activation parameters (the latter from Eyring plots). Students interpret experimental results and additional literature data to deduce likely operative decarboxylation mechanisms at varying acidities. KEYWORDS: Upper-Division Undergraduate, Laboratory Instruction, Organic Chemistry, Collaborative/Cooperative Learning, Aromatic Compounds, Carboxylic Acids, Isotopes, Kinetics, Mechanisms of Reactions, UVVis Spectroscopy
S
olvent kinetic isotope effects (SKIE) are phenomena providing penetrating insight into many organic reaction mechanisms.1 However, few student laboratory exercises incorporate SKIE in mechanistic elucidations.2 This is attributable to difficulty in finding an appropriate reaction that is economical in use of costly isotopes, pedagogically rigorous, and feasible within the time frame of an undergraduate laboratory period. We report a new collaborative learning laboratory exercise utilizing both SKIE and Eyring transition-state theory (ETST)3 to elucidate the mechanism of pyrrole-2-carboxylic acid decarboxylation (Scheme 1). This has been successfully integrated into a thirdyear physical organic chemistry course tailored toward biologically oriented undergraduates. Decarboxylation is the formal replacement of a carboxyl group (R-COOH) with a proton. It is a generic term for processes producing carbon dioxide from more complex molecules and the ultimate step in biochemical and chemical molecular degradation.4,5 It is typically necessary for a carboxyl group to lose a proton (forming a carboxylate ion R-COO) whereupon CO2 is lost.6,7 In concentrated acid, the carboxylate ion concentration is reduced through protonation. However, decarboxylation of pyrrole-2-carboxylic acid occurs rapidly under acidic conditions, which is an investigative focus of this experiment.8,9 Dunn and Lee9 proposed acid-catalyzed decarboxylation of pyrrole-2-carboxylic acid occurs via initial ring protonation followed by deprotonation and carboncarbon bond cleavage from zwitterion 1 (Scheme 1, path A). Forming the zwitterion facilitates loss of CO2 from 1 rather than HCO2þ, which is a very unfavorable, high-energy species. Support for this mechanism comes from a change in 12C/13C kinetic isotope effect (CKIE) from unity in neutral solution to 1.028 in concentrated acid, suggesting a change in rate-determining step from ring protonation to carbon carbon bond cleavage with increasing acidity. A recent report8 proposes the reaction proceeds via protonation and water addition to the carboxyl group forming 2 Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
(Scheme 1, path B), which is supported by independent theoretical calculations.10 Here, carboncarbon bond cleavage proceeds from the hydrated carboxyl initially generating protonated carbonic acid, which rapidly decomposes into CO2 and H3Oþ. The CKIE cannot distinguish between paths A and B, as carboncarbon bond cleavage can be a significant factor for either process in concentrated acid. However, changes in SKIE and ETST parameters provide additional support for the path B mechanism leading to protonated carbonic acid. It is important to note that protonation at an aromatic carbon atom occurs in paths A and B, rather than protonation of the nitrogen atom in pyrrole-2-carboxylic acid. Protonation at either carbon or nitrogen would lead to loss of aromaticity. Significantly, the carbocation formed by protonation at carbon is stabilized by charge delocalization, which would be unavailable if protonation were to take place at the nitrogen atom. These observations provide critical insight into long-standing issues involving enzyme catalysis. The reverse of decarboxylation (carboxylation) in biological processes continues to raise questions in enzymology.11,12 Loss of a carboxyl group can clearly occur without direct CO2 product formation.13 The same must be true in the reverse direction (the forward mechanism of a reversible reaction will be mirrored in the reverse reaction), suggesting enzymes may be capable of using readily available bicarbonate as a source of carbon dioxide.14 Hutchinson et al.15 reported an undergraduate physical chemistry exercise examining Arrhenius parameters associated with pyrrole-2-carboxylic acid decarboxylation, based on the path A mechanism. This experiment formed a strong foundation from which to expand and include SKIE and ETST measurements in our approach. Students now observe significant changes in SKIE and Eyring activation parameters under different experimental Published: May 05, 2011 1004
dx.doi.org/10.1021/ed100793r | J. Chem. Educ. 2011, 88, 1004–1006
Journal of Chemical Education
LABORATORY EXPERIMENT
Scheme 1. Two Proposed Mechanisms of Pyrrole-2-carboxylic Acid Decarboxylation
conditions and draw appropriate mechanistic conclusions.16 This exercise has thus evolved from simply determining reaction activation energies to accumulating substantial experimental evidence (including published 12C/13C kinetic isotope effects) for the reaction. From this and consultation of primary research literature, students are able to construct reasonable mechanisms for pyrrole-2-carboxylic acid decarboxylation.
’ EXPERIMENTAL OVERVIEW Decarboxylation of pyrrole-2-carboxylic acid involves two multistep mechanisms.17 In dilute acid, the reaction takes place via path A, where ring protonation is rate-determining. As the acid concentration is increased, ring protonation is facilitated. However, zwitterion formation becomes less favorable under these conditions and the mechanism changes to path B. In this instance, proton transfer and addition of water to the carboxyl group are fast, and carboncarbon bond cleavage is ratedetermining. The primary goal for students is to experimentally deduce that such a mechanistic change is occurring. The laboratory instructor prepares stock solutions of aqueous hydrochloric acid (5 and 4 wt %), deuterium chloride in D2O (5 and 4 wt %, 99 atom % D) and aqueous pyrrole-2-carboxylic acid (2.0 mM). Students must accurately control the temperature of these solutions. Decarboxylation of pyrrole-2-carboxylic acid is monitored through a decrease in absorbance at 262 nm using the initial-rates method after mixing the temperature-equilibrated reactant solutions.18 The observed rate law is given by eq 1. Pseudo-first-order conditions exist as the [H3Oþ] or [D3Oþ] is in a large excess over the concentration of pyrrole-2-carboxylic acid. rate ¼
d½pyrrole-2-carboxylic acid dt
¼ kobs ½pyrrole-2-carboxylic acid
ð1Þ
On the basis of the initial-rates method, absorbance versus time kinetic data are fit to a straight line.18 The first-order rate constant (kobs) is determined by dividing the slope of the fitted line by the initial absorbance obtained from pyrrole-2-carboxylic acid. Each student is assigned an HCl or DCl concentration and temperature to acquire kinetic data. Students that complete their measurements early are assigned a new set of parameters. Kinetic runs are performed in triplicate under thermostatted conditions and all calculated rate constants are submitted to the instructor for tabulation and distribution to the entire class. Eyring plots are constructed from a minimum of four points averaged from class data and typically show a good linear relationship (Figure 1).
Figure 1. Student Eyring plots for pyrrole-2-carboxylic acid decarboxylation using a temperature-controlled UV-spectrometer in 4 wt % (O) and 5 wt % (b) solutions of aqueous hydrochloric acid.
Figure 2. Logarithm of the observed first-order rate constant as a function of the Hammett acidity function (H0) in hydrochloric acid (O), perchloric acid (b), sulfuric acid (0), and 23 wt % deuterium chloride in deuterium oxide (X). Adapted with permission from ref 8. Copyright 2011 American Chemical Society.
SKIE are determined by dividing the first-order rate constant in HCl solution by the first-order rate constant in DCl solution under equivalent conditions.
’ HAZARDS Hydrochloric acid and deuterium chloride can cause severe burns; however, all stock solutions are prepared by the instructor. Pyrrole-2-carboxylic acid is a moderate skin, respiratory, and eye irritant. ’ RESULTS The experimental SKIE typically falls between 1.5 and 2.0 in both 5 and 4 wt % acid, and are in good agreement with the literature.17 Collaborative Eyring plots generally lead to activation parameters suggesting an increase in order in the reaction rate-determining transition state (ΔGq ∼ 95 kJ mol1, ΔHq ∼ 91 kJ mol1, ΔSq ∼ 13 J mol1 K1). These results support a mechanism involving rate-determining proton transfer under low acid concentrations. Students are provided with kinetic data at 23 wt % aqueous HCl, 23 wt % DCl in D2O and a plot of 1005
dx.doi.org/10.1021/ed100793r |J. Chem. Educ. 2011, 88, 1004–1006
Journal of Chemical Education observed reaction rates at different Hammett acidity functions (Figure 2). In 23 wt % acid, the SKIE is near unity and activation parameters reveal a significant change in the entropy term (ΔGq ∼ 92 kJ mol1, ΔHq ∼ 99 kJ mol1, ΔSq ∼ þ26 J mol1 K1). Absence of an SKIE reveals proton transfer is no longer part of the rate-determining transition state and the positive entropy change suggests loss of order from a dissociative process, which is a key feature in distinguishing paths A and B.17 In Figure 2, the reaction rate reaches a plateau at H0 < 2, in contrast to the decrease in reaction rate observed with other aromatic acids (corresponding to difficulty in forming the carboxylate in concentrated acid).7 Moreover, these latter reactions have nonunity SKIE values revealing importance of proton transfer in the rate-determining step that persists in concentrated acid.6 These examples provide students with direct evidence that pyrrole-2-carboxylic acid decarboxylation must occur by an alternative mechanism.
’ CONCLUSIONS The broad spectrum of available experimental evidence (multiple isotope effects, activation parameters, acidity functions) challenges students to widen their perspective in their approach to the mechanism. The nature of the results also allows introductory and advanced interpretations. A basic approach involves interpreting the mechanism based on the observed presence or absence of a solvent kinetic isotope effect under different reaction conditions. A more sophisticated approach allows students to compare the magnitude of isotope effects to other analogous processes found in the literature, which provides additional mechanistic insight.19,20 The postlaboratory formal report deals with directly comparing the original article by Dunn and Lee9 with experimental evidence obtained and provided in this laboratory exercise. Students additionally consider decarboxylation of pyridine-2carboxylic acid, which exemplifies a heteroaromatic decarboxylation reaction suppressed by acid.7 Pyridine-2-carboxylic acid is protonated on the ring nitrogen in concentrated acid, making the carboxylic acid a much weaker proton acceptor relative to pyrrole-2-carboxylic acid. Under comparative acidities, the mechanism involving the addition of water does not occur, a necessary contrasting example for student discussion. The cooperative, cost-effective methodology described herein makes this experiment fitting for a large upper-level reaction mechanisms course. Insufficient experiments exist at the undergraduate level featuring either Eyring activation parameters or solvent kinetic isotope effects as mechanistic probes. This procedure combines both of these concepts and others to examine a fundamental reaction mechanism involved in heteroaromatic decarboxylation. Students recognize that considering solvent and 12C/13C kinetic isotope effects along with activation parameters can yield important perspectives regarding organic reactivity.
LABORATORY EXPERIMENT
’ ACKNOWLEDGMENT We are grateful to the Department of Chemistry, University of Toronto for financial support via a graduate student Teaching Fellowship Program. ’ REFERENCES (1) For a discussion, see (a) Anslyn, E. V.; Dougherty, D. A. Modern Physical Organic Chemistry; University Science Books: Sausalito, CA, 2006; p 437. (b) Carroll, F. A. Perspectives on Structure and Mechanism in Organic Chemistry; Brooks/Cole: Pacific Grove, CA, 1998; pp 365366. (2) (a) El Seoud, O. A.; Bazito, R. C.; Sumodjo, P. T. J. Chem. Educ. 1997, 74, 562–565. (b) McGuiggan, P.; Eliason, R.; Anderson, B.; Botch, B. J. Chem. Educ. 1987, 64, 718–720. (3) Eyring, H. Chem. Rev. 1935, 17, 65–77. (4) Brown, B. R. Quart. Rev. 1951, 5, 131–146. (5) Kluger, R.; Tittmann, K. Chem. Rev. 2008, 108, 1797–1833. (6) Dunn, G. E.; Dayal, S. K. Can. J. Chem. 1970, 48, 3349–3353. (7) Dunn, G. E.; Lee, G. K. J.; Thimm, H. Can. J. Chem. 1972, 50, 3017–3027. (8) Mundle, S. O. C.; Kluger, R. J. Am. Chem. Soc. 2009, 131, 11674–11675. (9) Dunn, G. E.; Lee, G. K. J. Can. J. Chem. 1971, 49, 1032–1035. (10) Cheng, X.; Wang, J.; Tang, K.; Liu, Y.; Liu, C. Chem. Phys. Lett. 2010, 496, 36–41. (11) Kluger, R.; Mundle, S. O. C. Adv. Phys. Org. Chem. 2010, 44, 357–375. (12) Sauers, C. K.; Jencks, W. P.; Groh, S. J. Am. Chem. Soc. 1975, 97, 5546–5553. (13) Warren, S.; Williams, M. R. J. Chem. Soc. B 1971, 618–621. (14) Olah, G. A.; White, A. M. J. Am. Chem. Soc. 1968, 90, 1884–1889. (15) Hutchinson, K. M.; Bretz, S. L.; Mettee, H. D.; Smiley, J. A. Biochem. Mol. Biol. Educ. 2005, 33, 123–127. (16) For a discussion of Arrhenius and Eyring parameters, see Anslyn, E. V.; Dougherty, D. A. Modern Physical Organic Chemistry; University Science Books: Sausalito, CA, 2006; pp 366372. (17) Mundle, S. O. C.; Lacrampe-Couloume, G.; Sherwood Lollar, B.; Kluger, R. J. Am. Chem. Soc. 2010, 132, 2430–2436. (18) Casado, J.; Lopez-Quintela, M. A.; Lorenzo-Barral, F. M. J. Chem. Educ. 1986, 63, 450–452. (19) Willi, A. V. Z. Naturforsch., A: Phys. Sci. 1958, 13, 997–998. (20) Schubert, W. M.; Keeffe, J. R. J. Am. Chem. Soc. 1972, 94, 559–566.
’ ASSOCIATED CONTENT
bS
Supporting Information Laboratory notes for the students and notes for the instructors. This material is available via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. 1006
dx.doi.org/10.1021/ed100793r |J. Chem. Educ. 2011, 88, 1004–1006
