In the Laboratory edited by
The Microscale Laboratory
R. David Crouch Dickinson College Carlisle, PA 17013-2896
Evaluating Mechanisms of Dihydroxylation by Thin-Layer Chromatography A Microscale Experiment for Organic Chemistry Benjamin T. Burlingham* and Joseph C. Rettig Department of Chemistry, Mount Union College, Alliance, OH 44601; *
[email protected] At the center of modern organic chemistry is the concept of mechanisms. It is largely the mechanism, and its predictive power, that makes the art of organic chemistry into a science. In addition, the mechanism provides the organic chemist the conceptual framework to tie seemingly unrelated reactions into a cohesive discipline. The importance of the concept of mechanisms is reflected in the teaching of introductory-level organic chemistry, where a mechanistic approach is usually central to the course. Students are expected to understand the principles of functional-group reactivity and be able to express functional group transformation and the reactivity of reagents through the curved arrow formalism. Unfortunately, students often misunderstand not only how to use a mechanism, but the entire purpose and meaning of the concept of mechanisms. It has been noted that a mechanism is largely seen by students, even beginning graduate students, as a way to “get to the products” (1). In the effort to learn the formalism, students often reduce it to a puzzle designed to get to the right answer rather than a mechanism as a description, based on chemical principles, of how a reaction physically happens. Furthermore, a mechanism is often presented in texts as an immutable fact rather than the best reasonable proposal of a reaction sequence that fits the experimental data. In this regard, it might be helpful to expect students to be able to evaluate a mechanism before expecting students to be able to propose a mechanism. The experiment described herein may be used to demonstrate the concept of mechanisms as a description of a real phenomenon based on experimental evidence. Students are given the opportunity to evaluate a series of proposed mechanisms of dihydroxylation reactions in light of experimental data that they collect. Experiment In this experiment, students are initially posed the following question: Which of the proposed mechanisms of dihydroxylation is most consistent with experimental data? Multiple mechanisms are then presented for each of three dihydroxylation reaction conditions: basic permanganate, Oxone epoxidation– hydrolysis, and the Woodward dihydroxylation (Scheme I). The proposed mechanisms lead to a variety of stereochemical outcomes through anti addition, syn addition, or a mixture of anti and syn addition. To determine which proposed mechanism is consistent with experimental results, students must analyze the stereochemistry of the product(s) of each reaction and evaluate the proposed mechanism in light of experimental observation.
Teams of students then carry out the three dihydroxylations of cyclohexene on the microscale and analyze the products formed. Owing to the difficulty involved in separating and purifying the cyclohexane-1,2-diol products from aqueous medium, the products are not isolated; rather, the crude reaction mixture is analyzed by thin-layer chromatography (TLC) alongside authentic samples of cis- and trans-1,2-cyclohexanediol. TLC is a simple, yet powerful, tool for determining the product distribution in these reactions. Because the cis and trans diols are diastereomers, they may be separated by TLC and have significantly different R f values. Furthermore, when stained with p-anisaldehyde stain, the spots stain different colors, with the cis diastereomer stained red and the trans diastereomers stained blue (Figure 1). Student results are consistent, with all the reactions demonstrating stereoselectivity. The epoxidation– hydrolysis reaction produces the anti addition product (racemic 1,2-cyclohexanediol) and the permanganate reaction produces the syn addition (meso-1,2-cyclohexanediol) product. For the Woodward reaction, most students observe the expected cis product exclusively.1 The analysis of the data is straightforward because there are no other byproducts visible on the TLC plate.
A
1. Oxone, acetone 2. H3Oá
B
cold, basic permanganate
OH
OH
OH
OH
C
1. I2, AgOAc, wet glacial acetic acid
OH
2. basic hydrolysis
OH
Scheme I. Comparison of dihydroxylation reactions using (A) epoxidation–hydrolysis, (B) permanganate oxidation, and (C) the Woodward dihydroxylation.
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 7 July 2008 • Journal of Chemical Education
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In the Laboratory
from each mechanism. Students recognize that mechanism A is not a legitimate mechanism as written because the same two electrons are being used to form two different bonds in the product. Students propose the products of mechanism B to be the racemic mix of diols and the product of mechanism C to be the meso diol. When students see from the experimental data that the Oxone reaction produces the trans diol but not the cis diol, they are able to conclude that mechanism B is consistent with the data, but mechanism C is inconsistent with experimental evidence. Hazards
Figure 1. TLC analysis of dihydroxylation reactions: (A) co-spot of cis and trans diol, (B) authentic cis diol, (C) authentic trans diol, (D) Oxone reaction, (E) permanganate reaction, and (F) Woodward dihydroxylation. The plate was developed in ethyl acetate and stained with acidic p-anisaldehyde solution.
In addition to conducting the dihydroxylation reactions, students are asked to use the data to evaluate a set of mechanisms proposed by the instructor. As an example of the analysis that students are asked to perform, Scheme II represents the series of three possible mechanisms that are presented to students for the Oxone reaction. Prior to lab, students are asked to evaluate these mechanisms based on the principles of the arrow pushing formalism and to predict the product(s) that would be formed
Oxone is a corrosive oxidizer and may cause burns. Cyclohexene, acetone, and tert-butyl alcohol are flammable irritants. Concentrated hydrochloric acid, glacial acetic acid, and aqueous sodium hydroxide are corrosive. Potassium permanganate is a strong oxidizer and irritant. Silver acetate is an irritant. Iodine is corrosive, causes burns, and is harmful through inhalation or exposure to skin. Students should wear gloves and work in an adequate fume hood during the entire experiment. Students should avoid chemical contact through inhalation and exposure to skin and eyes. Discussion Many lab experiments ask students to propose or evaluate mechanisms based on experimental or calculated data (2–8). Experiments such as these require students to be experienced enough in mechanistic chemistry to propose a mechanism de novo or with a few guiding principles, and thus are only appropriate later in the organic chemistry curriculum or when a
A O
O
H
Cl
O O
O
O
H
Cl
H2O
B
deprotonation
OH2 O
O
O
H
Cl
O H
OH
C HCl, H2O
O
O
O
O
hydrolysis
Scheme II. Proposed mechanisms of dihydroxylation of cyclohexene with Oxone.
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Journal of Chemical Education • Vol. 85 No. 7 July 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
specific mechanism type is being investigated. In contrast, this experiment has been designed to fit into the classroom curriculum when students are first learning the purpose and formalisms of the concept of mechanisms, but prior to the investigation of any specific mechanisms (9). In this experiment, students are not asked to propose mechanisms but rather to evaluate previously proposed mechanisms on the basis of bonding principles and consistency with experimental data. In our curriculum, this lab has been designed as a guidedinquiry lab meant to teach data analysis and teamwork. The timing of this lab is important to its pedagogy. This lab is incorporated into the curriculum in the middle of the first semester. At this point, students have learned about the structure of organic molecules (including stereochemistry), nomenclature, acid–base reactions and are just being introduced to the basic concept of mechanisms and the arrow formalism. Students have not encountered any substitution, elimination, or addition reactions or mechanisms thereof. At this point in the curriculum, students have no knowledge of the expected products in these addition reactions. In this context, this lab works well as a guided-inquiry lab (10–15).2 The placement of this experiment early in the laboratory curriculum also affects what is expected from the students experimentally. Because of an emphasis on spectroscopy and separations earlier in the semester, this is the first time students have performed an organic reaction. Because students work in teams of three, each inexperienced student only performs one reaction, allowing ample time for completion of the reaction in a three-hour period. The short reaction times also allow for a post-lab discussion, either formally with the whole class or informally among teams. This experiment is also the first experience for students with TLC, so students are directly told which eluent to use and which authentic samples to include as references. The results of the TLC are unambiguous for most students and discussion about the separation of enantiomers versus the separation of diastereomers is possible. This series of reactions is ideal for teaching teamwork concepts early in the semester. No specific directions are given to students of how to work as a team, so students must plan how they will conduct the experiment. (Students generally choose the logical approach of having each team member conduct a different experiment and working together on the chromatography.) Because each member contributes in a different way to the experiment, each of the three team members is essential for success. At the same time, each team member learns the same fundamental techniques during the lab period. Furthermore, this experiment requires discussion of data analysis among teammates prior to conclusions being drawn. Based on formal reports submitted by teams, students demonstrated the ability to successfully evaluate mechanisms using TLC data. All groups properly interpreted the TLC data to determine which products were made during the reaction. All groups identified the one mechanism consistent with the permanganate reaction and the two mechanisms consistent with the Woodward dihydroxylation. Most groups also correctly evaluated the Oxone reaction mechanisms, but about 20% of the groups did not recognize that Ozone mechanism A is not an acceptable mechanism based on the arrows drawn.
Acknowledgments Thanks to Mount Union College and the Department of Chemistry for providing the funds and facilities necessary to complete this work. Notes 1. Some students (~10%) observe a small quantity of the trans product according to TLC data. The source of this variability was not investigated. 2. For a short discussion of criteria of guided-inquiry labs and how this lab may be recast to fit other curricula, see the instructors notes in the online supplement.
Literature Cited 1. Bhattacharyya, G.; Bodner, G. M. J. Chem. Educ. 2005, 82, 1402–1407. 2. Crouch, R. D.; Howard, J. L.; Zile, J. L.; Barker, K. H. J. Chem. Educ. 2006, 83, 1658–1660. 3. Ikeda, G. K.; Jang, K.; Mundle, S. O.; Dicks, A. P. J. Chem. Educ. 2006, 83, 1341–1343. 4. Mak, K. W.; Lai, Y. M.; Sui, Y.-H. J. Chem. Educ. 2006, 83, 1058–1061. 5. Grant, A.; Latimer, D. J. Chem. Educ. 2003, 80, 670–671. 6. Columbo, M. I.; Bohn, M. L.; Ruveda, E. A. J. Chem. Educ. 2002, 79, 484–485. 7. Krishnamurty, H. G.; Jain, N.; Samby, K. J. Chem. Educ. 2000, 77, 511–513. 8. Hessley, R. K. J. Chem. Educ. 2000, 77, 202–203. 9. Smith, J. G. Organic Chemistry, 2nd ed.; McGraw Hill: Boston, 2008; Chapter 6. 10. Allen, J. B.; Barker, L. N.; Ramsden, J. H. J. Chem. Educ. 1986, 63, 533–534. 11. Ricci, R. W.; Ditsler, M. A. J. Chem. Educ. 1991, 68, 228– 231. 12. Domin, D. S. J. Chem. Educ. 1999, 76, 543–547. 13. Gaddis, B. A.; Schoffstall, A. M. J. Chem. Educ. 2007, 84, 848–851. 14. Mohrig, J. R.; Hamond, C. N.; Colby, D. A. J. Chem. Educ. 2007, 84, 992–998. 15. Criteria for POGIL Laboratories. http://creegan.washcoll.edu/ gilabs/POGIL-lab_criteria.html (accessed Mar 2008).
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Jul/abs959.html Abstract and keywords Full text (PDF)
Links to cited URLs and JCE articles
Color figures
Supplement
Student handouts including prelab and discussion questions
Instructor notes
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 7 July 2008 • Journal of Chemical Education
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