In the Laboratory
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A Simple Combinatorial Experiment Based on Fischer Esterification An Experiment Suitable for the First-Semester Organic Chemistry Lab Peter A. Wade,* Susan A. Rutkowsky, and Daniel B. King Department of Chemistry, Drexel University, Philadelphia, PA 19104; *
[email protected] duced is required. Gung and Taylor’s recent parallel synthesis of azo dyes is similar in this respect (3). On the other hand, Wolkenberg and Su have published a combinatorial experiment involving synthesis of hydrazones from aldehydes and hydrazines (4). In their experiment, nine hydrazones are prepared as six mixtures by the students and are screened for antibiotic activity against E. coli. It is necessary for the instructor to prepare a culture of E. coli and the student samples need to incubate for a 24 hour period after the experiment. These features were a significant stumbling block to introduction of the experiment in our curriculum. There are a number of additional combinatorial experiments that have been published (5–9). Textbooks on combinatorial synthesis are also available (10).
A few years ago, we incorporated a new experiment into the sophomore organic laboratory that introduced students to the concepts, benefits, and limitations of combinatorial synthesis. Many of our undergraduate students are biology students who should be exposed to combinatorial synthesis, a common practice in the pharmaceutical industry. Our requirements for the lab were twofold: that the experiment be truly combinatorial (i.e., would produce more compounds for testing than the number of experiments conducted) and that the experiment be as operationally simple as possible. While there were a number of published combinatorial experiments available, none fully met our needs. Two published experiments, however, proved highly useful in developing a suitable experiment meeting our two criteria. A third recently published combinatorial experiment has also been noted as well as additional published experiments. Birney and Starnes have published a parallel combinatorial experiment involving Fischer esterification (1, 2). In their experiment, approximately 24 students are given an alcohol and an acid from which they individually prepare 24 esters. One of these esters is methyl salicylate and has the characteristic odor of wintergreen. The students screen each other’s esters by smell and identify the one that smells like wintergreen. Single pure esters are prepared and the same number of experiments as the number of compounds pro-
Combinatorial Synthesis of Esters We applied the observations of Birney and Starnes to a combinatorial experiment involving mixtures of esters. Initially, we were apprehensive about the ability to detect methyl salicylate in the presence of several other esters. Odor panels and olfactometers are often used in complex cases involving smells (11), and the question of whether students would be readily able to detect wintergreen in the presence of a number of other pleasant odors was not trivial. How-
Table 1. Carboxylic Acids and Alcohols with All Possible Esters That Can Be Formed by Esterification CH3OH methanol
Reactant COOH
CO2CH2CH2CH3
CO2CH3
benzoic acid
methyl benzoate
COOH
n-propyl benzoate CO2CH2CH2CH3
CO2CH3
OH salicylic acid
OH n-propyl salicylate
OH methyl salicylate
COOH
CO2CH2CH2CH3
CO2CH3
cinnamic acid
methyl cinnamate
COOH
n-propyl cinnamate CO2CH2CH2CH3
CO2CH3
CH3 o-toluic acid
www.JCE.DivCHED.org
CH3CH2CH2OH 1-propanol
CH3
CH3 methyl o-toluate
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In the Laboratory
ever, these apprehensions proved largely unfounded: the odor of wintergreen is distinct even in the four-component ester mixtures described here. None of the esters produced are reported to be toxic, so cautious smelling by students is acceptable practice. The starting materials are all inexpensive, with cinnamic acid costing the most ($18.70兾100 g from Aldrich). Eight esters are prepared and “tested” as components of six mixtures. In this experiment, 12 students are divided into two groups of six each. Each group member draws a number from 1 to 6 and is assigned a reaction to carry out based on that number. A list of the alcohols and acids used and the resulting esters produced is shown in Table 1. The students are issued acids and alcohols, identified only as CA1–CA4 (for the four acids) and AA or AB (for the two alcohols). They must then carry out the six reactions listed in Table 2. Table 3 shows how the data can be deconvoluted to correctly identify salicylic acid and methanol, alcohol AA and acid CA2 in the example given. The students are able to identify methyl salicylate by its characteristic odor as a reaction product in two of the mixtures produced. The acids used are close enough in molecular weight that equivalent weights can be used while equivalent volumes of the alcohols are employed. The four aromatic acids—benzoic, o-toluic, cinnamic, and salicylic acid—are all solids making it difficult to determine which is salicylic acid until after the experiment is over and the odor of wintergreen is detected. Students are easily able to screen the mixtures by smell (several hundred have now performed the experiment). In all cases, mixtures containing methyl salicylate were correctly identified. The odor of the four-component mixture containing propyl esters was sufficiently complex that in about 10% of cases, students were not certain that methyl salicylate was absent. It was therefore desirable to have confirmation of the esters present by another method. Accordingly, the mixtures were screened by GC analysis. All mixtures gave GC traces consistent with the number of components present (either two or four) and from the retention times, the GC traces added support to the odor results. Methyl salicylate was clearly indicated in the two mixtures containing it by comparison of retention time with a standard sample. The ratio of esters in the mixtures was also readily determined by GC analysis. Initially, propanoic acid instead of o-toluic acid was employed for the experiment. However, students sometimes “lost” the volatile propanoate esters during concentration of the mixtures so the use of propanoic acid was discontinued. Fischer esterification is an equilibrium process and this adds another dimension to the experiment. It is possible to calculate the theoretical molar ratio of methyl esters present in the four-component mixture assuming a common equilibrium constant Keq for all reactions (28:25:24:23 a small correction from 25:25:25:25 since the same weights, not moles, were used for the starting acids). However, Keq differs for reaction of methanol with each acid: in particular, Keq for reaction of salicylic acid is smaller than Keq for reaction with any of the other three acids. This results in lower quantities of methyl salicylate in the four-component mixture: integration gave a ratio of 29:29:11:31 for methyl benzoate, methyl o-toluate, methyl salicylate, and methyl cinnamate, respectively. The four-component propyl esters were similar: here, too, propyl salicylate was the least prevalent ester. 928
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Table 2. Reactions Performed by Each Group Member Student 1
Reaction CA1 with AA and AB (2 esters form)
2
CA2 with AA and AB (2 esters form)
3
CA3 with AA and AB (2 esters form)
4
CA4 with AA and AB (2 esters form)
5
CA1, CA2, CA3, and CA4 with AA (4 esters form)
6
CA1, CA2, CA3, and CA4 with AB (4 esters form)
Table 3. Example of Mixture Deconvolution AA ↓
AB ↓
CA1 →
CA1–AA
CA1–AB
CA2 →
CA2–AA
CA2–AB
CA3 →
CA3–AA
CA3–AB
CA4 →
CA4–AA
CA4–AB
Hazards Sulfuric acid is highly corrosive and should be handled with care. Acknowledgment We thank Anthony W. Wambsgans of Drexel University for helpful discussions. WSupplemental
Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online. Literature Cited 1. Birney, D. M.; Starnes, S. D. J. Chem. Educ. 1999, 76, 1560– 1561. 2. See also: Whitlock, C. R.; Bishop, P. A. Chem. Educ. 2003, 8, 352. 3. Gung, B. W.; Taylor, R. T. J. Chem. Educ. 2004, 81, 1630– 1632. 4. Wolkenberg, S. E.; Su, A. I. J. Chem. Educ. 2001, 78, 784–785. 5. Truran, G. A.; Aiken, K. S.; Fleming, T. R.; Webb, P. J.; Markgraf, J. H. J. Chem. Educ. 2002, 79, 85. 6. Duarte, R.; Nielsen, J. T.; Dragojlovic, V. J. Chem. Educ. 2004, 81, 1010. 7. Kittredge, K. W.; Marine, S. S.; Taylor, R. T. J. Chem. Educ. 2004, 81, 1494. 8. Miles, W. H.; Gelato, K. A.; Pompizzi, K. M.; Scarbinsky, A. M.; Albrecht, B. K.; Reynolds, E. R. J. Chem. Educ. 2001, 78, 540. 9. Hinks, J.; Swali, V. Educ. Chem. 2002, 39, 50. 10. (a) Weber, P. Combinatorial Strategies in Biology and Chemistry; John Wiley & Sons: Hoboken, NJ, 2002. (b) Wilson, S.; Czarnik, A. W. Combinatorial Chemistry. Synthesis and Application; Wiley: New York, 1997. (c) Terrett, N. Combinatorial Chemistry; Oxford University Press: Oxford, 1998. 11. Vuilleumier, C.; Cayeux, I.; Velazco, M. I. In ACS Symposium Series; American Chemical Society: Washington DC, 2002; Vol. 825, p 140.
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