In the Laboratory
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The Oxidation of Alkylbenzenes: Using Data Pooling in the Organic Laboratory to Illustrate Research in Organic Chemistry James C. Adrian Jr. and Leslie A. Hull* Department of Chemistry, Union College, Schenectady, NY 12308; *
[email protected] Basic organic chemistry textbooks present many synthetic reactions to students, most often without any reference to the original literature and the experimental results that support the generality of the reaction. More importantly, there is little in the textbooks to give the students a sense of how to structure a scientific inquiry to support the textbook assertion of the utility of the reaction. This is all right as long as the laboratory associated with the organic course is structured so that students are exposed to the experimental underpinnings of organic chemistry and come to understand through the laboratory how the science of organic chemistry is done. Unfortunately, in a typical experiment, the whole laboratory class performs the same reaction on the same substrate, illustrating only a single example of the reaction. None of us as professional chemists would accept data from a single reaction as a general result and permit publication of such an assertion. There have been descriptions of multi-period individual projects as a way of introducing research into the sophomore-level organic chemistry laboratory but these can take up significant portions of the laboratory and don’t provide a general approach (1). Description We have tried to remedy this problem by structuring several of our experiments in such a way that the students, by pooling their individual results, get a sense of how organic chemists design research projects. Oxidation of alkylbenzenes to benzoic acids by basic potassium permanganate solution is the experiment we chose to illustrate the design method for a research project to demonstrate the scope and generality of a reaction (2) (see scheme below).
R
Alklybenzene
Product Acid mp/°C
2-Chlorotoluene
138–140
3-Chlorotoluene
155–158
4-Chlorotoluene
239–241
3-Bromotoluene
155–158
4-Bromotoluene
252–254
4-Bromoethylbenzene
252–254
In the introduction to the exercise in our self-published lab text we outline the oxidation reaction of alkylbenzenes and ask the students to determine whether the product will be the corresponding substituted benzyl alcohol, benzaldehyde, or benzoic acid. Each group of two students in a laboratory class is assigned one of the alkylbenzenes listed in Table 1. Before performing the experiment the students must find the physical data (boiling points or melting points) for each of the derivatives that could be produced from their assigned alkylbenzene. In the prelab lecture we discuss the kinds of questions that can help to determine the scope and generality of the oxidation reaction. For example, What happens when your individual alkylbenzene is oxidized? Does the result of the reaction depend on the presence of other substituents on the ring? Does the result of the reaction depend on the position of the substituent? Does the result of the reaction depend on the alkyl chain length?
a. KMnO4/NaOH (aq)
CO2H
b. HCl
X
Table 1. Melting Points of Product Acids from Oxidation of Alkylbenzenes
X X = Cl, Br R = CH3, CH3CH2
The laboratory exercise also illustrates some important experimental techniques (reflux with stirring, filtration with the use of a filter aid, and others). We chose this reaction because it was not discussed in the students’ text and therefore the results of the reaction would not be immediately apparent; the text did discuss the potassium dichromate oxidation of alkylbenzenes in acid solution (3). Depending on the course text, one or another of the two oxidations can be used if only one is discussed (4 ). If both are discussed, it is also possible to arrange to do the oxidation before covering it in the text to preserve the discovery aspect of the experiment.
After performing the reaction and isolating product (all solids), the groups report their results to the class: the mp and the identity of the product. After all the groups have reported their experimental results, the class as a whole analyzes the data and discusses questions posed in the introduction to the exercise. Quickly the students report the following observations. A carboxylic acid is produced in every case. The presence of different substituents in the similarly substituted chloro and bromo derivatives produces the same result. The position of the substituent does not influence the product of the reaction. Chain length does not change the result, since the 4-bromotoluene gives the same product as the 4-bromoethylbenzene.
JChemEd.chem.wisc.edu • Vol. 78 No. 4 April 2001 • Journal of Chemical Education
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In the Laboratory
This last result (on alkyl chain length) turns out to be a surprise to the students. Virtually all the groups assigned the 4-bromoethylbenzene come into the class with physical data on the products resulting from oxidation of the methyl group of the ethyl chain. Thus, for example, the groups assigned the 4-bromoethylbenzene bring the mp for 4-bromophenylacetic acid (117–119 °C), instead of that for 4-bromobenzoic acid. They are quite puzzled when, on isolating their products, they get a mp of about 250 °C. Because of the way we have structured the in-class data table (the 4-bromotoluene result is adjacent to that of the 4-bromoethylbenzene result), it is possible to lead them quickly to a conclusion about what has occurred. Clearly we don’t have a very wide variety of substituents and need to qualify our conclusions about the general scope of the reaction. Nevertheless, the students get a sense of how to design a research project to test the generality of a specific reaction. They also get a sense of how the reactions reported in textbooks are derived from published experimental results. Hazards Potassium permanganate is a strong oxidizing agent. When handling it and its solutions, gloves should be worn.
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Halotoluenes and haloethylbenzenes are generally considered to be respiratory irritants and appropriate precautions should be taken when handling them to avoid prolonged exposure. W
Supplemental Material
Notes for the instructor and instructions for the students are available in this issue of JCE Online. Literature Cited 1. For example see the following. Davis, D. S.; Hargrove, R. J.; Hugdahl, J. D. J. Chem. Educ. 1999, 76, 1127. Ruttledge, T. R. J. Chem. Educ. 1998, 75, 1575. Kharas, G. B. J. Chem. Educ. 1997, 74, 829. 2. Organic Syntheses, Coll. Vol. I, 2nd ed.; Gilman, H.; Blatt, A., Eds.; Wiley: New York, 1941; pp 159, 392. 3. Carey, F. A. Organic Chemistry, 3rd ed.; McGraw Hill: New York, 1996; p 438. 4. For typical laboratory procedures see: Roberts, R. M.; Gilbert, J. C.; Martin, S. F. Experimental Organic Chemistry, A Miniscale Approach; Saunders College Publishing/Harcourt Brace College Publishers, New York, 1994; p 496. Nimitz, J. S. Experiments in Organic Chemistry, From Microscale to Macroscale; Prentice Hall: Englewood Cliffs, NJ, 1991; p 222.
Journal of Chemical Education • Vol. 78 No. 4 April 2001 • JChemEd.chem.wisc.edu
