In the Laboratory
Pyrolysis of Aryl Sulfonate Esters in the Absence of Solvent: E1 or E2? A Puzzle for the Organic Laboratory John J. Nash,* Marnie A. Leininger, and Kurt Keyes Department of Chemistry, Purdue University, West Lafayette, IN 47907; *
[email protected]
In recent years, there has been considerable interest in discovery-based (1) and puzzle-solving experiments (2) for the undergraduate organic laboratory (3, 4). These two approaches have some similarities, yet they can be quite different. In both types of experiments, students must draw conclusions and construct meaning for an open-ended question by collecting and analyzing data. A well-designed discovery-based experiment will require that the students make their own decisions about what experiments to do, how to carry out the experiments, and how to interpret the data. A puzzle-solving experiment, on the other hand, tends to have greater structure. For example, a question (i.e., puzzle) is presented, but students are provided with the experimental details describing how to collect the relevant data. Students then analyze and interpret the data to solve the puzzle. H
R2 R1
R4 R3
O O
S
OH O
O
%
R1
R3
2
4
O
á R
R
X aryl sulfonate ester
S
X aryl sulfonic acid
alkene
Scheme I. Thermal decomposition of an aryl sulfonate ester.
H
O
O
H3C N
S
H
O
CH3
H O H
menthyl N-acetylsulfanilate
E2
CH3
E1
CH3 á
CH3 á ...
Scheme II. Expected products for the two elimination pathways.
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Here, a new puzzle-solving experiment for the undergraduate organic laboratory is described. The puzzle is whether the thermal decomposition (i.e., pyrolysis) of an aryl sulfonate ester proceeds via an E1 or E2 elimination pathway (Scheme I). Thermal decomposition of aryl sulfonate esters of this type is thought to occur via an E1 mechanism in which the sulfonate ester C−O bond is initially broken to produce a carbocation (5). Subsequent proton abstraction by a base leads to the formation of an alkene. In addition, such aryl sulfonate esters containing a basic substituent (on the aromatic ring) decompose at relatively low temperatures, in the absence of solvent, to produce alkenes cleanly and in high yield (5). The aryl sulfonate ester that the students synthesize in this experiment, menthyl N-acetylsulfanilate, was chosen because it has only one (β) hydrogen atom anti-periplanar to the C−O bond (Scheme II). Thus, pyrolysis of menthyl N-acetylsulfanilate should only produce one alkene product if the elimination proceeds via an E2 mechanism.1 If the elimination proceeds via an E1 mechanism, then at least two alkene products would be expected (rearrangement of the carbocation could potentially lead to additional products). There are several other interesting aspects of this experiment. First, the pyrolysis of the (solid) aryl sulfonate ester is performed “neat” (i.e., no solvent is used). Solventless elimination reactions are unlikely to be encountered in most organic chemistry courses, and this adds a green chemistry component to the experiment. Second, whether the elimination occurs via either an E1 or E2 mechanism, the alkene(s) can only be formed via abstraction of a proton by a base. Since the pyrolysis is performed “neat”, it will probably not be immediately obvious to the students what substance functions as the base. The oxygen atom of the acetamido group on the aromatic ring is, in fact, sufficiently basic to abstract a proton. This can be rationalized by considering the zwitterionic form of the aryl sulfonate ester (Figure 1). Note that the geometry of the aryl sulfonate ester makes it virtually impossible for the oxygen atom of the acetamido group to abstract the proton intramolecularly. During the pyrolysis, it is more likely that the oxygen atom of one aryl sulfonate ester molecule abstracts a proton from another aryl sulfonate ester molecule, that is, an intermolecular proton abstraction.
ź
O CH3
H O
á
N
S
H
O
CH3
H O H
Figure 1. The zwitterionic form of the aryl sulfonate ester.
Journal of Chemical Education • Vol. 85 No. 4 April 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
Experimental Procedure The experiment is performed during two, three-hour lab periods. During the first lab period, the aryl sulfonate ester is synthesized and purified. During the second lab period, the aryl sulfonate ester is pyrolyzed, and the thermal decomposition products (i.e., the “pyrolysate”) are analyzed by using GC. Menthol (1.0 g) is dissolved in anhydrous pyridine2 (7 mL) in a dry round-bottom flask. A pre-weighed3 sample of Nacetylsulfanilyl chloride (ca. 1.65 g) is added, with magnetic stirring, over about 1 min. The flask is stoppered,4 and the mixture stirred at room temperature for 45 min. The reaction mixture is poured onto about 30 g of ice in a small beaker and stirred using a glass stirring rod until crystallization of the aryl sulfonate ester is complete and all of the ice has melted. The crude aryl sulfonate ester is isolated using vacuum filtration, recrystallized from ethanol/water and then allowed to dry in the lab drawer until the following week to give 1.81 g (80%) of a white solid (mp 86–88 °C). The aryl sulfonate ester (1.0 g) is placed in a round-bottom flask,5 which is then attached to a vacuum (water aspirator) distillation apparatus.6 The aryl sulfonate ester is heated,7 under vacuum, and the pyrolysate is collected.8 A small quantity (ca. 1 μL) of the (neat) pyrolysate is then analyzed by using GC. Hazards Menthol, pyridine, N-acetylsulfanilyl chloride, and ethanol may cause eye, skin, respiratory tract, and digestive tract irritation. Pyridine is flammable and has a strong stench. N-acetylsulfanilyl chloride is corrosive and moisture-sensitive. Ethanol is flammable. Menthyl N-acetylsulfanilate has not been reported in the literature; thus, toxicity data for this compound is not available. All chemicals should be dispensed, and used, in a fume hood. Discussion The students generally obtain good yields of the aryl sulfonate ester. The pyrolysis does not produce a large quantity of pyrolysate, but it is more than sufficient for a GC analysis. The gas chromatogram clearly shows that the pyrolysate contains several compounds (rather than one) and, as a result, the students have little trouble concluding that the reaction mechanism is E1. Finally, as part of their laboratory report, students are asked to write the mechanism for the reaction and predict the structures for the various products. While all of our students synthesize and pyrolyze the same aryl sulfonate ester, it should certainly be possible to modify the experiment so that different groups of students study different aryl sulfonate esters. Synthesis of the aryl sulfonate esters is relatively simple, and the synthetic procedure should not require extensive modification in order to use different alcohols. In this way, students could compare their results for different aryl sulfonate esters to determine the reaction mechanism. For example, pyrolysis of cyclohexyl N-acetylsulfanilate (i.e., replacing menthol with cyclohexanol in the synthesis) would be expected to form only one alkene product (cyclohexene) whether the mechanism were either E1 or E2. Thus, the results for this particular aryl sulfonate ester would be inconclusive, and additional data would be needed to determine the reaction mechanism.
Another possible modification might be to have the students analyze the pyrolysate by using GC–MS. GC–MS analysis of the pyrolysate has shown that this mixture contains at least nine different isomeric compounds (clearly, the carbocation does rearrange)! A comparison of the mass spectra for the isomers with a mass spectral library has shown that the two major products (comprising ca. 85% of the pyrolysate) are the expected alkene products based on an E1 mechanism. Even though the other isomers are formed in much smaller quantities, a GC–MS analysis could provide an interesting opportunity for students to see the diverse, and sometimes unexpected, behavior of carbocation intermediates. Acknowledgments We thank A. Rothwell for providing the GC–MS analysis of the pyrolysate, and M. Lipton, H. Morrison, and W. Robinson for helpful discussions. Finally, the comments from the Chemistry 255L graduate instructors were also instrumental in the development of this experiment. Notes 1. Another possibility might be that the reaction proceeds via a concerted, syn elimination. However, this mechanism has previously been ruled out for pyrolyses of aryl sulfonate esters similar to the one described here (5). 2. This is added to the student’s flask via syringe by the laboratory instructor. 3. Because N-acetylsulfanilyl chloride is quite moisture-sensitive, samples are pre-weighed under nitrogen (using a dry box) and provided to the students. Samples can also be prepared by using a (nitrogenfilled) glove bag. 4. The flask is stoppered loosely. There is little build-up of pressure in the flask during the reaction: the stopper is simply used to limit the quantity of moisture entering the flask. A drying tube can also be used. 5. The solid aryl sulfonate ester is pyrolyzed “neat”, that is, no solvent is added. 6. Because the students only have fairly large glassware available to them, a 50 mL round-bottom flask is used as the “pot” flask, and a 25 mL round-bottom flask is used as the “receiving” flask (these are connected to each other using a three-way adapter and a vacuum adapter). However, it should be possible to perform the pyrolysis equally well using microscale glassware. 7. The quantity of pyrolysate produced is rather low. We have found a heat gun very helpful for “driving” the product over into the receiving flask. 8. As the solid is heated, it will first melt. As the aryl sulfonate ester decomposes (to produce the aryl sulfonic acid), the liquid will return to solid.
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Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Apr/abs552.html Abstract and keywords Full text (PDF) with links to cited JCE articles Supplement Student handouts
Instructor notes
Journal of Chemical Education • Vol. 85 No. 4 April 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
