In the Classroom edited by
JCE DigiDemos: Tested Demonstrations
Ed Vitz
An Acid Hydrocarbon: A Chemical Paradox
Kutztown University Kutztown, PA 19530
submitted by:
Jeffrey T. Burke* Health and Science Division, Cumberland County College, Vineland, NJ 08362-1500;
[email protected] checked by:
Gary MacDonald Department of Chemistry, Kutztown University, Kutztown, PA 19530
Examples of chemical paradox have been previously addressed in this Journal (1–6). This article explores one more and investigates the use of paradox as a teaching and learning strategy. A simple demonstration is employed that, at least to college sophomores enrolled in organic chemistry, involves a paradox. The demonstration is conveniently performed in the laboratory, perhaps during a reflux period or some other “dead time” operation. The demonstration consists of introducing a sliver of sodium metal into a small vial of cyclopentadiene.1 Gas bubbles are observed streaming from the sodium metal. To many students, this seems reminiscent of the reaction of sodium metal and water so familiar from high school chemistry days, although not nearly as violent. Most students correctly conclude that the bubbles are hydrogen gas. This paradoxical reaction is best presented when the topic of aromaticity is studied in the classroom. Students are asked to think about what they have witnessed. Why would cyclopentadiene, a hydrocarbon, behave like water, a Brønsted acid? After all, sodium metal is preserved on the stock room shelf by storage under hydrocarbons. In light of this fact, it is indeed paradoxical, that cyclopentadiene, a hydrocarbon would behave like a Brønsted acid. What is so special about cyclopentadiene? The paradox is resolved, for most students, when the general theory of aromaticity, including the Hückel 4n + 2 rule is investigated in class. The topic of nonbenzenoid aromatics is addressed in most organic texts (7, 8). The paradoxical reaction in the vial is: H
Na
+
H H
H
H
H H
Naⴙ
+
ⴚ
H
H
H
+
1
2 H2
H
action. The aromatic nature of the cyclopentadienyl anion has been addressed elsewhere (7–9). The paradox is now resolved. A counterintuitive observation has provided an example by which a deeper and more general understanding of aromaticity, including nonbenzenoid aromatics is achieved. Needless to say, the whole issue of acidity and the measurement of organic acidity constants goes far beyond the release of hydrogen by metals. This complex topic has been addressed elsewhere (10). Procedure Cyclopentadiene is prepared in advance, by cracking its dimer, using an air-cooled, unpacked fractionating column. Cyclopentadiene is often used as the diene in Diels–Alder reactions. Therefore, detailed procedures for its preparation are found in many organic laboratory manuals. The demonstration is best carried out in a well-ventilated fume hood. Both cyclopentadiene and its dimer have objectionable odors. Two 20-mL glass scintillation vials are used. One vial is filled halfway with cyclopentadiene and the other with the dimer. A sliver of sodium metal is introduced into each vial. A reaction between sodium and cyclopentadiene is apparent as bubbles can be seen streaming from the surface of the sodium, while no bubbles are generated in the dimer. Although the bubbles are visible with the unaided eye, the use of a hand held magnifying glass will make observation easier and is highly recommended.2 About a dozen students can gather around the hood as the instructor introduces the sodium metal into the vials. Students can then move in closer, one at a time, to view the bubbles with the magnifying glass. The lack of bubbles from the dimer nicely underscores the importance of aromaticity as the driving force for the reaction between sodium metal and cyclopentadiene. Some skeptics may suggest that the bubbles of hydrogen are the result of “wet” cyclopentadiene. This plausible thought can be put to rest, however, by adding indicating Drierite before the introduction of sodium metal.3 Also as the sliver of sodium is consumed additional slivers can be added. In turn, they too bubble and at the same rate as the first. If the bubbles were the result of “wet” cyclopentadiene then they would eventually cease once any water had been consumed. Hazards
It is the relative thermodynamic stability of the aromatic cyclopentadienyl anion that provides the driving force for this rewww.JCE.DivCHED.org
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Although sodium metal, cyclopentadiene, its dimer, and the hydrogen gas generated are all flammable, the demonstra-
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tion presents only minimal risk if conducted on a semimicro scale in a fume hood. Nevertheless, material safety data sheets should be consulted prior to working with these items. Appropriate safety eyewear should be used. Unconsumed sodium metal can be destroyed with ethanol. Cyclopentadiene, its dimer, and the reaction mixture should all be disposed of in accord with local procedures regulating flammable materials. Acknowledgments The driving force for this work came as a direct result of the author attending the National Science Foundation Chautauqua Short Course—Paradox. Therefore, I wish to thank the NSF and especially the two Paradox instructors, Ralph Davis, Albion College, and Hans Christian von Bayer, College of William and Mary, for their leadership, enthusiasm, and expertise. Also, I am grateful to the anonymous reviewers for their helpful thoughts and suggestions. Notes 1. Cyclopentadiene cannot be purchased, but is instead prepared by cracking its dimer via distillation. Detailed instructions for the preparation of cyclopentadiene can be found in many Organic laboratory manuals. (See, for example, Hart, H.; Craine, L. Laboratory Manual Organic Chemistry A Short Course; Houghton Mifflin Co.: Boston, MA, 1991; pp 98–101.) The dimer, CAS number 77-73-6, can be purchased from Aldrich. On standing, cyclopentadiene slowly redimerizes. However, with refrigeration, it
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can be stored in reasonably pure form, for at least several days. 2. For larger audiences in a well-ventilated lecture hall, the scintillation vial may be covered with a glass plate and placed on an overhead projector. In the interest of safety, the use of more than a milliliter of cyclopentadiene and the dimer is not recommended. It is important to use slivers of sodium, as flat pieces will tend to migrate to the walls of the vial, making observation of the evolved hydrogen difficult. 3. Sometimes the cyclopentadiene obtained by distillation of the dimer is cloudy, indicating the presence of water. Allowing a cloudy distillate to stand over Indicating Drierite with intermittent swirling will remove the water.
Literature Cited 1. 2. 3. 4. 5. 6. 7.
Davidson, D. J. Chem. Educ. 1937, 14, 238–241. Feigl, F. J. Chem. Educ. 1944, 21, 347–349. Campbell, J. A. J. Chem. Educ. 1980, 57, 41–42 Chapple, F. H. J. Chem. Educ. 1998, 75, 342. Vitz, E. J. Chem. Educ. 2000, 77, 1011–1013. Bartell, L. S. J. Chem. Educ. 2001, 78, 1067–1069. Morrison, R. T.; Boyd, R. N. Organic Chemistry; Prentice Hall: Englewood Cliffs, NJ, 1992; pp 493–507. 8. Carey, F. A. Organic Chemistry; McGraw Hill: New York, 1992; pp 437–438. 9. Kasmai, H. S. J. Chem. Educ. 1999, 76, 830–834. 10. Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic Chemistry; Harper and Row, Publishers, Inc.: New York, 1976; pp 124–169.
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