Week 7: Identification of a Conjugated Diene from Eucalyptus Oil

This experiment was adapted by a group of 4 students from: Lehman, J. W., Lehman, J. W., Operational organic chemistry, & 3rd. (2004). Microscale operational organic chemistry : A problem-solving approach to the laboratory course. Upper Saddle River, N.J: Pearson Prentice Hall. This was an interesting find, I thought. I have done a couple of different Diels Alder experiments over the years: maleic anhydride and cyclopentadiene, as well as maleic anhydride and anthracene. I liked working with anthracene much more that cyclopentadiene.
2) Changes to published procedure
The amount of diene in the essential oil is very small. I ended up spiking the Eucalyptus oil with phellandrene from Aldrich. The experiment, as written, gives no indication of how much diene to expect from the essential oil. We did the experiment with 6 grams of essential oil and a great excess of maleic anhydride. Recrystallization was done with ethanol.
3) Positive
+The Diels-Alder reaction is one of those quintessential reactions that should be emphasized in the OChem curriculum.
+I liked the interface with natural products as well as the inquiry potential. The experiment presents four possible dienes in Eucalyptus oil. The essential oil is reacted with maleic anhydride and identification of the product by melting point reveals which diene was prevalent in Eucalyptus oil.
+Smells nice.
4) Neutral
±Not much new as far as technique here. The technical part comes with the methods of characterizing the product.
5) Negative
-The use of ether as a solvent is less-than-ideal here. We had a small fire in the lab when a student removed the round bottom from the reflux condenser.
-Melting point is a mundane way to characterize a product. GC or even IR are more appropriate here.
6) Inquiry & Extension ideas.
●This could possibly be done as a “solventless” reaction. The cineole in Eucalyptus oil could play the part of solvent.
●The inquiry aspect could be transferred to a “unknown” format. An unknown diene is reacted with maleic anhydride and the product characterized to reveal the structure of the diene.
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