Comparison of Traditional and Green Chemistry Methods for Extracting Essential Oils from Spices
One of my goals is for the students to be able to connect chemistry concepts to a larger context in which they consider sustainability in terms of environmental and social systems. This assignment connects aspects of green chemistry and environmental stewardship with some of the skills and theory involved in natural products chemistry and separation methods. Students obtain the essential oil from a spice, using steam distillation and then using liquid CO2 extraction. The oils are analyzed using IR (infrared spectroscopy) and GC-MS (gas chromatography, mass spectrometry). The two methods are compared for yield, purity, environmental impact and safety. A literature search is done to compare the methods in an industrial context and to obtain expected results. Student learning is assessed through a formal, journal-style report that includes experimental results and a discussion of the above comparisons.
Context for Use
Description and Teaching Materials
The AssignmentEach student chooses a spice from a selected set of spices. Readings are assigned from the lab textbook (PLKE) and handouts (see Appendix). Students complete a pre-lab assignment to be handed in before beginning the experiment. Half the class begins with macroscale steam distillation of their spice, followed by extraction with methylene chloride. Samples are prepared for IR and GC-MS. This usually takes one whole lab period. The other half of the class begins with liquid CO2 extraction of their spice. Again, samples are prepared for IR and GC-MS. Since this generally takes less than a whole lab period, this group of students can begin the spectroscopy. The student groups then switch methods for the second lab period. The third lab period is used for completing the spectroscopy. They are split into two groups to be most time efficient. The GC-MS runs take 10-15 minutes each, so can't be completed in one lab period. Having the students work in pairs could accommodate larger class sizes.
Students are assessed through a journal-style report that includes an abstract, introduction, materials and methods, results and discussion, conclusion and bibliography. Experimental results are presented and the two methods are compared by yield, composition, environmental impact and safety. The results are also compared to those reported in the literature in terms of these same categories. Students are required to cite at least three primary literature sources and sources are to be cited throughout the text. Handing in a rough draft for feedback is optional, but could be a required element. The reports are evaluated by section according to the guidelines given in the student handout, with a point value assigned to each section.
Student Handout 1 (Microsoft Word 2007 (.docx) 14kB Nov8 11)
Student Handout 2 (Microsoft Word 2007 (.docx) 1018kB Nov8 11)
Teaching Notes and Tips
Some spices work better than others, particularly with the liquid CO2 extraction. So far we have found that cloves, allspice and citrus peels work well. Caraway, cinnamon and fennel did not work so well. One issue is with yield; some spices have a higher essential oil content than others.
We have also had issues with spices going through the filter and ending up in the oil, requiring the student to dissolve the oil in solvent (usually methylene chloride), filter, and then evaporate the solvent. This reduces the integrity of the method. This has happened using the method given in the student handout for the liquid CO2 extraction, and with an alternate method (using a filter paper "cup"). We have had better success using a portion of a teabag (you can buy boxes of empty teabags), but still had some cases of spices going through. We are still searching for an optimized method.
If this is the first formal report, students will need extra guidance on how to organize the report and what to include. Students may also need help with finding appropriate literature sources. A rough draft is encouraged, and could be required. The students may also benefit from sending pieces of the report for feedback as they complete them.
A second assignment involving green chemistry can be given the second quarter of the lab sequence to further reinforce the learned concepts, connections and skills. In my class, the students carry out a solvent-free porphyrin synthesis and compare that method to more traditional methods for synthesizing porphyrins in a formal report.
References and Resources
Following is the major equipment for this experiment: GC-MS, IR, macroscale distillation apparatus, hotplates, separatory funnels, large plastic cylinders. Access to primary literature is needed to complete the report. The experiment could be done without IR since the GC-MS gives more detailed information about the composition of the essential oil. The IR is included for support of the GC-MS results and as an opportunity to practice IR interpretation. Although the experiment could be carried out without the GC-MS, detailed product analysis would not be possible, and so the depth and richness of the assignment would be reduced.
In Puget Sound, the Sound Citizen project at the Center for Urban Waters, University of Washington is a useful resource. http://depts.washington.edu/soundcit/