Chemical Education Today
Letters Vanillin
Anthocyanins
In the article “Vanillin: Synthetic Flavoring from Spent Sulfite Liquor” (Hocking, M. B. J. Chem. Educ. 1997, 74, 1055-105), the structural formula of methyl salicylate in Figure 1 on page 1055 appears to be incorrect. According to the Merck Index, eighth edition, and the CRC Handbook of Chemistry and Physics, 57th edition, salicylic acid is 2hydroxybenzoic acid. The methyl salicylate shown in Figure 1 should have the OH group in the ortho- position, not the para- (4-) position. My attention to the structural formula of methyl salicylate was triggered by the media attention given to the 100th anniversary of the synthesis of aspirin. For an example, see Marc Reisch’s article (Reisch, M. Aspirin is 100 years old. Chem. Eng. News 1997, 75(33), 12). Myths about aspirin being a natural product should be dispelled. One myth was that aspirin was discovered in extracts of willow bark. The chemical extracted was salicin which was easily converted to salicylic acid (Ed. note: see p 1234). James Burke, the TV “Connections” host stated another myth that aspirin was first isolated from the plant “Spirea ulmania”. The “a” of acetyl and the “spir” of Spirea were put together to invent the name aspirin for the newly synthesized acetylsalicylic acid. Salicylic acid was first isolated from Spirea. Likely, the substances in the plant were esters of salicylic acid. A worldwide demand for salicylic acid and its sodium salt to treat febrile conditions and rheumatism developed even though Reisch stated they “tasted awful” and “attacked the mouth and stomach linings”.
I enjoyed reading the JCE Classroom Activities # 2 experiment that examined anthocyanins in the October 1997 issue ( J. Chem. Educ. 1997, 74, 1176A–1176B). I applaud J. Chem. Educ. on the use of Classroom Activities that show students and teachers how complex principles of biological pigments and photosynthesis can be easily demonstrated in classrooms. I did, however, notice a misconception on page 1176B of that issue. The activity states that anthocyanins are found in beets as well as raspberries. This is not true. Although anthocyanins such as cyanin-glucoside are found in raspberries, blackberries, cabbages, cherries, passion fruit, apple skins, and hibiscus flowers (as well as many other plants), the red color of the beet root, as well as bougainvillea flowers, is due to nitrogen containing molecules called betacyanins. The chemical structures of these two types of molecules are quite different. Anthocyanins are the more stable of the two molecules and contain three linked six membered rings. Betacyanins contain one benzene ring connected to a nitrogen-containing five membered ring, which itself is connected to a nitrogen-containing six membered ring. In an experiment described by Harborne ( J. B. Harborne, Phytochemical Methods, A Guide to Modern Techniques of Plant Analysis, Chapman and Hall, New York, 1984), students can easily extract the betacyanins called betanidin and betanin from beet root and compare them to anthocyanins extracted from flowers and berries. A small amount of these plant materials are crushed in water using a mortar and pestle. Students can then differentiate these two different plant pigments using some simple chemical tests. For example, upon heating in acid for five minutes, the red color of the anthocyanins remains stable, while the red color from beet betacyanins vanishes. Upon addition of base to a fresh sample of the pigment-containing liquids, red anthocyanins turn blue-green, and red betacyanins change to yellow. This experiment can be used to illustrate the principles of biological extraction and the identification of unknown compounds that initially look similar to the eye. The stability and availability of anthocyanins has recently allowed them to be used as the pigment in a simple solar cell (see N. Cherepy et al. “Ultrafast Electron Injection: Implications for a Photoelectrochemical Cell Utilizing an Anthocyanin Dye Sensitized TiO2 Nanocrystalline Electrode”, J. Phys. Chem. B, 1997, 101, 9342–9351, G. Smestad, M. Grätzel, J. Chem. Educ. 1998, 75, 752–756). I am glad to see that these wonderful pigments have found uses in education.
Dean Calloway 27 Ridgewood Drive Columbus, MS 39701
The author replies: Dean Calloway is right. Methyl salicylate is methyl 2hydroxybenzoate, not the 4- (or para-) hydroxybenzoate shown in the article. A brief erratum was published in the December issue (J. Chem. Educ. 1997, 74, 1384). This further note should help ensure that few readers miss this correction. Calloway’s anecdote adds some interesting background about these already intriguing materials. Other literature sources state that salicylaldehyde (o-hydroxybenzaldehyde) is found in the volatile oils extracted from several species of Spirea. This would easily oxidize to salicylic acid in the presence of air. While the active analgesic agent is known to be salicylic acid, the “…awful” taste and the “attack [of ] the mouth and stomach linings” are both lessened by acetylation or methylation of the carboxyl group, which strongly lowers the acid strength. Thus both acetyl salicylic acid and methyl salicylate taste “better” and pass through the acidic medium of the stomach with relatively little change. They are hydrolyzed in the more alkaline intestinal tract where the active agent is absorbed into the bloodstream. The strongly acidic character of salicylic acid itself is used medically for the removal of warts and the reduction of callouses. Martin Hocking Department of Chemistry University of Victoria Victoria, B. C. V8W 3V6 Canada
Greg Smestad Solar Energy Materials and Solar Cells Pacific Grove, CA 93950 http://www.elsevier.nl/locate/solmat email:
[email protected] .
JChemEd.chem.wisc.edu • Vol. 75 No. 10 October 1998 • Journal of Chemical Education
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