It is important that students realize that the aroma of a piece of fruit arises from a complex mixture of substances, but that the molecules shown here are, in general, the predominant species in a given mixture. In a previous column we considered molecules for which the odor depended on the enantiomer that was being sampled (2). In the case of this month’s molecules there are no optically active species, so the differences in aroma must arise from other structural factors and interactions that do not depend on chirality. For students interested in the theory of odor, good starting points are the 2004 Nobel lectures by Linda Buck (3) and Richard Axel (4), and Luca Turin’s book The Secret of Scent: Adventures in Perfume and the Science of Smell (5).

Esters are not only pervasive in the food and fragrance industries, they are also found in many cosmetics where their primary purpose may have nothing to do with aroma. An interesting project for students in courses that deal with the intersection of chemistry, health, and society would be to look at the controversy surrounding the use of so-called parabens—esters of para-hydroxybenzoic acid—in many cosmetics, predominately as preservatives because of their antifungal properties (methyl paraben is included in this month’s molecules). This controversy is interesting not only for the scientific questions it raises, but also because it has largely been conducted through Internet and email communication. Such an exercise would be a good way of getting students to think critically about the questions that one should ask before adopting an extreme position, either advocating a ban on the use of parabens or blindly assuming that parabens must be safe because they have been used for a long time. Students could begin by reading the 2004 paper that reported the presence of parabens in breast tumors, and then developing, and researching a list of questions that will inform their decision (6). This may lead students in many different directions, including those outside of the range of a typical chemistry course, and this, I would argue, is a very good thing. Some of the questions may, as yet, have no answer. For example, as of this date there appears to be no report on the level of parabens in normal breast tissue, and therefore the statistical questions that students might raise may not be answerable at this time. They may find themselves faced with questions of economics and risk–benefit analysis, as well.

Literature Cited

1. Valentín, E. M.; Montes, I.; Adam, W. J. Chem. Educ. 2009, 86, 1315–1318.
2. Coleman, W. F. J. Chem. Educ. 2007, 84, 2018 (accessed Sep 2009).
3. Linda B. Buck, The Nobel Prize in Physiology or Medicine 2004; Nobel Lecture on Unraveling the Sense of Smell (accessed Sep 2009).
4. Richard Axel, The Nobel Prize in Physiology or Medicine 2004; Nobel Lecture on Scents and Sensibility: A Molecular Logic of Olfactory Perception (accessed Sep 2009).
5. Turin, Luca. The Secret of Scent: Adventures in Perfume and the Science of Smell; Harper Perennial: New York, 2007.
6. Darbre, P. D.; Aljarrah, A.; Miller, W. R.; Coldham, N. G.; Sauer, M. J.; Pope, G. S. J. Applied Toxicology 2004, 24, 5–13.

The molecules added to the collection this month include:

benzyl acetate (peach)
n-butyl acetate (pear)
ethyl acetate (nail polish remover)
3-methylbutyl acetate (banana)
n-octyl acetate (orange)
benzyl butyrate (flowers/jasmine)
ethyl butyrate (pineapple)
methyl butyrate (apple)
ethyl phenylacetate (floral scent)
ethyl propionate (rum)
isobutyl formate (raspberry)
methyl anthranilate (grape)
methyl salicylate (wintergreen)
methyl paraben