Stereoisomerism and Smell

The human senses of taste and smell, while poor compared to many other species of the animal kingdom, are still extremely sensitive. The tongue and nose can discern between thousands of tastes and scents, respectively, and the compounds in foods that are responsible for these effects are likewise plentiful. What one might not expect is that what might be perceived as an almost negligible change in chemical structure in a compound can drastically alter the smell it produces. Consider the two plants below:

Figure \(\PageIndex{1}\): Left, spearmint. Right, caraway fruits.

Spearmint has an easily identifiable aroma; caraway may not be as familiar, but exhibits a spicier scent and is often added rye breads and sauerkraut. One does not immediately find a similarity between these two plants, but surprisingly, the oils found in spearmint and caraway fruits are both abundant in the chemical carvone.

Why, then, does the same compound cause such a different smell in each plant? We must establish that the carvone in spearmint cannot chemically be the same as the carvone in caraway, or this discrepancy would not be observed. The difference lies in the stereochemistry of carvone in each plant. Carvone has two possible enantiomers, stereomers that are non-superimposable mirror images. The following image illustrates the two enantiomers:

Figure \(\PageIndex{2}\): R and S enantiomers of carvone.

One enantiomer is designated (R); the other is (S), but we will not get into the method of nomenclature at this point^*. What is important is to see that the mirror images are easily shown to be non-superimposable; there is no possible way to rotate or flip the (S) enantiomer to fit perfectly with the (R) enantiomer. We can then say that carvone is a chiral molecule; its enantiomers have different physical and chemical properties.

*For those who want an explanation, Wikipedia is your friend. Alternatively, a first semester organic chemistry class will provide a better source.

The chirality of carvone is directly translated into a discrepancy in smell because several olfactory receptors in your nose are chiral and will register certain enantiomers more strongly than others. Thus, (R) carvone smells like spearmint and (S) carvone smells like caraway.

Enantiomers are not so easily distinguished in all cases of chirality; we may have neither a difference in odor nor a chemical projection to help us. One method of separating enantiomers by visual observation is to observe the polarization of light each exhibits. One enantiomer will rotate light counterclockwise, and the other will rotate light clockwise.

From ChemPRIME: 8.9: Unsaturated Hydrocarbons

Contributors and Attributions

• Ed Vitz (Kutztown University), John W. Moore (UW-Madison), Justin Shorb (Hope College), Xavier Prat-Resina (University of Minnesota Rochester), Tim Wendorff, and Adam Hahn.