In short
Some molecules exist in two mirror-image forms, called enantiomers. They have the same formula and boiling point, yet they can smell different because smell receptors are chiral themselves. The carvones are the famous example, spearmint and caraway. Many other pairs differ less, or not at all, for most people, so never assume an effect either way.
What you will learn
- Explain chirality and what enantiomers are
- Interpret R, S, d and l on a label with caution
- Say why grade and isomer ratio matter when buying
Before this lesson: Structure-Odor Relationships
What chirality means
Your hands are mirror images, and you cannot superimpose them. A carbon with four different groups attached creates the same situation in a molecule. That carbon is a stereocenter, and it is usually marked in a drawing with a wedge or a dash. A molecule with one such center has two enantiomers.
Enantiomers have the same boiling point and vapor pressure, and they behave the same in most non-chiral situations. They differ only in interactions with other chiral things, including polarized light, some chromatography columns and receptor proteins.
Reading the labels
| Label | Meaning | Caution |
|---|---|---|
| R, S | Spatial arrangement, by a naming rule | Does not tell you the smell |
| d or (+), l or (−) | Direction a sample rotates polarized light | Not the same as R or S |
| Racemic, (±) | About equal enantiomers | Common for synthetics |
| cis, trans, E, Z | Geometry about a double bond or ring | These are diastereomers, with real physical differences |
A material can also be unspecified. The encyclopedia lists both limonene and limonene, unspecified because a CAS number may refer to one form or to a mixture. Read what you are buying.
What the evidence shows
- Carvones: Friedman and Miller's 1971 paper established that R-(−)-carvone and S-(+)-carvone are the constituents responsible for spearmint and caraway, and that they differ in odor.
- Many others are subtle. Laska (2004) tested 20 people on nine enantiomer pairs and found that as a group they could reliably discriminate five of them, including limonene and carvone, with marked differences between individuals. Differences are not universal.
- Impurities can mislead. A reported difference may reflect a trace impurity, especially in older work. Modern studies try to control for that.
- Strength can differ more than character. In some families, one enantiomer is reported to be much more powerful than the other.
Why it matters for a formula
Grades. A material may be sold with a higher share of one isomer. Hedione HC, for instance, is a high-cis grade of methyl dihydrojasmonate and it is sold as a distinct material from standard Hedione. The Lab's Hedione guide says more. Two materials that share a family name may therefore perform differently.
Substitution. If you change suppliers and the isomer ratio shifts, the smell can shift too, even with a matching CAS number. Ask for specifications.
Analysis. Standard GC columns do not separate enantiomers. A chiral column is needed for that. So an ordinary GC-MS report will not tell you which enantiomer you have. Level 7's GC-MS guide and the analytical track return to this.
Natural origin. Natural materials often have a characteristic enantiomer ratio. That is one way analysts investigate adulteration, as the analytical track will explain, and it requires specialized equipment.
Practical exercise: Smell a pair of enantiomers
Spearmint and caraway are widely sold as essential oils, and each is rich in one carvone, so you can make the point cheaply. Better still, use pure l-carvone and d-carvone.
- Dilute both to the same strength, and code them. Have a helper prepare triangle sets.
- Run six triangle trials. Report your score against the chance table from blind practice.
- Write how you would describe each.
- If you used oils rather than pure materials, note what else is in them, and why this is a weaker test.
- Review any isomer information on two supplier documents you own.
You have done this well if your conclusion distinguishes what you tested from a general claim about all enantiomers.
Common mistakes
- Assuming every chiral pair differs. Many are hard to tell apart.
- Reading d or l as R or S. They are unrelated conventions.
- Assuming a CAS number fixes the isomer. Check which form the number covers.
Key terms
- Chirality
- The property of a molecule that cannot be superimposed on its mirror image, like a left and a right hand.
- Enantiomers
- A pair of molecules that are mirror images of each other.
- Diastereomers
- Stereoisomers that are not mirror images, such as cis and trans forms. They generally differ in physical properties.
- Racemate
- A roughly equal mixture of two enantiomers.
Knowledge check
Written and reviewed by the RUŌOD Lab team. This is general education, not legal, regulatory, medical or safety advice. Example dilutions and exercises are for learning, are not validated products, and have not been safety-assessed. Completing a lesson confers no qualification. How we check these lessons.