Spearmint and caraway showed that your nose can tell the two hands of carvone apart. Smell receptors are handed, and each mirror-image pair fits them in its own way.
Linalool: lavender and coriander
Linalool, C₁₀H₁₈O, is an alcohol with a chain of eight carbon atoms and a C=C double bond near each end. The carbon holding its -OH group also carries a methyl group, a -CH=CH₂ group and the rest of the chain: four different groups, so linalool comes in two hands. Lavender makes mostly (R)-linalool, and coriander seeds make mostly (S)-linalool.
Drag the model to turn it. Pinch to zoom, or use the buttons.
Drag the model to turn it. Pinch to zoom, or use the buttons.
Your nose is more sensitive to one hand. In 2023, food chemists in Germany measured how much of each hand a trained panel needed in water to notice it: about 0.8 micrograms of (R)-linalool in a kilogram of water, and about 8 micrograms of (S)-linalool. A microgram is a millionth of a gram, so both are tiny amounts, and the (S) hand needs about ten times as much.
Pairs that smell the same
In 1999, scientists in Munich gave people ten mirror-image pairs to sniff. For each pair, they offered three samples, two of one hand and one of the other, and asked which one was the odd one out. As a group, the people picked it out for three pairs only. The other seven pairs smelled the same to them.
| Told apart | carvone, limonene, alpha-pinene |
|---|---|
| Smelled the same | menthol, camphor, fenchone, rose oxide, alpha-terpineol, beta-citronellol, butan-2-ol |
A receptor tells two hands apart when the groups around the stereocentre rest on matching spots in its pocket, the way one hand of a medicine fits all three spots and its mirror image fits only one. When a molecule's two hands fit the receptors it meets about equally well, both send the same pattern, and both smell the same. Butan-2-ol, the small alcohol whose ball-and-stick model you saw beside its mirror image in lesson 7, is one of these.
(+) and (−)
Menthol has three stereocentres, so chemists often name its hands by a measurement. Light that passes through a polarising filter, like the lenses of polarised sunglasses, vibrates in one direction only. One hand of menthol turns that direction a little to the right, written (+), and its mirror image turns it by the same amount to the left, written (−). The sign comes from the measurement, and either letter can go with either sign: (R)-linalool turns light to the left, so it is also (−)-linalool. The menthol in peppermint is (−)-menthol. Your nose finds the two menthols alike, and the next section shows a receptor in your mouth that tells them apart.