Spearmint and caraway

Section 3 of 4

Spearmint leaves and caraway seeds smell nothing alike. The smell of each comes mostly from the same molecule, carvone, in its two hands.

Carvone, C₁₀H₁₄O, is a ring of six carbons with a ketone group, C=O, a C=C double bond, a methyl group and a three-carbon branch. The ring carbon holding that branch also carries a hydrogen atom and two stretches of ring: one leads round to the C=O, the other to the C=C. They are different groups, so this carbon is a stereocentre, and carvone comes in two hands.

Chemists label the two hands R and S, from the Latin words rectus, right, and sinister, left. A set of rules about the four groups decides which hand gets which letter.

(R)-carvone, the spearmint hand A skeletal formula: a ring of six carbon atoms. The ring has a C=O group, a C=C double bond, and a -CH₃ group on the carbon next to the C=O. On the far side of the ring, one carbon carries a three-carbon branch with its own C=C double bond. That branch is joined by a hashed wedge, so it points away from you, behind the page. The branch is on the right of the ring. O
(R)-⁠carvone smells of spearmint. Spearmint oil is 50 to 80% (R)-⁠carvone.
(S)-carvone, the caraway hand A skeletal formula: a ring of six carbon atoms. The ring has a C=O group, a C=C double bond, and a -CH₃ group on the carbon next to the C=O. On the far side of the ring, one carbon carries a three-carbon branch with its own C=C double bond. That branch is joined by a hashed wedge, so it points away from you, behind the page. The branch is on the left of the ring: this drawing is the mirror image of the spearmint one. O
(S)-⁠carvone smells of caraway seeds, the seeds baked into some rye breads. Caraway oil is 60 to 70% (S)-⁠carvone.

The drawings use the wedges from lesson 3. Plain lines lie in the page, and the hashed wedge shows the branch pointing away from you, behind the page. A flat skeletal formula with plain lines only would look the same for both hands, so the wedge carries the handedness. In the mirror, the branch stays behind the page and moves from the right of the ring to the left. Open both 3D models and turn them: the two never fit on top of each other.

The two carvones boil at the same temperature, about 231 °C, and have the same density. To tell them apart you need something that is handed itself, and your nose is handed.

The smell receptors in your nose are proteins, built from L amino acids, so each receptor is handed. A molecule fits a receptor a little like a hand fits a glove: a right hand slips easily into a right glove and awkwardly into a left one. Some receptors respond more strongly to one carvone than to the other, so your brain gets two different messages, and you smell spearmint or caraway.

Quick check

(R)-⁠carvone and (S)-⁠carvone boil at the same temperature. How does your nose tell them apart?