terpene
Meaning of terpene: A repeating five carbon unit builds all of them, which is why pine, citrus peel and lavender share one family of scent compounds.
Definition of terpene
A terpene is a strongly scented compound built from repeating five carbon isoprene units, produced by conifers, herbs, flowers and fruit. Terpenes supply much of the smell and flavor of pine, citrus peel, lavender and cannabis, and they are the main constituents of essential oils. Plants make them largely to deter grazing animals and insects or to attract pollinators.
How are terpenes grouped?
By how many building blocks they contain. The unit is isoprene, a five carbon fragment, and the groups are counted in pairs of it, which is why almost every terpene has a carbon count divisible by ten. Chemists also draw a line between a terpene proper, made of carbon and hydrogen only, and a terpenoid, the same skeleton after oxygen has been added or the structure rearranged. Menthol, camphor and thymol are terpenoids by that definition, though everyday usage lumps them in with the rest.
| Group | Isoprene units | Familiar member |
|---|---|---|
| Monoterpene | Two | Limonene in citrus peel, pinene in resin |
| Sesquiterpene | Three | Farnesene, found in apple skin |
| Diterpene | Four | Taxol type compounds from yew bark |
| Triterpene | Six | Squalene, the starting point for making cholesterol |
Does aromatic mean the same thing here?
No, and the mismatch causes trouble. In ordinary speech aromatic means strongly scented, which terpenes certainly are. In chemistry it means something quite specific: a ring of carbon atoms sharing electrons in the way benzene does. Most terpenes contain no such ring, so a chemist would not call limonene aromatic even while smelling it across the room. Reading the word in its chemical sense in a description of a scented oil leads straight to the wrong picture of what the molecule looks like.
Do they do anything beyond scent?
Some do, and two examples are substantial. Paclitaxel, a diterpene compound first isolated from the bark of the Pacific yew, became a mainstay chemotherapy drug. Artemisinin, a sesquiterpene from sweet wormwood, is central to malaria treatment. Menthol has a well documented action of its own, switching on the nerve receptor that normally reports cold, which is why it feels cool without lowering any temperature. Against that, claims that particular terpenes in an essential oil or a cannabis variety produce specific health benefits at the small amounts present are far less well supported, and remain an active research question rather than settled ground. Terpenes are also volatile, and when compounds such as limonene and linalool stand exposed to air they oxidize into products recognized as skin allergens, so a fragrant natural oil is not automatically a gentle one.
Used in a sentence
The essential oil was analyzed to identify its major terpene components.
Good to know
Claims about specific health benefits of individual terpenes are considerably less well established than their documented role in producing a plant's characteristic scent and flavor.