Terpene Chemistry: The Building Blocks of Essential Oil Aroma
Terpenes are the primary chemical class found in essential oils. Understanding their structure explains why oils smell and behave the way they do.
Every essential oil is a complex mixture of organic compounds, but the dominant chemical class in most oils is terpenes. These are naturally occurring hydrocarbons built from repeating five-carbon isoprene units. Their structure determines their volatility, their scent character, and how they interact with the skin and body.
Monoterpenes
Monoterpenes are the lightest terpenes, built from two isoprene units (10 carbon atoms). They are the most volatile compounds in an essential oil, meaning they evaporate quickly and form the top notes of a scent. Limonene (found in citrus oils), pinene (pine, eucalyptus), and camphene are all monoterpenes. They tend to be bright, fresh, and short-lived in a blend.
Sesquiterpenes
Sesquiterpenes are built from three isoprene units (15 carbon atoms). They are heavier, less volatile, and form the base notes of a blend. Beta-caryophyllene, found in high concentrations in frankincense and black pepper, is a sesquiterpene that has been studied extensively for its interaction with the endocannabinoid system. Zingiberene, the primary compound in ginger oil, is another sesquiterpene.
Oxygenated terpenes
When terpenes are oxidised, they become alcohols, esters, ketones, aldehydes, or oxides. These oxygenated derivatives are often the most important compounds in an oil from a functional perspective. Linalool (a terpene alcohol found in lavender) and linalyl acetate (a terpene ester, also in lavender) are responsible for much of lavender's characteristic scent and are among the most studied aromatic compounds in the literature.
Why oxidation matters for storage
Terpenes oxidise when exposed to air, light, and heat. Oxidised terpenes can be skin sensitisers. This is why proper storage in amber glass, away from heat and light, is not just about preserving scent quality. It is about maintaining the safety profile of the oil. An old, improperly stored citrus oil may contain significant quantities of oxidised limonene, which is a known sensitiser.