The Molecular Chemistry of Essential Oils

Starting with the Terpene family…

Terpenes are a large group of hydrocarbons that are found in essential oils and are based on combinations of Isoprene units (C5H8). They consist of Monoterpenes, Sesquiterpenes and Diterpenes. There are other terpene groups but they are not found in essential oils.


Monoterpenes are the most basic of terpenes and are made of 2 isoprene units, such as myrcene, which is found in oils such as bay, verbena, pine and juniper.

Other common monoterpenes are limonene, ocimene, terpinolene, the pinenes, camphene, cymene, menthene, phellandrene, sabinene and thujene. Their general therapeutic properties are; strongly antiseptic, antibacterial, antimicrobial, soothe everyday aches and pains, stimulate body systems and support a healthy respiratory system.


Sesquiterpenes have a molecular formula that is 1 ½ times a monoterpene, they are formed by 3 isoprene units. This group make up the largest group of terpenes in the plants and are therefore extremely significant in the essential oil arena.

Examples of sesquiterpenes are: farnesene, bisabolene, caryophyllene, chamazulene, bourbonene, cadinene, cedrene, cubene, humulene, patchoulene, selinene,viridiflorene and ylangene to name just a few.

The general therapeutic properties of sesquiterpenes are; strongly antiseptic, antibacterial, calming and balancing, great for soothing everyday aches and pains, intensely cooling and soothing for hot conditions in the body.


Diterpenes are made up of 2 monoterpene units so 4 isoprene units. They are not as common in essential oils as they have heavier molecular weights and higher boiling points. If found at all, they are likely to be found in resinous oils such as camphorene in camphor oil.

Their therapeutic properties are generally similar to sesquiterpenes, and have been reported to have a balancing effect on the endocrine (hormonal) system of humans.

The Alcohol Group of Molecules

Not to be mistaken with alcohol you consume, contain an -OH group attached, such as geraniol, linalool also seen as linalol and citronellol. Examples of Alcohols are: Monoterpenols (monoterpene alcohols) such as borneol, lavandulol, myrtenol, nerol, terpineol and terpin-4-ol.

Sesquiterpenol examples are: alpha-bisabolol, Farnesol, alpha-santalol, cedrol, elemol, nerolidol. These molecules are present in larger quantities in oils such as patchouli, vetiver, valerian, ginger and carrot seed.

This group of molecules is thought of as the most potent of essential oil components. They have little toxicity and are renowned for their fragrant aroma profiles. They tend to be non-skin sensitising and are the most safe to use on the elderly and children. They are anti-infective, they boost and support the body’s immune system, a good general tonic, balancing, uplifting to the mood, warming and comforting. They are also stimulating and cleansing for the liver and digestive system as well as endocrine system’s glands.


Similar to alcohols, with the presence of the -OH group, phenols are known as substituted phenols* in aromatherapy as one or more of the hydrogen atoms is replaced by another group of atoms. Examples of substituted phenols are: carvacrol, thymol, eugenol, thymol.

The therapeutic properties of phenols include: antiseptic, antibacterial, anti-infectious, stimulating and boosting to the immune system and a stimulant for the nervous system uplifting and improving mood.


Aldehydes have very powerful aromas and are often used in perfumery. They are extremely reactive molecules and need to be stored carefully to prevent oxidation. Aldehydes such as citral, acetaldehyde, decanal, neral, piperonal, sinesal have anti-infectious, tonic, calming to the nervous system, vasodilator properties.


Ketones** are not common in essential oils and do not have any aroma. Examples of Ketones are carvone, menthone, ionone, fenchone, nootkatone, jasmone and camphor. Therapeutic properties of ketones are: calming, inducing deep rest, supports a healthy respiratory and digestive system, soothes everyday aches and pains, supports a healthy skin.


Acids are found in nature but very rarely in essential oils. For example formic acid is the sting from ants, acetic acid is found in vinegars and some aromatic waters and phenylacetic acid is found in neroli oil. Examples of acids are: Benzoic acid, cinnamic acid, palmitic acid, valerenic acid.

Acids importance is that in the presence of alcohols they form esters. Many esters are formed during the distillation process.


Esters are the most widespread group found in plant extracts and fragrances. Usually low in toxicity and are therefore safe to use except for methyl salicylate and sabinyl acetate both found in Spanish sage. Methyl salicylate is also found in wintergreen and birch and are often used in sports products.

Examples of esters are: benzyl acetate, linalyl acetate, geranyl acetate.

Therapeutic properties of esters are similar to those of the alcohol group; they also give off sweet fruity odours, calming and act as a tonic to the nervous system. soothing skin irritations, supports, soothes and cools heated body conditions.

Lactones and Coumarins

Lactones are only present in expressed oils and a few absolutes as their weight makes them too heavy to come over in the steam distillation process. Lactones are found frequently in nature but in low quantities in oils. Examples are achilline, nepetalactone, alantrolactone. Lactones support a healthy respiratory tract and can cool hot conditions.

Coumarins are a form of lactone such as citropten, umbelliferone, coumarin and can be skin irritants and photoxic so use with caution. They tend to be uplifting yet at the same time calms the body into a deep restful sleep. A well known furocoumarin is bergaptene found inn Bergamot which is phototoxic.


Oxides are found widely in camphoraceous oils such as eucalyptus, rosemary, tea tree and cajeput. The most commonly found oxide in aromatherapy is eucalyptol also known as 1,8-cineole. Examples of oxides are: bisabolol oxide, linalool oxide, pinene oxide rose oxide.

Therapeutic properties are: stimulating to the glands of the digestive and respiratory systems and supports a healthy mucous membrane.

*Safety: oils containing phenols must be used cautiously and in low dosages as the can be a skin and mucous membrane irritant as well as toxic to the liver.

** ketones should be used sparingly, with caution and not for prolonged periods. Do not use during pregnancy and avoid in people suffering from epilepsy and convulsions.

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