As you begin to use essential oils, reading and learning, you will encounter terms used to discuss their chemistry. Being familiar with these terms and knowing where to look for further definitions will enhance your experience with the essentials oils. Understanding some cursory chemistry will deepen your confidence and enables you to be better informed about the essential oils that you are using and those that you are interested in. An essential oil may contain upwards of 100 different chemical compounds, most of these being less than 1%. The synergy of those compounds makes each essential oil unique. The growing conditions, harvest and distillation can affect this already complex chemical signature. The material presented here is intended as an introduction, a brief view of some common categories and their generalities. It is by no means comprehensive.
There are 2 main types of chemical groups that make up most of the components of essential oils. Terpenes and their homologues and Phenylpropane derivitaves. Gattefosse categorized essential oils according to their functional groups; French aromatherapy is based on these distinctions.
Phenylpropane derivatives- These are by products of amino acid metabolism and based on a ring structure. More later…
Terpenes-
Isoprene (with 5 carbons) is the basic building block of terpenes. Terpenes are distinguished by the number of isoprene units. Syntheses within the plant cell takes isoprene to monoterpene and so on to larger and more complex molecules that aren’t mentioned here.
Monoterpenes- 10 carbons/2 isoprenes. Very common in essential oils. These oils tend to be clear, mobile and volatile. The names of terpenes end in –ene.
Sesquiterpenes-15 carbons/3 isoprenes. Less common in essential oils. These tend to be viscous, yellow or brown in color, and are usually used as fixatives and base notes. The names of sesquiterpenes also end in –ene.
Diterpenes- 20 carbons/4 isoprenes. These are resins, balsams and vitamin A.
Triterpenes- 30 carbons/6 isoprenes. These are resins, sterols, steroids and hormones.
Hydrocarbons- you may come across this term describing compounds in essential oils. A hydrocarbon contains only carbon and hydrogen, this term would apply to isoprenes, terpenes, monoterpenes etc…
Functional groups - Alcohols, esters, aldehydes, phenols, ketones, oxides…Just a little bit about organic chemistry…carbon needs to bond 4 times, this means that there are quite a bit of combinations possible. The carbon to carbon bond is the base of all organic molecules. Hydrogen slips into this equation very nicely. The carbon-carbon and carbon-hydrogen bond are typically very strong and being non-polar, makes hydrocarbons not very willing to react with other molecules, therefore very stable. Plants can add an oxygen to a hydrocarbon and make ‘oxygenated hydrocarbons’, structural elements of a molecule that are not the carbon-carbon or carbon-hydrogen. These oxygenated hydrocarbons are called functional groups. (acids, alcohols, aldehydes, ketones, esters, phenols, ethers, oxides, peroxides, furans and lactones). Because this carbon-oxygen bond is polar and therefore more reactive, chemical reactivity is greatly determined by ‘functional groups’. The smell, taste and effects of essential oils on living organisms is determined by these functional groups. In some essential oils, primarily the needle oils and the oils from citrus rind, hydrocarbons dominate, they are essentially monoterpenes without functional groups.
Functional groups with monoterpenes
alcohols- names end in –ol. They are generally mild, effective against micro-organisms, general tonics. The monoterpene alcohols are generally tolerated topically very well.
Linalol - coriander, rosewood, petitgrain, thyme linalool, Neroli, clary sage, Palmarosa
citronellol- geranium, rose
geraniol – Palmarosa
a terpinol – eucalyptus radiata, MQV, ravensare
terpinol 4 – tea tree, marjoram, mastic
menthol – peppermint, spearmint
Alcohols easily react with acids to form esters.
esters- names generally end in –yl –ate.
Strong fungicides, anti-spasmodic, this depends on the length of the acid chain.
(1c) Formic acid, found in geranium, mildly antispasmodic
(2c) acetates, found in inula, rosemary verbenon, abies siberica, bay laurel, cardamom
(5c) Roman chamomile
(7c) ylang, mandarin petitgrain, very strongly anti spasmodic
aldehydes – names end in –aldehyde or -al
Best used in low concentrations, diffused or diluted. Antispasmodic
Citronellal found in Eucalyptus citriodora, Citronella, Cymbopogon nardus, sedative, anti-inflammatory.
Citral found in lemongrass (Cymbopogon citrates), Indian verbena (Andropogon citratus), Lemon verbena, Melissa, antiviral.
phenols – names end in –ol. This makes it easy to mistake a phenols and alcohols.
Pure phenols are toxic. They are synthetic and carcinogenic, derived from mineral oil. Plant phenols have a hydroxyl group (OH) and a short carbon chain. Stimulating and irritating.
Anethole – irritant, anise seed, star anise seed, fennel
Asarone – carcinogen, calamus
Apiole – irritant, especially to mucous membrane, parsley seed
Eugenol – irritant, allspice, bay leaf, cinnamon leaf, clove leaf, stem and bud, pimento leaf, savory.
Methyl chavicol – mucous membrane irritant, toxic, carcinogenic in animals, basil, bay, marjoram, tarragon.
Safrole – carcinogenic, sassafras, brown and yellow camphor.
Thymol and carvacrol are the most common plant phenols, found in Thyme, Savory, Oregano and Ajowan.
ketones- names generally end in –one. Ketones may be able to cross the blood-brain barrier easily or more readily than other molecules. Ketones dissolve fats are and potentially neurotoxic, causing seizures, liver damage and fatality. However... ketones stimulate cell and tissue regeneration, and are mucolytic. Ketone containing essential oils should only be used by well informed individuals.
Methods of delivery by decreasing toxicity are
oral – rectal – vaginal – transdermal – inhalation
Low or relatively problem free ketone containing essential oils
Everlast and Rosemary verbenon are suitable for external use. Eucalyptus globulus and Vetiver are benign.
Low ketone content, use with caution.
Yarrow, not for children. Rosemary ct.camphor, Peppermint, Atlas Cedar. Eucalyptus polybractea and dives are nontoxic if mixed with other essential oils.
Moderate levels of ketones, only use these oils with extreme caution and education.
Sage, does appear to be less toxic than it’s ketone levels would lead one to expect, not for children. Spike lavender, best if mixed with other essential oils, do not confuse with lavandula angustifolia. Camphor is a neurotoxin.
High ketone levels –
Rue, santolina, Mugwort, pennyroyal are all not suitable for any but the very experienced. Thuja, wormwood and Hyssop all have their place in aromatherapy, again…not for the inexperienced. Lavandula stoechas should not be mistaken for lavandula angustifolia.
oxides and ethers – names usually end in –oxide, -ether. Oxides and ethers are both compounds that have an oxygen atom located between two carbon atoms. Ethers have this series in a linear arrangement, oxides utilize an oxide that is within a ring.
The most common oxide is cineole aka eucalyptol. Very antiviral and expectorant, found in eucalyptus and melaleuca
linalool oxide- antiviral and expectorant, great for bronchial infections, found only in Hyssop decumbens
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Sesquiterpene alcohols
The larger terpene molecule is less affected by the accompanying functional group. There is more variation within the chemical structure and the effect and therefore there are no categories as with the monoterpenes. Researched examples show that the sesquiterpene alcohols are antiallergenic, stimulating to the liver and glands. They are anti-inflammatory and reduce congestion in veins and the lymph system. They are moderately antimicrobial.
a Bisabol-matricaria recutita
Zingiberol- ginger
Patchouli alcohol – Patchouli
a Santalol – Sandalwood
Viridiflorol – naiouli, Peppermint, Sage
Other functional groups effect sesquiterpenes in very different ways and we are not able to generalize here.
Phenylpropanes- Derived from the amino acid phenylalanine. The breakdown of phenyl alanine produces cinnamic acid, which is then a building block for other components of essential oils.
Eugenol – Sensitising, antiseptic and stimulating, found in Clove (Eugenia carophyllata)
Cinnamic aldehyde – antiseptic and irritating, sensitizing. Found in Cinnamon as cinnamomum ceylanicum and Cassia as cinnamomum cassia
Saffrol- Sassafras albidum and Cinnamomum camphora (white camphor)
Myristicin- Myristicin fragrans
Apiol – Petrosetinum sativum (Parsley)
Estragol aka menthol chavicol- antispasmodic found in Tarragon Artemesia dracunculus and Basil Ocicum basilicum
Anethol – antispasmodic, found in Anise.
