Essential Oil Chemistry Basics

Just like any molecule, the chemical structure of essential oils really dictates how they’ll act. For example, will they be harsh on your skin, what potential therapeutic effects might they have, what should they be dissolved in, are there safety concerns, and so on. Essential oils are incredibly complex molecules that have some pretty interesting actions. I decided to write the following blog post for all of you who are keen on learning a bit of the chemistry that makes essential oils what they are. In this post, I’m not going to go through anything past structure since there’s so much to know on that front. You definitely don’t have to know all of the biochemistry here to use essential oils, but having a bit of understanding will only enhance your understanding of their end effects. With that said, I think that anyone who’s a practitioner or formulator whose using essential oils should learn some of the chemistry because it will give you a good grounding on essential oil characteristics (and the ‘why’) down the line. That’s my two cents! :)

What is an essential oil?

Imagine walking through a thicket of mint, you’d smell the aroma emanating from the mint. If you pick up a mint leaf and crunch it with your fingertips, you would notice some oil being released from the plant and a burst of aroma. Now, say you put the leaf under the lens of a strong microscope. You would notice fat globules interspersed throughout the matrix of the plant. That’s the essential oil. Quite simply, essential oils are the aromatic volatile components of aromatic plants. In nature, they serve as a defense against pests, a means for communication between plants, and a tool to attract pollinators.

Plants have several strategies for warding off predators, including defensive chemicals known as secondary plant metabolites (called secondary because they’re not needed for primary cell functions, e.g. photosynthesis). Essential oils form part of this plant arsenal.

Molecules that make up essential oils

Essential oils are made up of a wide diversity of different molecules, with lots of crossover from plant to plant. For example, the molecule 1,8-cineole can be found in the makeup of essential oils from rosemary, tea tree, eucalyptus, spike lavender and more. Essential oils with a higher content of this particular molecule tend to be good decongestants yet not the greatest for young children. The molecule Limonene can be seen in most citrus essential oils. This light molecule makes for essential oils that are nice ‘top notes’ in perfumery, good cleaners, yet shorter on the shelf-life side of things. Essential oils are incredibly complex and can contain hundreds of individual constituents like these.

The chemicals that make up essential oils include:

Terpenoid molecules: most essential oils consist of these molecules. (Very common)

  • Made via mevalonic acid biosynthetic pathway (same as for carotenoids and sterols as well as cholesterol and steroids in animals). Isoprene units (5 carbon molecule) is the starting point to make terpenoids, which are easily linked together to form longer branched carbon chains.

  • Terpenes in essential oils are typically either Monoterpenes, which have two isoprene units (i.e. 10 carbon units), or sesquiterpenes, which have three isoprene units (i.e. 15 carbon units). These can be further modified with ‘functional groups’ (when they have a functional group added, they’re called terpenoids)

Examples Monoterpenes:

Source:  Medicines

Source: Medicines

  • Limonene (C10H16 monocyclic): Grapefruit (93%), Sweet Orange (89%), Lemon (70%), Bergamot (38.4%).

  • Alpha-pinene (C10H16 bicyclic): Cypress (45%), Pine (44%), Spanish Rosemary (22%), Eucalyptus (15%).

Since these are smaller molecules, these will evaporate a lot more rapidly, dissolve slightly in ethanol and are often thought of as ‘top’ notes in perfumery (i.e. first smells you pick up in a blend). Combine with oxygen from the air over time which will create harshness/higher skin irritancy in the essential oil. Important to keep containers tightly sealed and away from light and heat. These oils tend to have shorter shelf lives compared to other essential oils.


Examples Sesquiterpenes:

  • Beta-caryophyllene (C15H22 bicyclic): Black Pepper (34%), Patchouli (20%), Ylang Ylang (10.5%).

  • Beta-Farnesene (C15H22 open chain): Germane Chamomile (27%), Juniper berry (10.5%).

Don’t dissolve readily in ethanol like monoterpenes. Not as volatile, often thought of as middle or base notes in perfumery. Woody oils tend to be sesquiterpenoid in character.

Phenolic (and phenylpropanoid) molecules: There are only a few phenolic compounds found in essential oils.

  • Made via shikimic acid pathway (same as for tannins in tea). They have a distinctive benzene or aromatic ring.

  • Phenols have a hydroxyl (-OH) group attached to the ring. Phenylpropanoids typically have a methyl ether functional group attached to the ring, and a propenyl tail (3-carbon chain with one c=c bonded to the ring by one end.

Source:  Frontiers

Source: Frontiers

Due to the exposed oxygen in the molecule, essential oils with a lot of phenol constituents (phenolic essential oils) are potent antioxidants, potential to be very therapeutic, but also potentially harsher on your skin compared to other essential oils. These essential oils should be diluted lower than many of the other essential oil groups.

Non-Terpenoid aliphatic molecules: Found in citrus peel oils.

  • ‘Aliphatic’ describes molecules made up of carbon chains that are linear, not forming any ring structure. Usually only found in trace amounts in essential oils but usually still have noticeable odors despite this when they have oxygenated functional groups.

Heterocyclic molecules: these contain atoms other than carbon in a closed ring. Found in only a few oils.

  • Made up of carbons that are arranged in a ring, either with nitrogen or oxygen included. Very uncommon, found mainly in ‘heady’ floral oils such as neroli and jasmine. Rarely seen in essential oils that have been steam distilled as they’re soluble in water.


Note: there is a lot of variation in compositions of essential oils from different species. Same species will also have variations depending on harvest time, geo-climatic locations, extraction process, etc. ‘Chemotype’ describes essential oils with different compositions of the same species of plant. E.g. Spanish rosemary (chemotype 1) has higher levels of camphor compared to Tunisia rosemary (type 2), which has higher amounts of 1,8-cineole. If you want to know what the essential oil may do, including usage or safety concerns, knowing the chemotype of an essential oil can be very important.

Table 1. Essential oil composition of Artemisia vulgaris L. from Wilmington Delaware and Woodbridge N.J.

Table 1. Essential oil composition of Artemisia vulgaris L. from Wilmington Delaware and Woodbridge N.J.


Functional Groups

Functional groups are found attached to the monoterpene or sesquiterpene skeletons. Note, there can be more than one functional group in a molecule. The functional group(s) give molecules their distinct properties. Again, I’m not going to go into any of the potential skin effects of these groups, but we’ll definitely explore that in a future blog post. With that said, I recommend making generalizations for effects across a certain functional group with caution.

Note: The overall polarity of an essential oil influences how it acts. Essential oils with oxygen-containing functional groups (i.e. alcohols, phenols, aldehydes and ketones) are polar at the oxygen atom site. The presence of an exposed oxygen (electronegative) makes the molecule more interactive, which may make the essential oil more of an antioxidant, potentially more therapeutic, and also potentially harsher. Non-polar molecules have no strongly electronegative atoms (e.g. terpenes)

1-prim-sec-tert-e1410979431652_grande.png

Alcohols: Have hydroxyl (-OH) group attached to one of their carbons. Note, phenols are a type of alcohol, but since they aromatic ring, they’re classified differently than other alcohols.

Examples monoterpenols:

  • Linalol (C10H17OH open chain): Rosewood (90%), Lavender (37%)

  • Terpinen-4-ol (C10H19OH monocyclic): teatree (40%)

  • Menthol (C10H19OH monocyclic): Peppermint (45%)

  • Geraniol (C10H17OH open chain): Citronella (25%), Geranium (20%)

Examples sesquiterpenols:

  • Farnesol (C15H25OH open chain, unsaturated): Jasmine (10%), Ylang ylang (2%)

  • Patchoulol (C15H24OH tricyclic, saturated): patchouli (40%)

Examples of phenols:

  • Thymol (C10H12OH): Thyme (40%)

  • Carvocol (C10H12OH): Oregano (60%)

  • Eugenol (C9H8OCH3OH): Allspice (80%), Clove bud (70%)

The oxygen in these essential oils make them more of an antioxidant, sometimes more therapeutic and also sometimes a lot harsher on your skin, particularly with the phenol group. The exposed oxygen site is also 'polar' (like water) while the rest of the molecule is nonpolar (won't dissolve in water). Generally, these essential oils, especially the monoterpenols, will have a slight solubility in alcohol, in addition to oil, which essential oils generally dissolve in. The monoterpenols have a slight solubility in water. In addition, the alcohol-rich essential oils will oxidize over time and create a harsher oil. It's important to keep these oils away from light, heat, and air.

 

1200px-Aldehyd_-_Aldehyde.svg_5fd949d7-8004-4a5f-a8b6-e0c39c2b60e4_large.png

Aldehydes: Have a carbonyl group on an end carbon unit. Derived from alcohols via oxidation. Namin ends with either the word ‘aldehyde’ or ‘-al’.


Examples:

  • Citronellal (C9H17CHO monoterpenoid): Citronella (35%)

  • Neral (C9H15CHO monoterpenoid): Lemongrass (35%), Melissa (35%)

  • Geranial (C9H15CHO monoterpenoid): Lemongrass (50%), Melissa (20%)

 Note: Since carbonyl groups are polar, small molecules here are slightly soluble in water as well as in ethanol and oil. Can be irritating to the skin due to their chemical structure, low dilution recommended.

 




acetone-56a12be23df78cf7726816ce_large.jpg

Ketones: Contain carbonyl group like the aldehydes, but always on a carbon atom that's bonded to two other carbons. Naming ends with “-one”.

Examples:

  • Menthone (C10H18O monocyclic): Peppermint (30%)

  • Camphor (C10H16O bicyclic): Rosemary (15-30%), Sage (22%), Spike Lavender (15%)

Note: Slightly soluble in water as well as in ethanol and oils. These molecules are relatively stable and can pose serious problems upon ingestion because they're resistant to liver metabolism. Several ketones can have toxic effects at very low dosages (e.g. camphor at about 3g). In addition, certain ketones, such as camphor, are unsafe for young children (should be kept away from their faces).

 

Acids and estersAcids in plants known as carboxylic acids, mostly non-terpenoids allowing them to be soluble in water. Most are extracted into hydrosols in distillation. These groups are easily combined with alcohols to form esters. Esters are often formed or broken down during steam distillation. Naming for esters are after the name of both parent molecules (e.g. linalool +acetic acid becomes linalyl acetate)

linalyl-acetate1_grande.png

Examples:

  • Linalyl acetate (C10H17OCOCH3): Lavender (40%), Bergamot (25%), Clary Sage (50%)

  • Benzyl benzoate (C7H8OCOC7H8): Jasmine absolute (16%), Ylang ylang (7%)

Note: Generally quite stable and generally recognized as safe.

  • Phenyl Methyl Ethers

  • Cyclic ethers or oxides

Example: 1,8-cineole/eucalyptole - in eucalyptus, spike lavender, rosemary, sage, etc

Note: most ethers, cyclic ethers & oxides are soluble in ethanol. Of the functional groups, cyclic ethers and oxides possibly produce the strongest odors and can give essential oils their distinctive smell at as low of a percentage as 0.3% (rose oxide in Rosa damascena oil). Certain oxides, particularly 1,8-cineole (i.e. eucalyptol) are known to have neurotoxic effects in children - shouldn't be used near the faces of young children. This essential oil constituent can also cause respiratory irritation, particularly in people with asthma.

Lactones: Can be mono- or sesquiterpenoids and always have a carbonyl group (c=o) next to an oxygen atom that's part of a closed ring. Not found in as many essential oils compared to the previous functional groups. Tend to end in -lactone, but can also end in -in, or -ine.

Coumarines Have a lactone ring joined to a benzene ring that can have several functional groups attached. Tend to end with, -in, but can also end with -one in naming. Furanocoumarins (also known as psoralens) such as bergaptene, have 5-membered furan ring attached to the coumarin. Many of these constituents make for oils that are photosensitivity causing (in most of the citrus family).

Note: Solubility is an important consideration for the essential oils. In general, essential oils are able to dissolve into vegetable oils (like dissolves like). Monoterpenes and sesquiterpenes are non-polar hydrocarbons and therefore not soluble in water. However, oxygen-containing functional groups (e.g. hydroxyl groups) can result in a slight solubility depending on how big the molecule is, the boundary between the oil and water layers will be less distinct. Here, it’s possible to get to a temporary emulsion of water with certain monoterpenoids with -OH functional groups by using hot water and shaking vigorously (e.g. geranium oil which has a high percentage of geraniol, a monoterpenol). Ethanol is both a polar and a non-polar solvent due to it’s -OH on the one end and c2h5 on the other. Perfumers use ethanol because it can dissolve many essential oils and also water, allowing for a subtle emulsification. Most monoterpenoids are moderately soluble in ethanol. Bigger molecules such as sesquiterpenoids are too big to be soluble.


If you made it through this post, you probably have come to a better understanding of the complexity of essential oils and the importance of knowing a bit about their chemistry. For each of the molecule types, we’re really only just brushing the surface here. With that said, if you decide to do further research on essential oils, some of the above will give you some good search terms to learn more. Keep in mind, we’ll be exploring each of the topics further on The Eco Well blog.


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  • Svoboda, K. (1996). The Biology of Fragrance. Aromatherapy Quarterly. 49:25-28.

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  • Verallo-Rowell, Vermén M., Stephanie S. Katalbas, and Julia P. Pangasinan. (2016) "Natural (Mineral, Vegetable, Coconut, Essential) Oils and Contact Dermatitis." Current allergy and asthma reports 16.7: 1-11.