Chemistry of Essential Oils

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What are essential oils?

Taken directly from this critical review, “essential oils are complex mixtures of volatile compounds particularly abundant in aromatic plants and are mainly composed of terpenes. These volatile molecules include monoterpenes (hydrocarbon and oxygenated monoterpens), sesquiterpenes (hydrocarbon and oxygenated sesquiterpens), and phenolic compounds. Thanks to their chemical composition, essential oils possess numerous biological activities (antioxidant, anti-inflammatory, antimicrobial, etc…) of great interest in food and cosmetic industries, as well as in the human health field.

The term essential oil dates back to the sixteenth century and derives from the drug Quinta essentia, named by Paracelsus von Hohenheim of Switzerland. Essential oils or “essences” owe their name to their flammability.

he French Agency for Normalization: Agence Française de Normalisation (AFNOR) gives the following definition (NF T 75-006): “The essential oil is the product obtained from a vegetable raw material, either by steam distillation or by mechanical processes from the epicarp of Citrus, or “dry”” distillation. The essential oil is then separated from the aqueous phase by physical means.

Essential oils are soluble in alcohol, ether, and fixed oils, but insoluble in water. These volatile oils are generally liquid and colorless at room temperature. They have a characteristic odor, are usually liquid at room temperature and have a density less than unity, with the exception of a few cases (cinnamon, sassafras, and vetiver). They have a refractive index and a very high optical activity. These volatile oils contained in herbs are responsible for different scents that plants emit. They are widely used in the cosmetics industry, perfumery, and also aromatherapy. The latter is intended as a therapeutic technique including massage, inhalations, or baths using these volatile oils. The last key will serve as chemical signals allowing the plant to control or regulate its environment (ecological role): attraction of pollinating insects, repellent to predators, inhibition of seed germination, or communication between plants (emission signals chemically signaling the presence of herbivores, for example). Moreover, EOs also possesses antifungal or insecticide and deterrent activities.

Chemistry of Essential OIls

Essential oils are produced by various differentiated structures, especially the number and characteristics of which are highly variable. Essential oils are localized in the cytoplasm of certain plant cell secretions, which lies in one or more organs of the plant; namely, the secretory hairs or trichomes, epidermal cells, internal secretory cells, and the secretory pockets. These oils are complex mixtures that may contain over 300 different compounds.

They consist of organic volatile compounds, generally of low molecular weight below 300. Their vapor pressure at atmospheric pressure and at room temperature is sufficiently high so that they are found partly in the vapor state. These volatile compounds belong to various chemical classes: alcohols, ethers or oxides, aldehydes, ketones, esters, amines, amides, phenols, heterocycles, and mainly the terpenes. Alcohols, aldehydes, and ketones offer a wide variety of aromatic notes, such as fruity ((E)-nerolidol), floral (Linalool), citrus (Limonene), herbal (γ-selinene), etc.” (PMID: 28930135).

Let’s take a look at each individual organic compound that makes up an essential oil (this information was taken directly from doTERRA University – please visit their site for the original article).

Terpene Hydrocarbons

The most widely represented class of hydrocarbons in essential oils is the terpenes.

The basic unit of every variety of terpene is a five carbon molecule called an isoprene

Isoprenes do not exist in nature as free molecules. Instead, multiple isoprene units are joined together by chemical bonding to form molecules composed of 10, 15, 20,…etc carbons.

This class of molecule can be identified from its scientific name, which typically uses the suffixe -ene.

Monoterpenes

When a chemical bond forms between 2 isoprene units, a 10 carbon molecule called a monoterpene results.

Monoterpenes can assume many shapes including cyclic, straight, and branched varieties.

Common physical characteristics of monoterpenes are a low molecular weight, clear in color, free flowing, non-viscous, high volatility, and strong aroma.

Monoterpenes are present in varying amounts in nearly every essential oil. Common types are limonene, pinene, terpinene, and cymene,

Main health effects:

  • Cleansing1
  • Antioxidant2*
  • Protective to cells and promotes normal cell growth3*
  • Repels insects4
  • Stimulating and enhancing effects on mood and cognitive health5
Essential OilMain Monoterpene Constituent (s)Amount
Grapefruit5limoneneApproximately 97%
Wild Orange1,5limoneneApproximately 95%
Lemon1,5limonene,  pineneApproximately 95%
Lime1,5limonene, terpineneApproximately 85%
Frankincense3*,5pinene, limoneneApproximately 75%
White Fir5limonene, pineneApproximately 75%
Juniper Berry1,5pinene, sabinene, myrceneApproximately 75%
Cypress5pinene, careneApproximately 70%
Bergamot1,5limonene, terpineneApproximately 50%
Black Pepper2*,5limonene, careneApproximately 50%
Coriander5pinene, terpineneApproximately 45%
Dill1,5limonene, phellandreneApproximately 40%
Rosemary4,5pinene, campheneApproximately 35%
Melaleuca1terpineneApproximately 35%
Marjoram5terpineneApproximately 30%
Fennel5limoneneApproximately 25%

Sesquiterpenes

A total of 3 isoprene units combine together to form a sesquiterpenes with 15 carbs.

They have a higher molecular weight, less volatile, and less prevalent in essential oils.

Common types include cedrene, zingiberene, himachlene, and caryophyllene.

Main health effects:

  • Cleansing1
  • Digestive health2*
  • Aides in healthy circulation3*
  • Improves the appearance of skin4
  • Promotes grounding and balance of emotions5
Essential OilMain Sesquiterpene Constituent(s)Amount
Cedarwood4,5cedrene, thujopseneApproximately 70%
Patchouli4,5bulnesene, guaieneApproximately 65%
Vetiver5vatirenene, seychellane, cubebeneApproximately 65%
Ginger2*,5zingiberene, sesquiphellandrene, curcumeneApproximately 55%
Ylang Ylang5germacrene, caryophyllene, farneseneApproximately 55%
Myrrh1,2*,4,5myrrh sesquiterpenoid, elemeneApproximately 55%
Helichrysum4himachalene, curcumeneApproximately 40%
Melissa5germacrene, caryophylleneApproximately 40%
Black Pepper3*caryophylleneApproximately 25%

Diterpenes

A combination of 2 monoterpene units for a total of 20 carbs.

These molecules are difficult to extract through distillation due to their high molecular weight.

These are the largest molecular weight (molecular mass in the low 300s) volatile aromatic compounds that can be collected under normal distillation conditions.

Diterpenes that may be found in essential oils include camphorene, cafestol, kahweol, cambrene, and taxideme.

Oxygenated Compounds

The next layer of complexity in essential oil chemistry is the addition of functional groups to terpene backbones.

Additional non-carbon atoms alters the properties of a terpene by changing its reactivity.

Oxygenated groups are the most common type of functional group found in essential oils.

Alcohols

Among the most widely studied and valuable functional groupd are alcohols.

This group consists of a hydroxyl group (-OH).

The name will end with the suffix -ol.

ALcohols typically have pleasant aromas that are well tolerated.

Common types are menthol, linalool, and santalol

Main health effects:

  • Powerfully cleansing, protects against environmental threats1
  • Uplifting effects on mood2
  • Supports restful sleep3
  • Supports a healthy cardiovascular system4*
Essential OilMain Alcohol Constituent(s)Amount
Sandalwood2,3santalolApproximately 90%
Basil2linaloolApproximately 80%
Rose2citronellol, geraniol, nerolApproximately 60%
Marjoram 2,4*linalool, terpinenolApproximately 55%
Geranium2citronellol, geraniolApproximately 50%
Melaleuca1terpineolApproximately 50%
Peppermint2mentholApproximately 50%
Cilantro1linalool, decenolApproximately 45%
Jasmine2phytol, linaloolApproximately 45%
Lavender2,3linaloolApproximately 35%
Coriander2linalool, geraniolApproximately 30%
Clary Sage2,3linaloolApproximately 25%
Patchouli2patchoulolApproximately 25%
Cedarwood2cedrolApproximately 20%

Phenols

Consist of an alcohol group attached to an aromatic benzene ring.

Aromatic benzene ring is a 6-carbon ring structure with 3 double bonds.

The structure contributes to the powerful antioxidant properties of phenols as well as many other beneficial properties.

Phenol constituents are very potent, which may cause irritation to the skin.

Common types are thymol, carvavrol, eugenol, and anteole

Main health effects:

  • Protects against environmental threats1
  • Antioxidant2*
  • Repels insects3
Essential OilMain Phenol Constituent(s)Amount
Oregano1,2*thymol, carvacrolApproximately 90%
Clove2*,3eugenolApproximately 80%
Thyme1,3thymol, carvacrolApproximately 50%

Aldehyde

Aldehydres occur at the end of a carbon chain and include an oxygen with a double bond and a hydrogen (-CHO)

Name ends in teh suffix – al or aldehyde

Found naturally in plants in small quantities

Known for their distinctly potent fragrances and are often key contributors to the overall aroma of an essential oil.

Aldehydes can cause skin irritation when used topically with sensitive skin types

Common types are cinnamaldehyde, geranial, and  neral.

Main health effects:

  • Protects against environmental threats1
  • Calming to the emotions2
  • Helps support healthy gastrointestinal function and digestion3*
Essential OilMain Aldehyde Constituent(s)Amount
Cassia1,2cinnamaldehyde, benzaldehydeApproximately 90%
Lemongrass3*geranial, neralApproximately 75%
Cinnamon1cinnamaldehydeApproximately 70%
Cilantro3*decenalApproximately 45%
Melissa1,2geranialApproximately 35%

Ketones

Very similar to aldehydes as they contain an oxygen with a double bond, but occurs somewhere in the iddle of a carbon chain rather than on the end.

The name has the suffix -one.at the end

Common types are carvone and menthone

Main health effects:

  • Improves the appearance of skin1
  • Some have calming properties2
  • Supports healthy respiratory function3*
  • Promotes healthy digestion4*
Essential OilMain Ketone Constituent(s)Amount
Dill2,4*carvoneApproximately 50%
Peppermint 3*,4*menthoneApproximately 15%
Rosemary 2,3*,4*camphorApproximately 10%
Fennel 2,3*,4*frenchoneApproximately 5%
Lavender 1,2octanoneApproximately 5%

Esters

Formed when an esterification reaction occurs between an alcohol and a carboxylic acid functional group

It is a combination of these 2 functional groups and includes a carbon atom with a double bond to an oxygen molecule, a single bond to an oxygen molecule, and a single bond to another carbon (-COO or -COOC)

Suffix ends in -yl or -ester

Common types include methyl salicylate, linalyl acetate, and neryl acetate

Main Health Effects:

  • Calming, relaxing, soothing, and balancing effects on mood1
  • Reduces the appearance of skin blemishes2
  • Provides a soothing and warming sensation when applied topically3
Essential OilMain Ester Constituent(s)Amount
Birch3methyl salicylateApproximately 99%
Wintergreen 1,2,3methyl salicylateApproximately 99%
Roman Chamomile 1,2isobutyl acetate, isoamyl angelate,Approximately 75%
Clary Sage 1linalyl acetateApproximately 65%
Lavender 1,2linalyl acetateApproximately 50%
Helichrysum 1,2neryl acetateApproximately 40%
Bergamot 1,2linalyl formateApproximately 40%
Jasmine 1,2benzyl acetate, benzyl benzoateApproximately 40%

Oxides

Include an oxygen ato that is part of the hydrocarbon ring structure

Name generally ends with -oxide or -ole

COmmon types include eucalypol, rose oxide, linalool oxide, and pinene oxide

Main Health Effects:

  • Protects against environmental threats1
  • Promotes feelings of clear breathing2
  • Supports healthy respiratory function3*
Essential OilMain Oxide Constituent(s)Amount
Eucalyptus 1,2eucalyptolApproximately 75%
Rosemary 3*eucalyptolUp to 40%
Peppermint 3*eucalyptolUp to 10%
Thyme 1eucalyptolUp to 10%

Essential oils have a complex composition. One single oil may contain hundreds of individual constituents from a variety of chemical families. Although this complexity may seem overwhelming to all but the most advanced scientist, it is actually a vital part of the therapeutic value of essential oils. Every different class of constituents offers unique biochemical properties that allow them to react in a certain way. Together, a variety of constituents work together as a complete synergistic unit to maximize the functionality of the oil and in turn, enhance its applicable beneficial properties. In plants, this allows the plant to be dynamic and adaptive in its environment by conferring certain protective and reproductive benefits. In humans, this allows essential oils to have endless applications to health and wellness. A single essential oil may have many functions because of their complex chemical profiles. For example, lemon oil is renowned for its powerful purifying properties and is useful as a cleaning agent. On the other hand, it has also been demonstrated to have powerfully uplifting effects on mood.

Certified Pure Therapeutic Grade® essential oils should always have certain ratios of the same core constituents, as these major constituents are largely what determine how the oil will interact in the body. Because essential oils are pure botanical extracts, the ratio of constituents will fluctuate slightly depending on the geographic location, weather conditions, soil conditions, insect presence, precipitation, temperature, distillation conditions, etc. Quality control tests should be used to monitor the chemical profile of each batch of oil and determine if it fits within the proper ranges that will result in beneficial health effects.

Synergy

s the science behind essential oils continues to grow, we have a better understanding of the whole oil’s physiologic functionality. An isolated constituent reacts differently than does the entire profile of constituents. An isolated constituent also acts differently when in the presence of other constituents. Although an isolated constituent may demonstrate a specific chemical or beneficial property, this may not necessarily be reflective of the oil as a whole. In nature, no constituent ever works alone, but instead interacts with all other constituents to create a more beneficial whole essential oil. There is also increased safety in variety. A complex profile of constituents covers a broader range of health applications, while also preventing a concentrated abundance of one constituent to minimize the risk for toxicity.

Several studies have demonstrated the principle of synergy. Let’s look at a few pertinent examples.

Example 1: Effectively disinfecting surfaces, and other commonly used items is a critical part of maintaining health. One study examined the effectiveness of pure eucalyptus oil compared to isolated 1, 8-cineole, a constituent recognized as the active cleansing agent in many essential oils, at eliminating microorganisms. The eucalyptus oil was shown to be significantly more effective than isolated 1, 8-cineol against potential environmental threats. These notable differences in efficacy suggest that the minor constituents in essential oils contribute a synergistic effect that improves the beneficial effects of an oil as a whole. (Hendry ER, Et Al. 2009)

Example 2: Research surrounding the ability of essential oils to support healthy cell function is continuing to expand. One study tested the effectiveness of mandarin essential oil compared to its major constituent limonene. Although both the mandarin oil and isolated limonene showed a powerful inhibitory effect on the damaged cells, the whole essential oil was more effective overall. Researchers attributed this to the mandarin oil’s minor constituents, which contributed a synergistic effect to limonene’s ability to support healthy DNA and cell growth. Overall, the essential oil is more effective than just one of its isolated constituents, in turn allowing the whole essential oil to exhibit a more powerful effect than an isolated constituent. (Manassero CA, Et Al. 2013)

Oils blends with known health benefits are produced under the direction and guidance of experts familiar with how the blending process alters the chemical profile of the oils. However, oils can also be effectively blended in your own home based on your personal preferences and desired benefit. It is important to keep in mind that blending oils may alter their aroma and beneficial properties.

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