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Chemistry of Essential Oils

Essential oils are aromatic hydrophobic volatile liquid extracted from plants. They are known to have many properties and play protective and communicative mechanism in the plant. But where from essential oils get their properties? What are their constituents that make them special? What is chemistry of essential oil?

Essential oils, like all organic compounds, are made up of hundreds of compounds. But there are particularly few major chemical components that are responsible for their characteristic properties. These can be categorised into two groups:

  • Hydrocarbons: Hydrocarbons are present in essential oils mainly in the form of terpenes. Some of them are monoterpenes, sesquiterpenes and diterpenes.
  • Oxygenated Compounds: Mainly present in the form of phenols, alcohols, aldehydes, ketones, esters and oxides.

In addition, essential oils may contain other compounds lie alkanes, ethers, carboxylic acids, lactones, coumarins and furanoids. It is important to have a basic understanding of the chemistry of essential oils to understand the adulteration and properties of essential oils. Let’s discuss in detail these main constituents.


Terpenes

Terpenes are main constituents of essential oils. Terpenes are hydrocarbon molecules with ten carbon atoms and a varying number of hydrogen atoms.

Some examples of the terpenes presents in essential oils are limonene, pinene, camphene, cadinene, caryophyllene, cedrene, dipentene, phellandrene, terpinene, sabinene, and myrcene among others. Limonene has antiviral properties and is found in 90 per cent of citrus oils. Pinene has antiseptic properties and is found in high proportions in pine and turpentine oils.

Terpenes are further classified by the number of terpene units in the molecule, such as monoterpenes, sesquiterpenes, diterpenes. There are estimated 1,000 monoterpenes and 3,000 sesquiterpenes found in essential oils.

Monoterpene

Monoterpene are found in all essential oils and have a structure of 10 carbon atoms. These compounds oxidised in the presence of heat and air. Essential oils that have large amount of monoterpene do not last long on exposure to air.

Some examples of monoterpenes found in essential oils are myrcene, l-limonene, d-limonene, alpha-pinene, b-ocimene and beta-phellandrene. Myrcene is significant in oils of ylang-ylang, parsley, wild thyme, cardamom, hops, bay and cannabis. Limonene is found in many oils but is majorly found in citrus oils. Cymene is found in thyme and oregano oils. Rose, geranium, and citronella are the oils rich in citronellol.

Some oils high in monoterpene are pine, orange. ginger, frankincense, geranium, lavender, sweet marjoram and ylang-ylang.

Broadly, monoterpene are responsible for a number of properties like anti-inflammatory, antiseptic, antiviral, antibacterial, analgesic and stimulating in essential oils.

Sesquiterpenes

Sesquiterpenes consist of 15 carbon atoms and have lower volatility and higher boiling points than monoterpenes.

Some examples of sesquiterpenes present in essential oils are chamazulene found in German chamomile. Another sesquiterpene often found in chamomile and rose, as well as other floral oils is farnesene. Oils high in sesquiterpenes are German chamomile, cedarwood, sandalwood, myrrh, geranium, patchouli and vetiver.

The therapeutic properties of sesquiterpenes include analgesic, antibacterial, anti-fungal, anti-inflammatory, anti-spasmodic, cooling, and sedative.


Phenols

Phenols are compounds consisting of a hydroxyl group (—OH) bonded directly to a carbon atom that is part of an aromatic hydrocarbon group. Simplest of them is monohydroxybenzene (C6H5OH), also known as benzenol, or carbolic acid.

Some common phenols found in essential oils are eugenol, thymol, carvacrol, methyl eugenol, anethole, methyl chavicol, myristicin, safrole and apiol among others. Eugenol is found in clove, cinnamon, basil and West Indian bay; thymol is found in thyme and carvacrol is found in oregano and savoury. Oils high in phenols are basil, clove bud, oregano and thyme.

Phenols have antioxidant, antiseptic, anti-bacterial and stimulating properties. However, they are also known to be skin irritants.


Alcohols

One of the most useful groups of compounds, tending to have good antiseptic and antiviral properties with an uplifting quality; they are also generally non-toxic.

Some of the most common terpene alcohols are linalol, citronellol, geraniol, borneol, menthol, nerol, terpineol, farnesol, vetiverol, benzyl alcohol and cedrol among others. Linalol is found in rosewood, citronellol is found in rose, lemon eucalyptus and geranium and geraniol is found in palmarosa.

Monoterpene Alcohols

These oils have good antiseptic, anti-viral and anti-fungal properties with very few side effects such as skin irritation or toxicity and have an uplifting energizing effect. Examples of these alcohols are linalool, citronellol and terpineol. Linalool is found in lavender, terpineol is found in juniper and tea tree oil and citronellol is found in rose and geranium.

Sesquiterpene alcohols

Sesquiterpene alcohols are not common in essential oils. Some of them are known to have stimulant, anti-allergen and anti-inflammatory properties. Some examples of sesquiterpene alcohols found in essential oils are bisabolol and a-santalol, found in German chamomile and sandalwood, respectively. Some oils rich in sesquiterpene alcohols are ginger, patchouli, vetiver, carrot seed, everlasting and valerian.


Aldehydes

Aldehydes are organic compounds where a carbon atom shares a double bond with an oxygen atom, a single bond with a hydrogen atom and a single bond with another atom or group of atoms. The double bond between carbon and oxygen is characteristic of all aldehydes and is known as the carbonyl group.

Some of the aldehydes found in essential oils are citral, citronellal, neral, benzaldehyde, cinnamaldehyde, cuminic aldehyde and perill aldehyde. Some of the essential oils high in aldehydes are cinnamon bark, melissa, lemongrass, lemon verbena, lemon-scented eucalyptus, citronella etc.

Aldehydes are unstable and easily oxidise in the presence of oxygen and even low heat.

Aldehydes have anti-fungal, anti-inflammatory, anti-septic, disinfectant and sedative properties. They can also cause skin irritation and hence should be used with dilution.


Ketones

Ketones are considered to be toxic constituents but not all ketones are toxic.

Some example of ketones present in essential oils are thujone, pulegone, jasmone, fenchone, camphor, carvone, menthane, methyl nonyl ketone and pinocamphone. Thujone is found in mugwort, tansy, sage and wormwood, pulegone is found in pennyroyal, jasmone is found in jasmine and fenchone is found in in fennel.

Ketones has calming and sedative properties. They are helpful in cell regeneration, easing the secretion of mucus.

Essential oils high in ketones need to be used with care in pregnancy.


Esters

Esters are formed by reaction between alcohols and acids. Esters are very fragrant and fruity substances.

Some examples of esters found in essential oils are linalyl acetate, geranyl acetate, bornyl acetate, eugenyl acetate and lavendulyl acetate. Linalyl acetate is found in lavender, clary sage and bergamot and geranyl acetate is found in sweet marjoram. Essential oils high in esters are birch and wintergreen.

Esters are known to be relaxing, calming and balancing. They have sedative, anti-fungal, anti-microbial and antispasmodic properties.


Oxides

An oxide is a chemical compound that contains at least one oxygen atom and one other element. They are derived from alcohols, terpenes or ketones which have been oxidized.

Some of the oxides present in essential oils are 1,8 cineole, linalol oxide, ascaridol, bisabolol oxide and bisabolone oxide. Cineol is the most well known oxide. It is the principal constituent of eucalyptus oil. It is also found in rosemary, laurel, tea tree and cajeput oils. Linalol oxide is found in hyssop.Oils high in oxides are German chamomile, eucalyptus globulus, ravintsara, and rosemary.

Main therapeutic effect of oxides, especially cineole, are that of expectorant, anti viral and mucolytic effects.


Essential oils on average contain hundreds of different substances. With the help of advances in technology and science, we have been successful in finding about the constituents of essential oils. However, please understand that humans can not create the magic of essential oil even if we combine all constituents of essential oil in proportion. The so created oil do not have the same therapeutic effect as the natural and pure essential oil.

References

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