Table of Contents
1. Introduction
2. Basic Chemical Structure
2.1 Hydrocarbons
Monoterpenes
Sesquiterpenes
2.2 Oxygenated Compounds
3. Extraction Methods
4. How essential oils are analysed
5. Discussion
6. Conclusion
7. Bibliography
1. Introduction
Essential oils are used as alternative medical treatments, fragrances for perfumes and also as flavours in food and beverages (Djilani & Dicko, 2012). They are made up of fragranced mixtures that are found in different parts of plants such as the seeds, stems or flowers. (doTERRA, 2014). In this essay the following topics of essential oils will be discussed; the chemical structure, how they are extracted from plants and how to determine their components through analysis.

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2. Basic Chemical Structure
Essential oils are extracted from plants as the compounds that make up essential oils are created naturally by chemical reactions that occur in different plants. Essential oils are released when oils are chemically extracted from the herb or when the herb is compressed (EBSCO, 2014).There are two groups of chemicals that are made from nature; they are classified into primary and secondary metabolites. Primary metabolites can be divided further into carbohydrates, lipids, nucleic acids and proteins. Secondary metabolites are divided into alkaloids, polyketides, shikimates and terponiods (Baser & Buchbauer, 2010). The secondary metabolism of a plant is responsible for their scent and also plays a role in the defence system of the plant protecting it from pests Essential oils are made up of hydrocarbons and their derivatives oxygentated compounds that are created from plants secondary metabolism (Chamorro et al., 2012).
As it can be seen in Figure 1 in the biosysthesis of secondary metabolites, water and carbon dioxide are converted to glucose through photosynthesis. Phosphoenolpyrutave is a main element in the shikimate group of natural products. Decarboxylation of phosphoenolpyrutave produces acetate which is esterified with coenzyme-A to produce acetyl CoA. Acetyl CoA is a starting point for Mevalonic Acid, which is the starting compound for terpenoids. Figure 1 shows the chemical structure of the biosynthesis of secondary metabolites. (Baser & Buchbauer, 2010).

Figure 1: Biosythesis of secondary metabolites (Baser & Buchbauer, 2010).
Essential oils molecules are prepared mainly from carbon, hydrogen, and oxygen (essentials, 2014). Essential oils can be divided into two groups hydrocarbons and their derived oxygenated compounds (Martin, 2014). The hydrocarbon group is divided into monoterpenes and sesquiterpenes. Hydrocarbon chains are kept together by carbon atoms that are linked together. At different points in the chains atoms are attached to make up alternative oils (essentials, 2014). The oxygenated compounds can be broken down into smaller compounds such as Phenols, Alcohols, Aldehydes, Ketones, Esters, Lactones, Coumarins, Ethers and Oxides (EsotericOils, 2014).
2.1 Hydrocarbons
Terpenes are made up of isoprene units. Isoprenes are five-carbon molecules. Isoprenes are assembled in different formations to make up terpenes (Cyberlipid, 2014). Terpenes are a group of molecules that is based on a number of isoprene units in a head to tail fashion.

Figure 2 : Isopene Unit (Cyberlipid, 2014)
Modified terpenes (terpenoids) are where methyl groups have been moved or removed or oxygen atoms added to the structure. Terpenes are easily decomposable under different effects such as air light and moisture which can cause spoilage of the oil. Terpenes have a narrow boiling range, making them difficult to purify. (Parry, 1922). Monoterpenes and sesquiterpenes are the main components of essential oils as they are adequately volatile (Cyberlipid, 2014), (Baser & Buchbauer, 2010).
Monoterpenes
Monoterpenes have 2 isoprenes units linked together. Monoterpenes are very volatile due to the molecular structure (Chamorro et al., 2012). Monoterpenes increase the therapeutic functions of other components in the oil and stop the collection of toxins (Martin, 2014).
An example of an essential oil that is a monoterpene is menthol which is a cyclic monoterpene. Menthol has cooling properties and has a unique scent of the oil residue that is extracted from it (Kamatoua et al., 2013). There is a difference in the content of methone and menthol detected, as the plant mature there is a higher content of menthol and menthyl acetate content (Baser & Buchbauer, 2010).

Figure 3: Steoreoisomers of menthol
(-) –menthol is the most commonly used synthetic and natural form. Menthol has microbial properties but is not a principal compound in essential oils as it is only a component in of a restricted number of aromatic plants (Kamatoua et al., 2013).
Sesquiterpenes
Sesquiterpenes have 3 isoprene units linked together. They are common in essential oils and are less volatile than monterpenes so they blend well with volatile oils (Martin, 2014). An example of a sestquiterpene is Zingiberene that is used in the oil. It is acquired by fractional distillation under reduced pressure (Parry, 1922).

Figure 4: Structure of Zingiberene (Chemistry, 2014).
2.2 Oxygenated Compounds
Oxgenated compounds or terpenoids are derivates of terpenes. They have a stronger aroma and are normally more stable as they do not oxidise as easily under different conditions (Fresholi, 2014). Some examples of oxygentated compounds are alcohol, ketones and esters. Alcohols can further be divided into monoterpene alcohol and sesquiterpene alcohols (EsotericOils, 2014). Alcohols are found in in their free state and in the form of esters in essential oils. Methyl alcohols which are soluble in water which are main components in essential oils are removed during the distillation process so they are not actually found in the essential oil but can be found in the distilled water. But when in ester form methyl alcohols can be found in essential oils such as winter green. (Parry, 1922). Alcohols are beneficial components that have antimicrobial and antiseptic properties.
Esters are naturally occurring in plants. Esters such as terpineol attribute to the aroma in essential oils (Pharmacognosy, 2012). They also have calming and sedative properties (Oils, 2007). Linalyl acetate is a naturally occurring ester that is found herbs and flowers. It is used to make Lavender oil (Hermitageoils, 2014).Ketones have skin healing properties and also helps break down fats (Oils, 2007). Verbenone is an example of a natural ketone that is extracted from plants. It is an ingredient in the oil Rosemary
Other examples of oxygenated compounds are aldehydes that have anti-inflammatory and anti-infectious agents such as geraniol found in Rose Geranium. Oxides that have anti-inflammatory properties such as eucalyptole are found in Eucalyptus. Phenols have anti-pathogenic properties (Oils, 2007). Thymol is an example of a phenol, it is a compound of thyme and ajowan seed oil (Parry, 1922) and ethers help regulate hormones and the central nervous system such as chavicol found in basil (Oils, 2007).
3. Extraction Methods
There are different extraction methods that are used for the extraction of components for essential oils. There are two main types of extraction methods these are distillation and expression. Solvent extraction and CO2 extracts are also other techniques that can be used for the release of essential oils from plants. Different distillation methods that are used are steam distillation, water distillation and steam and water distillation. For distillation process the material from which the material is being extracted is placed on a grid in the still, the steam or/and water depending on which method is being used breaks through the plant material and removes the volatile compounds in it. The volatile compounds rise up into the condenser which cools the vapour into liquid form. This oil liquid will be form a separate layer with water and can be drawn off separately from the water (NAHA, 2014).
Expression of essential oils is done through a technique known as ecuelle a piquer. This technique involves placing the rind of a fruit in a container with spikes that puncture the peel while it is being rotated. This technique allows the essential oils that are contained in the fruit to be released when it is punctured. Centrifugal force can then be used to separate the fruit juice from the essential oils (NAHA, 2014).
Solvent extraction is used when the plants are too fragile to go through the distillation process. In solvent extraction the odoriferous lipophilic is extracted from the plant along with other tissues in the plant. This causes a thick solution to be extracted that contains waxes, fats and other odoriferous material. This solution is then mixed with alcohol which extracts the aromatic compounds (NAHA, 2014).
Hypercritical carbon dioxide (CO2) extraction involves putting C02 under pressure to turn it into a liquid from a gas. This liquid is then used as an inert liquid solvent which can extract aromatic compounds from the plant by diffusing through it. C02 can contain some elements that are not found in the corresponding essential oils.
During extraction methods some main considerations must be addressed such as the cost, pesticide residue on the plants and also the safety and therapeutic benefits of the essential oil being produced. These considerations help decide which extraction method is most beneficial (NAHA, 2014).
4. How essential oils are analysed
Terpenes are structurally varied therefore the methods used for analyses have to account for a great number of molecular compounds (Baser & Buchbauer, 2010). With essential oils there are cheaper versions available on the market. Sensory tests are preformed first to help determine if the oil is poor quality. The clarity, colour, odour and viscosity are looked at to determine the quality. When these sensory tests are completed, physical parameters are measured through refractive index, optical rotation and their specific gravity (Lyth, 2014). Chromatography techniques are then used in the separation and identification of compounds (Baser & Buchbauer, 2010).Gas chromatography/Mass Spectrometry is widely used in the separation and identification of compounds in essential oils.
In gas chromatography the identification and quantification of the different chemical compounds in essential oils can be detected. Each individual compound can be identified by the retention time of the peaks. The data collected can then be compared against standards to determine the purity. (Lyth, 2014).One of the main objectives in separation is the resolution of the compound with a short retention time. To achieve this, the appropriate parameters must be provided. Chiral stationary phases allow for the separation of compounds with optical isomerism. When identifying essential oils in gas chromatography the temperature must be changed as essential oils contain votaile compounds and less volatile compounds. The temperature must start off low and then be raised every minute until 200C to obtain elution of heavy terpenoids. This allows for shorter elution times, separate and narrow peaks (Baser & Buchbauer, 2010)
5. Discussion
The chemistry of essential oils is important as they have such diverse effects. The different structures of the compounds is what causes the effects of the essential oil (EBSCO, 2014).It also depends on what part of the plant is being used and also the development stage of the plant. There are also changes in the amount of oil yield and the different compounds that are observed between the process of the bud of a flower forming and full flowering (Baser & Buchbauer, 2010). But a plant’s compounds can have different effects depending on the country of origin, the type of soil it was grown in and also the climate conditions of where it was grown. These factors can all effect natural variations in the essential oils that are extracted from the plant (Lyth, 2014).
Even though essential oils are known for their healing properties it is difficult to prove that they work because of the unique aroma of essential oils it is also difficult to have a control group. A lot of published studies on essential oil fail to reach accurate and reliable scientific standards (EBSCO, 2014)
6. Conclusion
Essential oils are used in a variety of industries, but it is important to know the chemistry behind the process of producing them. This helps produce high quality and pure products that will produce better results. The extraction of them from the plant and also where the plant was growing and the conditions that it grew in are important factors when analysing the product for quality and purity.
7. Bibliography
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