Chapter 3 Families of compounds that occur in essential oils
The chemistry of essential oils is organic and vast. To avoid confusion a formal system was developed: the IUPAC (International Union of Pure and Applied Chemistry) system. This names compounds based on the arrangement of the component atoms into functional groups, e.g. alcohols contain –OH.
There are two main types of component in essential oils: hydrocarbons (carbon and hydrogen only) and oxygenated hydrocarbons, which also contain oxygen. These are subdivided into groups based on their structures (Table 3.1). In this chapter, the general physicochemical and therapeutic properties associated with each group are given but it must be emphasized that not all members will have every property; for example acetic acid (vinegar) and the polyacids in dietary fat are all of the form X–COOH but differ drastically. Interactions with other groups in the molecule and in the oil can also affect properties.
Table 3.1 Two major classes of compounds found in essential oils
THE TERPENES
The terpenes are a large group of naturally occurring hydrocarbons (made up of carbon and hydrogen only) found in essential oils. They are based on the isoprene unit with the molecular formula C5H8 (Fig. 3.1). Isoprene is a chain structure described as aliphatic or acyclic, which means a compound with its carbon atoms in chains not closed rings:
Figure 3.1 Isoprene (C5H8): The monomer, or single unit, that builds up into the terpenes.
Courtesy Spiring Enterprises Ltd.
The systematic or IUPAC name is 2-methylbuta-1,3-diene. This means that the longest chain of carbon atoms is 4 (like butane), giving the ‘buta’ part of the name; it is an alkene (‘ene’ represents an alkene) with two (‘di’) double bonds starting on carbon atoms 1 and 3, giving the name buta-1, 3-diene, and the methyl group is attached to the carbon atom 2 (‘2-methyl’). (When no ambiguity can arise, the name may also be written 2-methyl-1, 3-butadiene.) From this point forward these more systematic names will be used.
For most practical purposes it is called by the ‘trivial’ name of isoprene and has been known for a long time as it forms the basis of another important natural product – rubber.
The isoprene unit acts as monomer or single unit that builds up in repeating units to make the groups of terpenes found in the essential oils. Their names usually end in -ene.
There are several groups of terpene hydrocarbon based on the number of isoprene units incorporated.
Monoterpenes
Monoterpenes are made up of two isoprene units, joined head to head. They have a molecular formula of C10H16 (Fig. 3.2).
Figure 3.2 Monoterpenes (C10H16). (A) Myrcene, an acyclic monoterpene. (B) α-Pinene, a dicyclic structure. (C) d-Limonene, a monocyclic structure.
Courtesy Spiring Enterprises Ltd.
Myrcene
Myrcene is an example of a monoterpene and is found in essential oils of bay, verbena, pine and juniper, and in many others.
Ocimene
Found in essential oil of basil.
One new bond formed, so three double bonds remain.
Myrcene and ocimene are not arranged in ring structures: they are acyclic. However, many monoterpenes link up to form rings or cyclic structures, for example in limonene.
Terpinene
Found in essential oils of tea tree and juniper.
α- (alpha-), β- (beta-) and γ- (gamma-) terpinene are isomers, with the position of the double bond varying.
Other cyclic monoterpenes, e.g. pinenes, form ‘bridged’ structures, but the molecular formula is still C10H16.
Sesquiterpenes
Sesqui means half as much again, so sesquiterpenes have a molecular formula one and a half times a monoterpene: they are made up of three isoprene units. The molecular formula is C15H24 (Fig. 3.3).
Figure 3.3 Sesquiterpenes (C15H24). (A) Farnesene (3,7,11-trimethyl-2,6,10-dodecatriene) (acyclic). (B) Caryophyllene (bicyclic).
Courtesy Spiring Enterprises Ltd.
Farnesene
A branched chain hydrocarbon found in oils of citronella, German chamomile, yarrow, rose and cassie absolute.
Bisabolene
A cyclic structure (with a carbon ring in the molecule) found in myrrh oil and German chamomile.
Caryophyllene
Cyclic and has a strong woody, spicy odour, found in oil cloves, lavender, sweet thyme and ylang ylang.
Chamazulene
A bicyclic unsaturated hydrocarbon with the molecular formula C14H16. It does not belong to the sequiterpenes but is historically included with them. It is an azulene which are compounds derived from sequiterpenes. Chamazulene is formed by the breakdown of matricine during the steam distillation process.
Diterpenes
Diterpenes are made up of two monoterpene units (four isoprenes): 2 × C10H16 = molecular formula, C20H32.
Diterpenes are not so common in essential oils as their higher molecular weight and boiling point prevents them coming over in the extraction process of steam distillation. They are most likely to occur in resins.
Camphorene
Cyclic compound, found in the high boiling fraction of camphor oil, boiling point (b.p.) 177–178 °C.
Properties of diterpenes
The molecular structure of the diterpene camphorene is given in Figure 3.4.
Physical and chemical properties
Diterpenes are similar to sesquiterpenes but the larger molecules and molecular weights give them higher boiling points and lower oxidation rates.
Triterpenes
Six isoprene units combine to give a molecular formula C30H48 for the triterpenes. Their derivatives – the triterpenoids – include sterols (found in all plant and animal cells), steroids (many hormones) and saponins (important plant products).
Tetra- or quadraterpenes
Eight isoprene units combine to give a molecular formula C40H56 for the tetra- or quadraterpenes. Their derivatives include the carotenoids, which are important starting materials for vitamin A, fat-soluble vitamins D, E and K, cholesterol and sex hormones.
Polyterpenes
Polyterpenes are compounds comprising several hundred isoprene units and give rise to natural rubber. Poly means ‘many’, so that rubber is made up of many repeating isoprene units. The name for a compound made up of many such repeating units is a polymer. There are many both naturally occurring and synthetically produced polymers of importance with many applications.
The vast majority of these compounds are produced in plants but are obviously outside the scope of aromatherapy.
AROMAFACT
SUMMARY: In essential oils, most constituents are terpenes and terpenoid molecules. The method of extraction can influence the terpene content. In citrus oils extracted by expression (squeezing or pressing the plant material), the terpenes present are similar to those found in the living plant tissue; for example, orange, lemon, mandarin and grapefruit essential oils may be made up of up to 90% of the monoterpene limonene. When extraction is by steam distillation the action of the hot water and steam on thermolabile (heat-sensitive) molecules present in the plant is responsible for the formation of the bulk of the terpene content. Solvent extraction often produces absolutes that are very low in terpenes or do not contain them at all, when compared to a distillation of the same material, for example as found in lavender and rose products.
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