Monoterpenes
Two isoprene units (ten carbon atoms) joined together head to tail, make what is known as a monoterpene (more often referred to simply as a terpene), which is a class of chemical compounds contained in essential oils. See Figure 3.5.
FIGURE 3.5: Cyclic monoterpene and chain monoterpene (each made up of two isoprene units)
Monoterpenes occur in practically all essential oils and their effects, although weak, are antiseptic in the air, bactericidal, stimulating, expectorant and slightly analgesic. Some are antiviral and others break down gall stones. As they may be slightly irritating to the skin, oils containing a high percentage of these should always be used in a carrier of some sort. All the citrus oils (except bergamot) contain a high proportion of terpenes, especially dextro-limonene (see Figure 3.6) and, although there has been one report of hypersensitivity to this material, it may generally be regarded as safe (allergic eczema involving orange peel has been attributed to limonene, but the case has not been proved). In fact, dextro-limonene is thought to be a quencher – i.e. it quenches any hazardous effects an oil may have. For example, when an oil containing a large percentage of dextro-limonene, e.g. mandarin, is added to lemongrass (a skin irritant, because of its high aldehyde content), the limonene in it quenches the irritant effect, rendering the lemongrass safe to use.10
FIGURE 3.6: Limonene, a cyclic monoterpene
Sesquiterpenes
Now they start to look more complicated – don’t worry! It is not necessary to learn these molecules – I show them only so that you can see how they get bigger and heavier, which explains why some are more volatile than others (see chapter 2).
If three isoprene units join together head to tail they make a longer and heavier chain molecule known as a sesquiterpene (15 carbon atoms), which is another class of chemical compounds. See Figure 3.7.
FIGURE 3.7: a) Sesquicitronellene, a chain sesquiterpene b) α-bisabolene, a monocyclic sesquiterpene
An enormous number of essential oils contain sesquiterpenes, such as bisabolene, found in black pepper and lemon oils. Another sesquiterpene worth remembering, because it occurs in practically all plants which belong to the labiate family, is beta-caryophellene. (Azulene is not a true sesquiterpene, though it often is shown as such and occurs in several oils – see chamomile, chapter 3).
Sesquiterpenes are slightly antiseptic, bactericidal, slightly hypotensive, calming and anti-inflammatory; some are analgesic and/or spasmolytic (relieve muscle spasm or cramp).
Diterpenes
When four isoprene units join together this larger molecule is known as a diterpene (see Figure 3.8). There are not many essential oils with diterpenes as the complete molecule is rather heavy to come over in the distillation process.
FIGURE 3.8: α-camphorene, a diterpene
Diterpenes are slightly bactericidal, expectorant and purgative, some are antifungal and antiviral and appear to have a balancing effect on the hormonal system.11
Now that we have an idea of what terpenes are, we can begin to look at them more closely.
The terpenes, of which there are many in nearly every oil, seem to be quite weak in their effects (though not insignificant). However, if the concept of a whole oil is to be recognized as important – which it is – they could well have a secondary use as diluents or quenchers to any possible side effect the oil may have if they were not there, as explained earlier.
Chain Building Blocks (Aliphatic)
Each of the terpene classes above (having 10, 15 and 20 carbon atoms respectively) may be regarded as ‘chain skeletons’, which can attract specific functional groups of atoms to form alcohols, aldehydes, ketones and (organic) acids, thus forming a myriad of different molecules, all having different shapes and therefore different therapeutic properties.
Ring Building Blocks (Aromatic)
These are the second building blocks for essential oils. Carbon atoms do not always join together in a straight or branched chain. Sometimes six of them will join together in a ring to form what is known as a benzene (or aromatic) ring. It is so called because the basic molecule formed is named benzene (and so many substances based on this molecule are aromatic). Nowadays it is more often called a phenyl ring, though all three names are used.
The shape formed by these six carbons may be regarded as a ‘ring skeleton’, and to it can be attached the same functional groups as can join a chain skeleton, to give yet another range of molecules – phenols (not alcohols this time), aldehydes, ketones and (organic) acids.
The phenyl ring can be represented diagrammatically in more than one way; these are easy to draw and to recognize, and using one of them saves writing out the whole formula (see Figure 3.9).
FIGURE 3.9: Benzene ring or aromatic ring or phenyl ring
I show both diagrammatic forms for the sake of interest, but I recommend use of the second one as it is the most up to date and the least confusing.
A word of warning! When two isoprene units are joined together they may appear to be rather like the phenyl ring. However, if you look carefully, you will be able to see that the monoterpene has 10 carbon atoms, whereas the phenyl ring has only six. See Figure 3.10.
FIGURE 3.10: Monoterpene chain (10 carbons) and phenyl ring (6 carbons) [hydrogen atoms (H) omitted]
You may by now have a slight idea of the complexity and multiplicity of essential oil constituents, and I trust you also have a good basic understanding of the chemistry behind the formation of the two main ‘skeletons’ namely the aliphatic chain and the aromatic ring. These form the basis of all essential oils.
These skeletons are known as hydrocarbons because they are made up only of hydrogen and carbon atoms. Within the framework described there are many, many hydrocarbon relations, whose molecules are basically the same, but with slight variations. I will not confuse you by including all of these – you will meet their names in many aromatherapy books. We will concentrate only on the main ones!
The Addition of Oxygen
So far we have become familiar with two of the essential building blocks of life, namely carbon and hydrogen (forming hydrocarbons), and now we are going to introduce another element, oxygen. Oxygen is usually found as part of a functional group. There are many such