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 groups, each altering the carbon chain or phenyl ring molecule to which it is attached, to form alcohols, aldehydes, ketones, etc. (as explained above).
Of the molecules we are going to look at, those having therapeutic effects – largely without toxicity – are alcohols and esters (which are interrelated with organic acids). Ketones, aldehydes and phenols, also effective therapeutically, need to be treated with respect as they are very powerful.
Let us take these molecules one at a time without worrying about remembering the name of each functional group – it is much more important to remember the ‘family’ name of the chemical formed by the addition of a functional group – alcohol, ketone, etc.
There are many different alcohols, aldehydes, (organic) acids, and so on in a single essential oil – essential oils are extremely complex mixes. In fact there is no direct, simple relationship between any one chemical constituent and the therapeutic qualities of a whole essential oil.12
Alcohols
The hydroxyl group (-OH) consists of one hydrogen atom and one oxygen atom; it is not very happy or stable as it has a free arm longing to link with another free arm. When this free hydroxyl group attaches itself to one of the carbons in an aliphatic terpene chain (by displacing one of the hydrogens) it forms an alcohol. Thus a range of alcohols may be formed (none of these is the same as that which occurs in wine and spirits! That is ethyl alcohol, which does not occur in essential oils).
Monoterpenols
When the terpene to which the hydroxyl group attaches itself is a monoterpene, the resulting alcohol is called a monoterpenol and these, like all alcohols, are comparatively easy to recognize as they all end in ‘ol’, for example terpineol, geraniol, linalool (sometimes spelt ‘linalol’) and menthol. They are strong bactericides, anti-infectious, antiviral, stimulating, warming, good general tonics and seen to be free of any hazard, including skin irritation. Because of this, essential oils containing a high percentage of alcohols may generally be regarded as good oils to use on children and the elderly. See Figure 3.11.
FIGURE 3.11: a) Linalool; b) α-geraniol; c) lavandulol, a chain monoterpene; d) α-terpineol, a cyclic monoterpenol
Sesquiterpenols
Should the free –OH group attach itself to a sesquiterpene molecule, it becomes an alcohol called a sesquiterpenol (see Figure 3.12).
