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Some terpenes are hydrocarbons, being made up solely of carbon and hydrogen atoms in a chain. Because they are in a chain they are termed aliphatic. Although they are not classed as aromatic, they do have some aroma, and play a part in the therapeutic effect of the whole oil. Perfumers are interested in the individual chemicals within an essential oil and sometimes those oils with a large percentage of terpenes are partially or completely de-terpenated – meaning that some or all of the terpenes are removed from the natural oil (an oil so treated is also known as a folded oil – see fractionation in chapter 2. As already explained earlier in the book, the big essential oil companies sell most of their oils to the perfume and food industries, whose requirements far outweigh those of aromatherapy suppliers. Unless the latter specifically state that they do not want a de-terpenated or folded oil, this is what they will probably be given, and where then is the synergy concept of a whole, natural essential oil for the aromatherapist?

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.