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). These are decongestant to the circulatory system, tonic and, like the monoterpenols, also non-irritating. Some have specific actions such as being stimulating to the heart or regenerating to the liver.
FIGURE 3.12: a) farnesol, a chain sesquiterpenol; b) elemol, a monocyclic sesquiterpenol
Diterpenols
These are formed in the same way, by a free –OH group attaching itself to a diterpene molecule. Being heavier, they are not very volatile, so not many of them vaporize and come through the distillation process into the essential oil. However, they are very important, as their structure is somewhat similar to that of human hormones (steroid) and they appear to have a balancing effect on the hormonal system.11 Sclareol (just small enough to vaporize) is a diterpenic alcohol found in clary essential oil and is claimed to be a hormone balancer. See Figure 3.13.
FIGURE 3.13: Sclareol, a diterpenol
So far we have seen the results of attaching the –OH group to molecules made up of hydrocarbon chains of various lengths. Now let us see what happens when this same –OH group joins a hydrocarbon phenyl ring.
Phenols
When the –OH group attaches itself to a carbon in the phenyl ring, the new molecule is known as a phenol, not an alcohol. See Figure 3.14. Phenols are very active and even stronger in their action than alcohols. Unfortunately for ease of recognition, phenols also end in ‘-ol’, so it is necessary to learn the major ones. These include carvacrol, eugenol, thymol – there are many; some are present as phenolic ethers which can be neurotoxic. The latter mostly end in ‘ole’, e.g. trans- and cis-anethole – trans-anethole is less toxic than cis-anethole. These are slightly more complicated and we will not go into their chemistry now. N.B. An exception to the ‘ole’ ending is the phenolic ether asarone, in carrot oil.
Powerful antiseptics and bactericides, phenols are stimulant both to the nervous and the immune systems but if present in significant amounts, the oil containing that particular phenol should be used with discretion, in low concentrations and for short periods of time. Phenols can be skin irritants if used incorrectly.
FIGURE 3.14: Phenol
Aldehydes
To make an aldehyde, both the carbonyl group (=0) and a hydrogen atom (-H) attach themselves to a carbon atom in either a chain or a ring molecule. See Figure 3.15.
Aldehydes are very important to the perfumer because they very often have a powerful aroma. There is no mistaking an aldehyde as they are referred to by the use of that word, or their names end in ‘al’.
The properties of aldehydes fall somewhere between alcohols and ketones though they are not such consistent and anti-infectious agents as alcohols and tend to be skin irritants to some degree (see chapter 6).
FIGURE 3.15: Benzaldehyde
A few aldehydes are skin sensitizers, which means they may, on certain people, cause a reaction such as a skin rash, which may appear whenever the same aldehyde is met with in any other oil.
FIGURE 3.16: Cinnamic aldehyde
Cinnamic aldehyde (Figure 3.16), when isolated from cinnamon bark oil, can be a skin sensitizer, but when the whole oil (diluted and tested on humans) is used, it has been found not to be so.9 It should nevertheless be used with care, as on certain hyper-sensitive skins, unless well diluted, it can be a skin irritant. Citral, an aldehyde (consisting of geranial and neral) present in lemon oil (5 per cent), is a powerful irritant when isolated; in the whole oil its possible hazards are cancelled by the presence of the other 95 per cent of constituents, most of which are terpenes. Many of the known (and unknown) constituents of an oil act as ‘quenchers’ (i.e. they prevent side effects occurring). Mixing an oil containing citral with an oil containing an equal amount of dextro-limonene can negate its irritant properties altogether (see under monoterpenes earlier in this chapter).
Aldehydes are anti-inflammatory, anti-infectious, tonic, hypotensive, calming to the nervous system and generally temperature reducing.
If an oil containing aldehydes is stored for any length of time or in poor conditions, unwanted acids can form from the precious therapeutically active aldehydes, rendering the oil almost useless for aromatherapy purposes.
Ketones
We now meet the carbonyl group again, =O, which attaches itself to a carbon on either a chain or a ring molecule, creating a ketone (see Figure 3.17).