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).
FIGURE 3.17: a) menthone; b) acetone (ketones)
These are easy to identify as they always end in ‘one’. (N.B. A false friend is asarone, which is a phenolic ether in carrot oil.) A common one, not occurring in essential oils, but known to most people, is acetone (nail polish remover). Fortunately, ketones are not present in the majority of essential oils – I say ‘fortunately’ because many of them are not ‘user friendly’ and have to be employed with knowledge and care, as several are neurotoxic. There is no scientific proof that all ketones are hazardous and generally (and fortunately) those which are known to be very much so occur in oils not used by aromatherapists, e.g. tansy, buchu, thuja, rue, etc.
Oils used in aromatherapy which contain significant amounts of ketones are aniseed, caraway, hyssop, pennyroyal and sage, and in France, only pharmacists may sell hyssop and sage oils. It is thought that the ketone in hyssop (pinocamphone) may provoke an epileptic fit in someone predisposed to them and that a few ketones, e.g. thujone and pulegone, if used in overdose, may provoke a miscarriage.
At the EOTA conference (June 1990) at Brunel University, Dr K. Tyman said that ‘there are 4 different thujones and a change in molecular shape means a change in effect’. Dr Tyman made what I feel to be a most important statement, which explains why sage, supposedly very hazardous because of its high thujone content, does not in practice appear to be so.
He explained that because the thujone molecule can arrange itself in four different ways (alpha-thujone, beta-thujone, etc.) it is feasible to expect all four to have different pharmacological effects; therefore it may be that not all thujone molecule shapes will necessarily present a hazard.
Carvone, for example, present in caraway and dill oils (the latter is an ingredient in gripe water for babies), is assumed to have the hazards of all ketones, yet there is no evidence to prove this. In fact, it is believed that some ketones (like carvone) may be completely harmless. There are two forms of carvone, laevo-carvone and dextro-carvone,
