charge. These electrons are whizzing round and round the nucleus, rather like the earth orbits around the sun – ceaseless, never still. See Figure 3.1.
FIGURE 3.1: Hydrogen atom
The electrons orbit the nucleus at various distances from it. To feel really happy, the atom likes to have two electrons in the first ‘orbit’ (called a shell) around the nucleus – the second and further orbits or shells like to have eight.
As you can see from the diagram, a hydrogen atom is short of one electron – oxygen is short of two and carbon is short of no less than four! So each searches for and joins with other atoms capable of sharing electrons and therefore satisfactorily completing the number necessary for its stability. The atom is then content!
A simpler way to represent the ‘discontented’ or unstable atoms is to give them ‘arms’, i.e. – and =. These arms are called ‘bonds’ because they unite one atom to another.
Molecules
Once there are two or more atoms joined together, the group then becomes a molecule and Figure 3.2a shows a complete molecule of hydrogen, sharing the electrons.
You will notice that hydrogen only needs one more atom like itself to become stable. Oxygen, on the other hand, needs two hydrogen atoms to become a stable molecule of water (H2O). See Figure 3.2b. If we give the carbon atom four hydrogen atoms it will become a stable molecule of methane, (CH4) – Figure 3.2c, which is a gas; if we give the carbon atom two oxygen atoms (remember that oxygen has 2 arms) it will become a molecule of carbon dioxide (CO2), also a gas – the one we breathe out (Figure 3.2d).
FIGURE 3.2: a) hydrogen molecule; b) water molecule; c) methane molecule; d) carbon dioxide molecule
The bonds making up the water and the methane molecules are called ‘single bonds’, those making the carbon dioxide molecule are called ‘double bonds’, because there are two parallel bonds. Double bonds give a molecule a certain amount of rigidity, but they can separate fairly easily to provide an opportunity for other atoms to join in and share electrons as we shall see later on.
Now it begins to get interesting! Carbon atoms have a special ability to keep joining with other carbon atoms to form long straight or branched chains. Each time a carbon atom (with two hydrogen atoms) joins the chain, the molecule so formed is bigger and heavier than the one preceding it (see Figure 3.3).
We are nearly there!
FIGURE 3.3: The chain increases by the addition of CH2 each time
Isoprene Units
An isoprene unit is a molecule comprising five carbon atoms in a branched chain and is one of the two basic building blocks for essential oils. See Figure 3.4.
FIGURE 3.4: Isoprene unit
Terpenes
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
