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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!