ABSTRACT

The term “essential oil” is a contraction of the original “quintessential oil.” This stems from the Aristotelian idea that matter is composed of four elements: re, air, earth, and water. The fth element, or quintessence, was then considered to be spirit or life force. Distillation and evaporation were thought to be processes of removing the spirit from the plant, and this is also reected in our language since the term “spirits” is used to describe distilled alcoholic beverages such as brandy, whiskey, and eau de vie. The last of these again shows reference to the concept of removing the life force from the plant. Nowadays, of course, we know that, far from being spirit, essential oils are physical in nature and composed of complex mixtures of chemicals. One thing that we do see from the ancient concepts is that the chemical components of essential oils must be volatile since they are removed by distillation. In order to have boiling points low enough to enable distillation, and atmospheric pressure steam distillation in particular, the essential oil components need to have molecular weights below 300 Da (molecular mass relative to hydrogen = 1) and are usually fairly hydrophobic. Within these constraints, nature has provided an amazingly rich and diverse range of chemicals (Hay and Waterman, 1993; Lawrence, 1985) but there are patterns of molecular structure that give clues to how the molecules were constructed. These synthetic pathways have now been conrmed by experiment and will serve to provide a structure for the contents of this chapter.