Camphor, C10H16O, is a ketone, having the following structural formula:
There are a considerable number of essential oils which are phenols. Thymol, C6H3·(CH3)·(C3H7)·OH, in oil of thyme, and carvacrol, its isomer, in oil of hops, are familiar examples.
Coumarin, the anhydride of cinnamic acid,
; is an example of an acid substance which is classed as an essential oil, even though it is a solid at ordinary temperatures. It has an odor and flavor similar to that of vanillin, the essential flavoring material of the vanilla bean, and is often used as a substitute for the latter in the preparation of artificial flavoring extracts.
Of the essential oils containing sulfur, there are two common examples; oil of mustard, allyl isosulfocyanide, C3H5NCS, and oil of garlic, allyl sulfide (C3H5)2S. The latter is present in onions, garlic, water cress, radishes, etc., the difference in flavor of these vegetables being due to the fact that the allyl sulfide is united with other different groups in the glucoside arrangement, in the different plants. Similarly, mustard oil is not present in mustard seeds as such, but as a glucoside which, when hydrolyzed by the enzyme myrosin which is always present in other cells of the same seeds, yields C3H5NCS, KHSO4, and C6H12O6.
THE RESINS
The resins were formerly supposed to be the mother substances from which the terpenes are derived. It is now known, however, that they are the oxidation products of the terpenes. Their exact structure is still a matter of some uncertainty, as their peculiar "resinous" character makes them very difficult to study by the usual methods of chemical investigations.
Resins are divided into two classes: (a) the balsams, and (b) the solid or hard resins. Canada balsam and crude turpentine are familiar examples of the first class. They consist of resinous substances, dissolved in or mixed with fluid terpenes. Ordinary resin, or colophony, consists chiefly of a monobasic acid having the empirical formula C20H30O2, known as sylvinic acid, whose exact structure is not known. Its sodium salt is used as the basis for cheap soaps.