These may be divided, according to their chemical composition, into two major groups; (1) the hydrocarbon oils, or terpenes, and (2) the oxygenated and sulfuretted oils.
The terpenes are of three different types, namely: (a) the hemiterpenes, C5H8, unsaturated compounds of the valerylene series, of which isoprene (found in crude rubber) is the best-known example; (b) the terpenes proper, C10H16, which constitute the major proportion of the whole group; and (c) the polyterpenes (C5H8)n, of which colophene and caoutchouc are the most common examples.
Eleven different terpenes having the formula C10H16 have been isolated from various plant juices, and their molecular arrangement carefully worked out. The following three examples will serve as typical of the general structural arrangement of these hydrocarbons:
A discussion of the evidence which supports these formulas as properly represented the molecular arrangements of the various isomeric forms would be out of place here, as its only particular interest is in connection with the medicinal effects of the different compounds. It is clear, however, that they are six-membered hydrocarbon rings, with additional hydrocarbon groups attached to one or more of the carbon atoms in the ring.
Different modifications, or varieties, of the terpenes constitute the main proportions of the oils of turpentine, bergamot, lemon, fir needles, eucalyptus, fennel, pennyroyal, etc.
The oxygenated essential oils may be either alcohols, aldehydes, ketones, acids, esters, or phenols, derived from either five-membered or six-membered closed-ring hydrocarbons. They are usually present in the plant oil in mixtures with each other or with a terpene. Since most of them have pronounced physiological or medicinal properties, their structure has been well worked out, in most cases; but it seems to be hardly worth while to present these matters in detail here, as they are of interest chiefly on account of their medicinal properties rather than their botanical functions.
Borneol, C10H17OH, and menthol, C10H19OH, are typical alcohols. The latter is a crystalline substance, which melts at 42°, which is present in peppermint oil, both as the free alcohol and as an ester of acetic acid.
Amyl acetate, CH3·COOC5H11, and linalyl acetate, CH3·COOC10H17, the latter occurring in the oils of lavender and bergamot, are typical esters classed as essential oils.
As examples of the aldehyde oils, benzoic aldehyde, C6H5CHO, "oil of bitter almonds," and cinnamic aldehyde, C6H5CH=CHCHO, found in the oils of cinnamon and cassia, may be cited.