The volatile oils are widely distributed in the vegetable kingdom, more particularly among the phanerogams. However, volatile oils have also been isolated from several of the cryptogams.
In most cases the oils exist preformed in the various organs of the plant, the leaves, flowers, fruits, stems and roots. They occur secreted either in glands or in canal-like, intercellular receptacles. Only a few are formed by hydrolysis during the process of preparation from other substances in the plant, e.g. bitter almond oil from amygdalin, mustard oil from sinigrin. Nothing definite is known with regard to their function or their relation to the other constituents of the plant organism. It appears, however, to be well established that they are excretions formed during the life process of the plant, which are of no further importance in the processes of metabolism. It does not follow from this that they are therefore useless, for they act as a means of attracting insects that bring about fertilization, they also protect plants against enemies. These oils are not definite chemical units, but mixtures of substances belonging to many series and classes of compounds.
Not infrequently the oils obtained from different organs of the same plant differ in their chemical composition and in consequence reveal more or less striking differences in their properties. Thus the oil obtained from the root of the cinnamon shrub differs greatly in its composition from that of the bark, which in turn bears but little resemblance to that from the leaves. Moreover, the oils obtained from the same parts of the same species often reveal considerable differences, which are explained by differences in the stage of development of those parts, or by differences in the conditions of climate and soil. Even from identical material, different oils can be obtained by variation in the method of distillation.
Among the various constituents of an oil, one frequently attracts attention as the principal bearer of the odor. In many instances this constituent is the most valuable. The desire to obtain it in a concentrated and pure form may have given the first impetus to the scientific investigation of essential oils. So long as organic chemistry was in its infancy, these investigations could not be crowned with great success. Only after the methods of investigation had been improved hand in hand with the progress of science, was an insight into the nature of volatile oils obtainable. This insight revealed the fact that the oils are as a rule mixtures of a number of complex substances. How systematic research, more particularly in recent decades has afforded an insight into this field, has been revealed in the historical part. In the following pages the results of investigations are to be recorded so far as they concern the better characterized constituents found in volatile oils. Occasionally reference will also be made to substances that have been obtained only by synthesis. For reasons readily apparent a detailed account of their chemical behavior had to be omitted. Only those properties that pertain to their isolation, recognition and identification are enumerated. For further information the larger handbooks of organic chemistry will have to be consulted. To all who are concerned with the scientific investigation of volatile oils, the classic work of Otto Wallach1) "Terpene und Campher", also the detailed handbook of Semmler2) will prove particularly useful. The latter affords a ready oversight over the enormous literature on the constituents of volatile oils, and discusses in detail all constituents found in volatile oils and not only particular classes of compounds.
The analysis of a volatile oil is difficult because most of the constituents are liquid, and can, therefore, be separated only by fractional distillation. This operation, imperfect at best, is often rendered more unsatisfactory by the fact that certain constituents are not volatile without decomposition. For this and other reasons it is best to subject an oil to a preliminary examination, the results of which often suggest modifications which simplify the examination considerably. The preliminary examination consists primarily in the determination of the physical properties of the oil and of its elementary composition; also in a study of the behavior of the oil toward certain group reagents, whereby the presence or absence of certain classes of chemical compounds can be ascertained.
1) O. Wallach, Terpene und Campher. Zusammenfassung eigener Untersuchungen a. d. Gebiete der alicyclischen Kohlenstoffverbindungen. Leipzig 1909.
2) F. W. Semmler, Die atherischen O/e nach ihren chemischen Be-standteilen. Leipzig 1906-1907.
Of the physical constants, the specific gravity, the optical properties and the behavior of an oil toward heat and cold allow conclusions to be drawn as to its composition. A specific gravity, e. g. of less than 0,90 indicates the presence of a large amount of terpenes or of compounds of the fatty series. The oil of Pinus Sabiniana with d15o = 0,70, heracleum oil with d15o = 0,80 to 0,88 and oil of rue with d15o = 0,833 to 0,840 belong to those with a very low specific gravity. These consist mainly of derivatives of the fatty series. Oil of orange, sp. gr. 0,848 to 0,857, consists largely of the terpene limonene; turpentine oil, with a specific gravity of 0,850 to 0,875, almost completely of hydrocarbons C10H16 of the terpene series. A specific gravity higher than 0,90, as is the case with most oils, indicates a mixture of several classes of compounds; whereas a specific gravity of more than 1,0 indicates the presence of compounds of the aromatic series, or if the compound contains sulphur or nitrogen, of sulphides, nitriles or isosulphocyanides.
The optical properties, rotation and refraction, are of less importance unless pure chemical compounds are to be examined or adulterations to be looked for. The optical activity of an oil indicates the presence of a compound or compounds with one or more asymmetric carbon atoms; a high index of refraction the presence of a substance or substances with double bonds, thus enabling the investigator to draw conclusions as to the presence of aliphatic, cyclic or dicyclic compounds.