When exposed to low temperatures, a number of oils deposit one or more of their constituents in crystalline form. Some oils, like rose oil, contain crystals even at ordinary temperature, others, like orris oil and oil of ambrette seeds are butyraceous in consistency. These substances have been designated stearop-tenes or camphors and are paraffins, higher members of the series of fatty acids, such as lauric, myristic and palmitic acids, and derivatives of aromatic and hydroaromatic hydrocarbons. As an exception the methyl nonyl ketone of oil of rue should be mentioned.
Most oils, in case they can be distilled under ordinary pressure without decomposition, begin to boil above 150°. Exceptions to this rule are e. g. those containing sulphur, and such volatile oils which, like the distillation product of the turpentine from Pinus Sabiniana or the oil of Pittosporum resiniferum, contain low boiling hydrocarbons of the paraffin or olefin series. In the absence of oxygenated constituents, a boiling point below 200° indicates the presence of terpenes, between 250 and 280° that of sesquiterpenes, above 300° of poly terpenes.
In their elementary composition the volatile oils do not manifest great variety. All contain carbon and hydrogen; most of them also contain oxygen in larger or smaller quantity; few contain nitrogen or sulphur, or both. The presence or absence of oxygen can be determined by elementary analysis only. The presence of only a small amount of this element, up to 5 p. c, indicates a high hydrocarbon content. The presence of sulphur, which can be oxidized in sealed tubes to sulphuric acid by means of concentrated nitric acid indicates mustard oils, sulphides, or poly-sulphides. Nitrogenous compounds are converted into cyanides by heating with metallic sodium or potassium and are recognized by means of the Prussian blue reaction. The nitrogen content of an oil is mostly due to nitriles. If sulphur is also found, mustard oils are present which, as a rule, betray their presence by their characteristic odor. however, are modified thereby and at times destroyed. Sulphur derivatives can often be isolated in the form of their mercury compounds or as thioureas.
After the elementary composition of an oil has been ascertained, a few group reagents can be applied to learn whether special attention should be given to one class or another of chemical compounds.
If an oil shows an acid reaction, it contains acids or phenols. Small amounts of fatty acids occur occassionally as decomposition products of esters present. Larger amounts reveal their presence by the diminution of volume when the oil is shaken with an aqueous solution of caustic or carbonated alkali. The presence of an ester or lactone can be ascertained when an oil is heated with alcoholic potassa of known strength and titrated back with standard acid. This test presupposes the absence of free acids and aldehydes. Alcohols can be converted into acetic esters by heating the oil with acetic acid anhydride. Subsequent saponification will then reveal the presence or absence of an alcohol in the oil. Aldehydes and ketones can be recognized by their addition products with alkali bisulphites, or by their condensation products with hydroxylamine or semicarbazide in aqueous alcoholic solution. The latter contain nitrogen and as a rule are difficulty volatile with water vapor. Ethers, which are sometimes present as phenol ethers, can be recognized by means of Zeisel's method,.
After these preliminary tests have revealed the presence of a substance belonging to one of the above mentioned classes, it is sometimes possible to separate it without fractional distillation; provided, however, that this method of separation does not change the other constituents of the oil. The analysis of the oil having been thus simplified, fractionation of the non-reacting portion can be resorted to. It should also be noted that these methods of separation never effect a perfect isolation because the other constituents of the oil prevent a part of the sub-stance to be isolated from reacting. When, therefore, the non-reacting portion of the oil is fractionated small amounts of this substance should not be overlooked. Thus it is possible with these group reagents to separate aldehydes and many ketones with acid sulphite solution - a reaction that can at times be facilitated by the addition of alcohol. The crystalline addition product is washed with alcohol and ether and the aldehyde or ketone regenerated by the addition of alkali or dilute acid. Free acids and phenols can be shaken out with aqueous alkali; indifferent substances are then removed from the aqueous solution by shaking it with ether, and then the acid or phenol is set free with dilute mineral acid. If acids and phenols are both present, the former are separated with carbonate solution. Basic substances, such as indole and anthranilic esters can be separated by shaking out with dilute acids. Lactones yield salts of the corresponding hydroxyacids when heated with alcoholic potassa. They are precipitated from the alkaline solution as lactones or hydroxyacids by the addition of a mineral acid. Esters if present are saponified by this treatment with alkali. Aldehydes and ketones,
If none of these short cuts is possible, the oil is fractionated either under ordinary or diminished pressure. If esters are present, the oil is first saponified. It may be taken for granted that the apparatus to be used and the methods to be employed are known. A good guide for distillation under diminished pressure will be found in a small monograph on this subject by Anschutz.1) The various fractions obtained upon a careful fractionation are examined for compounds, in part according to the methods already mentioned. A trained sense of smell will prove an important additional factor. Suspected compounds are, if possible, converted into crystalline derivatives and thus purified and identified.
Ketones that will not combine with acid sulphites, such as menthone, camphor, fenchone, carvone, are converted into oximes or semicarbazones. Inasmuch as the oximes frequently suffer rearrangement with acids, the semicarbazones are at times to be preferred because the ketones can in most cases be regenerated. Certain ketones, such as carvone and pulegone, can be separated by means of their sodium sulphite compounds.
Alcohols are characterized by their capacity to form esters, also phenylurethanes with phenyl/socyanate (carbanil). They can be purified by means of difficultly volatile esters of monobasic acids, such as benzoic acid; or by means of acid esters of dibasic acids, such as succinic acid, phthalic acid, etc. Some of the primary alcohols can also be purified by converting them into calcium chloride addition products from which they are easily regenerated by means of water.
From the hydrocarbons traces of oxygenated compounds can be removed by repeated treatment with metallic sodium. If low boiling hydrocarbons are to be distilled under diminished pressure, a liquid alloy of potassium and sodium is preferable.
As to the compounds themselves that are found in volatile oils, they belong in part to the aliphatic, in part to the aromatic and hydroaromatic series and are distributed over a large number of classes. The hydrocarbons, especially those of the formula C10H16, are widely distributed. Of greater importance, however, are the oxygenated substances, because they are mostly the bearers of the characteristic odor of the oil in which they are contained. In addition to the hydrocarbons there have been found alcohols, aldehydes, acids, esters, ketones, phenols, phenol-ethers, lactones, quinones, and oxides; further, bases, sulphides, mercaptanes, nitriles, and Isothiocyanates.
1) Die Destination unter vermindertem Druck. 2. ed. Bonn 1895.