Chemical Classification. Volatile Oils.
In considering the question of classifying the volatile oils, two methods of arrangement naturally suggest themselves, viz. : a classification according to the botanical natural orders to which they belong, and a chemical classification based on the most important chemical constituents of the oils themselves. While the first of these is the more readily made, it suffers from the disadvantage of being cumbrous and less readily understood except by the botanist. On the other hand, the second plan shows at a glance the sources of the valuable odoriferous and medicinally and technically important constituents for which the volatile oils are largely used.
That it has not been generally adopted is no doubt due to the fact that many of the oils contain several different constituents of value, and it is therefore difficult to make an assignment of some of them to individual chemical groups. Any chemical system of classifying volatile oils must be either exceedingly complex and cumbersome or, if conciseness is attempted, there are too many inconsistencies and discrepancies.
For instance, one of the favored methods of classification is to divide them into four groups, i.e., the terpenes, oxygenated oils, nitro-generated oils, and sulphurated oils. If this be critically studied it is seen that oil of lemon, the terpene class, owes its real identity and value to citral, an aldehyde present in only a very small proportion (less than 5 per cent) and that oil of bitter almond, which is given as the example of the nitrogenated class, owes its classification in this respect only to the incidental presence of hydrocyanic acid, which is frequently removed from the oil, especially when it is used for flavoring purposes.
Moreover the sulphurated which the oxygenated class is the repository of over 90 per cent, of oils of diversified character. It is therefore more suitable to follow the foregoing outline adding such explanatory matter and examples as will serve to make it intelligible and valuable for either study or reference purposes. It is believed that the chemical classification adopted here will be found to have practical value for reference.
Group I. Oils containing mainly Terpenes and Sesquiterpenes.
Copaiba Balsam; caryophyllene.
Cubeb; cadinene and cubeb camphor (an oxygenated constituent).
Dog Fennel; pellandrene.
Ginger; Sesquiterpenes and phellandrene.
Hemp; Cannabene and other terpenes
Hops; humulene and tetrahydrocymene.
Myristica; terpenes and myristicol (an oxygenated constituent). Orange; limonene with small amount of citral and citronellal (both oxygenated constituents).
Turpentine; pinene and sylvestrene
Group II.� Oils containing mainly Alcohols and their esters,
A. Aliphatic saturated alcohols:
Oil of Heracleum; octyl alcohol and octyl ester.
B. Aliphatic unsaturated alcohols:
Coriander; linalool and d-pinene.
Geranium; geraniol and geranyl esters
Lavender; linalyl acetate and geraniol esters.
Lime; linalyl acetate and limonene.
Linaloes; linalool and geraniol.
Petit Grain; linalool and linalyl acetate.
Rose; geraniol and citronellol with esters
C. Monocyclic and dicyclic alcohols:
Fir Cones; bornyl acetate and terpenes.
Golden Rod: borneol and bornyl esters.
Juniper Berries; Juniper camphor and cadinene.
Patchouli; patchouli camphor and cadinene.
Peppermint; menthol and menthone.
Pine Needles; bornyl acetate and terpenes.
Rosemary; borneol and bornyl acetate.
Sandalwood; santalol and esters.
Savin; sabinol and sabinyl acetate.
Valerian; Borneol and bornyl esters.
D. Aromatic Alcohols and Esters:
Group III. � Oils containing Aldehydes as characteristic
Oil of Citron peel; citral and limonene.
Citronella; citronellal and geraniol.
Lemon; Citral and citronellal (this oil might also properly be classed in Group I, as the terpenes constitute over 90 per cent, of the oils).
Lemon grass; citral, citronellal, and methyl-heptenone.
B. Aromatic aldehydes:
Cassia; cinnamic aldehyde.
Cinnamon; cinnamic aldehyde and eugenol.
Cumin; cumic aldehyde.
Meadow Sweet; salicyl aldehyde.
Group IV.� Oils containing Ketones as characteristic constituents,
Oil of Rue; methyl-nonyl-ketone.
B. Aliphatic unsaturated aldehydes:
C. Monocyclic and dicyclic ketones:
Caraway; carvone and limonene.
Dill; carvone and limonene.
Peppermint; menthone and menthol.
Sage; thujone with borneol and cineol.
Tansy; thujone with borneol.
Thuja; thujone and fenchone.
Wormwood; thujone and thujyl alcohol.
Group V. � Oils containing Esters,
Oil of Angelica; methyl-ethyl-acetic esters.
Calamus; heptylic and palmitic acid and esters.
Cardamom; acetic esters and cineol.
German Chamomile; caproic acid esters.
Roman Chamomile; butyric, angelic and tiglic esters.
B. Aromatic acid esters:
Sweet Birch; methyl salicylate.
Gaultheria; methyl salicylate.
C. Undetermined acids:
Elecampane; alantic acid and lactone.
Group VI. � Oils containing Phenols and Phenol Ethers.
Oil of Ajowan; thymol and cymene.
Anise; anethol and methyl-chavicol.
Betel; chavicol and methoxy-chavicol.
Fennel; anethol and fenchone.
Marjoram; carvacrol and linalool.
Savory; carvacrol, pinene, and cymene.
Star Anise; anethol, methyl-chavicol and safrol.
Sweet Basil; Methyl-chavicol and d-linalool.
B. Diatomic phenols and their ethers:
Bay; eugenol, methyl-eugenol and methyl-chavicol.
Camphor; safrol, eugenol and camphor.
Cascarilla Bark; eugenol and terpenes.
Cinnamon Leaf; eugenol and cinnamic aldehyde.
Cloves; eugenol and sesquiterpene.
Pimenta; eugenol and sesquiterpene.
Sassafras; safrol and camphor.
C. Triatomic phenols and their ethers:
D. Tetratomic phenols and their ethers:
Group VII. � Oils containing Neutral Bodies.
Oil of Cajuput; cineol, terpineol and terpenyl acetate.
Eucalyptus; cineol, pinene, and aldehydes.
Laurel Leaves; cineol and pinene.
Myrtle; cineol, d-pinene and dipentene.
Wormseed; cineol and dipentene.
Group VIII.� Oils containing Sulphur.
Garlic; diallyl-disulphide and allylpropyl sulphide.
Mustard; allyl-thiocyanate, allyl-cyanide and carbon disulphide. Onion; allyl-propyl sulphide.
Properties.Volatile oils are slightly soluble in water. Agitated with this fluid they render it milky, but the major portion separates upon standing, leaving the water impregnated with their odor and taste. This impregnation is more complete when water is distilled with the oils, or from the plants containing them. Trituration with insoluble powders, such as talc, magnesium carbonate, kieselguhr, etc., renders them more easily soluble, probably in consequence of their minute division. The intervention of sugar also greatly increases their solubility, and affords a convenient method of preparing them for internal use.
The hydrocarbon oils are scarcely soluble in diluted alcohol, and, according to De Saussure, the solubility of volatile oils generally in this liquid is proportionate to the oxygen which they contain. The volatile oils dissolve sulphur and phosphorus with the aid of heat, and deposit them on cooling. By long boiling with sulphur they form brown, unctuous, fetid substances, formerly called balsams of sulphur. They absorb chlorine, which converts them into resinous oxidation products and then combines with these. Iodine produces a similar effect. They are decomposed by the strong mineral acids, and unite with some organic acids. When treated with a caustic alkali, some of them are saponified or otherwise decomposed. Several of the metallic oxides, and various salts which easily part with oxygen, convert them into resinous oxidation products.
The volatile oils dissolve many of the proximate principles of plant and animal tissues, such as the fixed oils and fats, resins, camphor, and many of the alkaloids when in the free state. Exposed to air and light, many of them absorb oxygen and become what are termed ozonized oils which possess oxidizing properties.
Adulterations. The volatile oils are often sophisticated. Among the greater adulterations are fixed oils, resinous substances, chloroform, hydrocarbon oils, and alcohol, but the most dangerous are those made by mixing the pure oil with the cheaper volatile oils and terpenes and fractions from other oils like limonene.
The presence of the fixed oils may be known by the permanent greasy stain which they leave on paper, while that occasioned by a pure volatile oil disappears entirely when exposed to heat. They may also in general be detected by their comparative insolubility in alcohol. Both the fixed oils and resins are left behind when the adulterated oil is distilled with water. If alcohol be present, the oil will become milky when agitated with water in a graduated tube, and after the separation of the liquids the water will occupy more space and the oil less than before.
The following method of detecting alcohol was proposed by Beral. Put twelve drops of the suspected oil in a perfectly dry watch-glass, and add a piece of potassium about as large as the head of a pin. If the potassium remains for twelve or fifteen minutes in the midst of the liquid, there is either no alcohol present, or less than 4 per cent. If it disappears in five minutes, the oil contains more than 4 per cent, of alcohol; if in less than a minute, 25 per cent, or more. Borsarelli employs calcium chloride for the same purpose. This he introduces in small pieces, well dried and perfectly free from powder, into a small cylindrical tube, closed at one end, and about two-thirds filled with the oil to be examined, and heats the tube to 100� C., occasionally shaking it. If there is considerable proportion of alcohol, the chloride will be entirely dissolved, forming a solution which sinks to the bottom of the tube; if only a very small quantity, the pieces will lose their form, and collect at the bottom in a white adhering mass; if none at all, they will remain unchanged. (J.P.G.,xxvi,429.)
J.J.Bernoulli proposes as a test dry potassium acetate, which remains unaffected in a pure oil, but will be dissolved if alcohol be present, and form a distinct liquid. (See A.J.P., xxv, 82.) Distillation, catching the first portion, and testing for alcohol by the iodoform reaction, will detect very small additions of alcohol.
The adulterants most difficult of detection are the synthetic terpenes or alcohols or those occurring as by-products in some departments of the essential oil industry.
Sometimes volatile oils of little value are mixed with the more costly. The taste and odor afford in this case the best means of detecting the fraud. The specific gravity of the oils is rarely of any value as a test of purity. The optical activity is one of the most valuable of the physical constants used in detecting adulteration and in determining the purity of volatile oils. Thus, the oils of juniper, lavender, and rosemary rotate the plane of polarization to the left, while American oil of turpentine rotates it to the right; and if this should be added to one of the other oils it lowers the optical activity and thus offers one means for its detection.
Unfortunately, the French oil of turpentine, obtained from the Pinus Maritima, acts strongly in the opposite direction. But the very strength of its left-rotatory power might lead to its detection by the abnormal increase of this power which it would impart to the oils in question.
Synthetic or artificial volatile oils are now largely manufactured. They vary greatly at times in their resemblance to the natural products. They will be considered under their respective titles elsewhere, a number having' received official recognition.
Volatile oils may be preserved without change in small, well-stoppered amber-colored bottles, entirely filled with the oil, and secluded from the light.
They may also be preserved by mixing with them about 5 per cent, of a bland fixed oil which will remain undissolved when the volatile oil is added to alcohol and for which allowance must be made, of course, in the volume of the oil used in formulas, etc. (LaWall, Proc. A. Ph. A., 1910, 1121.)