Vanillin.—Vanilla is the not entirely ripe, pod-like, capsular fruit (wrongly called pod), of a tropical orchid (Vanilla planifolia, Andrews), which is cultivated in Mexico, the West Indies, and South America. It is extensively used for flavoring, and its odoriferous substance is highly valued in perfumery. The cross-section of the capsule is thick and fleshy, filled with very small, black, lustrous seeds stuck together by a gummy balsam with which they are coated. The capsule has a sourish taste and has no value, the seeds, or rather the balsam enveloping the seeds, being the substance on which the odor and taste of vanilla depend. When the vanilla fruit becomes ripe, the capsule opens and empties its content of seeds in the form of a balsam-like mass.
The lustrous black-brown surface of vanilla is frequently coated with white, delicate crystals, which were formerly taken for benzoic acid. Bley and Vee first recognized them as a peculiar substance, which was further examined by Gobley and Stokkebye. This substance, to which Gobley applied the term vanillin, is the chief odoriferous substance of vanilla. It is deposited upon the vanilla-crystals, when the latter are densely and closely packed together and for some time exposed to a heat of about 77° F. Of vanillin, vanilla contains 1.5 to 2.75 per cent.; the Mexican variety containing 1.69 to 1.32 per cent., the Bourbon No. I, 2.48 to 1.91 per cent., Bourbon No. II, 1.55 to 0.75 per cent., and the Java, 2.75 to 1.56 per cent. It is singular, that the highly valued Mexican vanilla has, generally speaking, a lower content of vanillin than the other varieties.
At present, vanillin is prepared artificially. Tiemann and Harmann first showed that by the oxidation of coniferin, a glucoside occurring in the cambial sap of the Coniferæ, a product, perfectly identical with the vanillin prepared from vanilla, is obtained. The coniferin is obtained by barking the pine or silver fir, scraping together the sap under the bark together with a portion of the liber and pouring it into a vessel. The sap is then pressed off, boiled to separate the albumin, filtered, evaporated to one-fifth its volume, and set aside to crystallize. One hundred quarts of sap are said to yield from 1 to 2 pounds of coniferin-crystals. By now allowing an aqueous coniferin-solution to run into a heated mixture of 10 parts potassium bichromate, 15 parts concentrated sulphuric acid, and 80 parts water, and heating for 3 hours in a flask with back-flow cooler, a liquid is obtained from which ether takes up a yellow oil. After treating the latter with animal charcoal, dissolving in ether and evaporating the latter, there remain colorless, acicular crystals of the odor and taste of vanilla. These crystals consist of vanillin contaminated with some vanillic acid. To separate the latter, purify with acid sodium sulphite and recrystallize. After this operation, vanillin represents a nearly white crystalline powder which melts at from 176° to 177.8° F. In this form it is brought into commerce as a complete substitute for vanilla, 5.64 drachms of it corresponding to about 1 pound of vanilla. A medium-sized pine tree is said to yield vanillin of the value of 80 marks ($19.20).
Vanillin may also be prepared by oxidation from eugenol. Oil of cloves is diluted with three times its volume of ether and agitated with weak caustic potash solution to fix the eugenol on the potash. By acidulating the alkaline solution and shaking with ether, the eugenol is collected. After distilling off the ether, the eugenol is converted with acetic anhydride into aceteugol, and the latter oxidized with dilute, moderately-warmed potassium permanganate solution. The filtrate is made slightly alkaline, concentrated, then compounded with acid and the vanillin extracted with ether.
Vanillin (C8H8O3) forms small colorless prisms of a strong vanilla odor, a warm, vanilla taste, and an acid reaction. It is readily soluble in hot water, alcohol, ether, chloroform, fat and volatile oils, as well as in solutions of caustic alkalies and alkaline carbonates. It melts when heated to from 176° to 177.8° F.; at a higher temperature it sublimates without leaving a residue.
According to a notice published in the "Deutsch-Amerikanischen Apotheker Zeitung," vanillin adulterated with benzoic acid has occurred in the United States. A sample subjected to examination is said to have been nothing but benzoic acid perfumed with vanillin. Such an adulteration can be detected with the microscope, since vanillin crystallizes in acicular crystals, and benzoic acid in lamina, which can be readily recognized. Pure vanillin melts at 176° F., while the melting points of such mixtures are considerably higher, it being in one case at 249° F. By extracting such mixture with thin sodium carbonate solution, benzoic acid passes into solution. After neutralizing with hydrochloric acid, the filtrate yields with ferric chloride a fawn-brown precipitate of ferric benzoate, and on adding hydrochloric acid in excess, the benzoic acid, which dissolves with great difficulty in cold water, is precipitated. By treating the latter, or the ferric benzoate, with dilute sulphuric acid and magnesium, the benzoic acid is reduced to benzaldehyde, which is recognized by its characteristic odor of oil of bitter almonds.
Nitrobenzol is obtained by treating benzol, or a mixture of it, with toluol and their higher homologues, with strong nitric acid, or a mixture of nitric and sulphuric acids, washing the product of reaction with water and soda, caustic soda or ammonia, expelling the unaltered hydrocarbons with steam and rectifying the residue. Three varieties distinguished by their boiling points and odor occur in commerce. The nitrobenzol or oil of mirbane (essence de mirbane) is the so-called light nitrobenzol, which boils at from 401° to 415° F. The heavier varieties boil at a higher temperature and have a more or less disagreeable odor; they are used in the manufacture of aniline and aniline colors.
Pure oil of mirbane is pale yellow, the finest qualities being colorless and almost as clear as water. It has an agreeable odor resembling that of oil of bitter almonds, a specific gravity of 1.186 to 1.2 = 25° Bé., and congeals at 37.4° F. to a crystalline mass. It is scarcely soluble in water, sparingly so in alcohol and with difficulty in watery spirit of wine; it is miscible in all proportions with ether, benzine, volatile oils, and most fat oils.
Oil of mirbane is largely manufactured in England, but the German product is now generally preferred, it being purer and does not impart to soap perfumed with it a yellowish tinge. The finest oil of mirbane is prepared from pure crystallizable benzol, and again purified by washing with potassium bichromate and sulphuric acid, and by rectification with steam.
Pure nitrobenzol suffers no change by boiling with soda lye, while the poorly rectified product colors the lye yellow or brown.