Vanillin, the methyl ether of protocatechuic aldehyde, occurs widely distributed in the Vegetable Kingdom but mostly in very small amounts. Moreover, in the living plant it does not appear to occur as such, but is presumably the product of fermentation of a glucoside. It is the characteristic constituent of vanilla beans. It has also been found in the flowers of Nigritella suaveolens, in the oil of Spiraea Ulmaria, in Peru balsam, and in oil of cloves. It appears to be contained in the wood substance of numerous plants and can frequently be recognized by means of its characteristic agreeable odor, especially after proper preliminary treatment of the material to be tested. Of plants and plant materials in which its presence may be assumed, the following may be mentioned: Siam benzoes, Asa fetida, umbelliferous opopanax, the rosin of species of larch, mate, potato skins, fresh linden bark, cork, bulbs of dahlia, asparagus shoots, crude beet sugar, and the aqueous extract of the seed of Lupinus a/bus. Traces of vanillin can also be obtained from the sulphite wastes of the manufacture of cellulose, also from saw-dust by heating it under pressure with water to 180°.
1) Bertram and Kursten, Journ. f. prakt. Chem. II. 51 (1895), 316.
Artificially vanillin was first obtained from coniferin, a glu-coside found in the cambial juice of conifers.1) Other methods have been devised since then. At present it is obtained principally by the oxidation of Isoeugenol.
Vanillin constitutes colorless, prismatic needles which are readily soluble in the common organic solvents. It is also readily soluble in boiling water, but difficultly soluble in cold water. Both the aqueous and alcoholic solutions are colored blue by ferric chloride. Its melting point is usually recorded as 80 to 81°. Schimmel &Co., however, observed 82 to 84° in connection with pure preparations. When carefully heated it sublimes without decomposition. It boils at 285° (in a current of carbon dioxide), or at 170° under 15 mm. pressure.
Of crystalline derivatives the following may be mentioned: the methyl ether, m. p. 42 to 43°; the ethyl ether, m. p. 64 to 65°; the acetyl derivative, m. p. 77°; the benzoate, m. p. 75°; and the oxime, m. p. 121 to 122°. Upon oxidation its yields vanillic acid, m. p. 207°. Its isolation can be accomplished by means of alkali bisulphite, p-bromphenylhydrazine or 777-nitrobenzylhydrazine. For the quantitative determination of vanillin compare the chapter "The examination of volatile oils".
In commerce vanillin is frequently found adulterated. The following list, of substances has thus far been found as adulterants, viz., acetisoeugenol, antifebrine, benzoic acid, coumarin, terpin hydrate, and sugar. The melting point, its solubility in water, alcohol and ether, and its behavior toward acid sulphite are the principal criteria in testing vanillin for its purity.