Benzaldehyde occurs rather frequently in volatile oils. Its existence in these oils is due to the presence, in parts of plants, of certain glucosides which, upon hydrolysis, yield benzaldehyde, hydrocyanic acid, and glucose. Such glucosides are amygdalin, laurocerasin, prulaurasin and sambunigrin.
1) Rupe and Lotz, Berl. Berichte 89 (1903), 2796; Rupe and Schlochoff, ibidem 4377.
2) Tiemann, Berl. Berichte 32 (1899), 824.
Bitter almond oil consists almost entirely, wild cherry bark oil and cherry laurel oil in large part of benzaldehyde. Its presence has also been established in the oils of Ceylon cinnamon and cinnamon leaf, in indigofera oil, in the oils of Roman and French cassie blossoms, in neroli oil, niaouli oil, patchouli oil, etc. Almost invariably hydrocyanic acid is present with the benzaldehyde. (Compare under hydrocyanic acid where the names of all plants yielding benzaldehyde and hydrocyanic acid upon distillation are recorded.)
As first material for the artificial preparation of benzaldehyde, toluene, C6H5CH3, from coal tar is used. This hydrocarbon is chlorinated to either benzylchloride, C6H5CH2C1, or benzal-chloride, C6H5CHC12, and the chlorides are converted into benz-aldehyde.
Benzaldehyde is a colorless liquid and has the characteristic odor of moistened comminuted almonds. Contrasted with bitter almond oil containing hydrocyanic acid, it is comparatively harmless.1) It is readily soluble in the ordinary solvents and is dissolved even by water in the ratio of about 1 :200. It is reported to boil at 179°. The following constants have been observed in the laboratory of Schimmel & Co.:
B. p. 177,3° (733 mm.), 45° (5 mm.); d15o1,050 to 1,055; aD + 0°; nD20o 1,544 to 1,546; soluble in 8 vols, of 50 p. c, 2,5 to 3 vols, of 60 p. c, and 1 to 1,5 vols, of 70 p. c. alcohol.
Of the numerous derivatives that have been prepared, particularly in condensation reactions, the following may serve for the identification of benzaldehyde: the acid sulphite addition product, the semicarbazone melting at 214° and the phenyl-hydrazone melting at 156°. Atmospheric oxygen readily oxidizes benzaldehyde to benzoic acid which frequently crystallizes out when bottles are not completely filled with the aldehyde, The addition of 10 p. c. alcohol retards this oxidation, the addition of a smaller amount accelerates it.2)
1) Report of Schimmel & Co. October 1893, 6. 2) Report of Schimmel & Co. April 1895, 12.
Because of its method of preparation, artificial benzaldehyde almost invariably contains more or less chlorinated products, which impair its odor. Hence these render it unsuited for perfumery purposes and for the production of liqueurs. However, by means of careful purification, artificial benzaldehyde can be made absolutely free from chlorine. A good product should, therefore, be free from chlorine and the absence of halogen should be ascertained by proper tests. (Compare the chapter: "The examination of volatile oils".)