It is claimed that the amygdalin is brought into solution better by treating the bulk of the almond powder with boiling water. The emulsin of one part of the almond powder suffices to hydrolyze the amygdalin of 12 parts of the powder.
During the process of distillation care should be exercised not to allow the poisonous vapors of hydrocyanic acid to escape into the laboratory. In order to prevent this the condensation should be very effective, the connection between condenser and receiver should be sealed with either bladder or parchment paper and any hydrocyanic acid that does not condense should be conducted by means of a tube from the end of the receiver out into the open air.
Inasmuch as benzaldehyde is readily soluble in water, more particularly in water containing hydrocyanic acid, the bulk of the oil is obtained only after cohobation of the aqueous distillate.
The yield of oil from bitter almonds varies from 0,5 to 0,7 p.c, from apricot kernels 0,6 to 1 p.c.
Inasmuch as in many applications the presence of the hydrocyanic acid is objectionable, the oil may be deprived thereof. This is done by shaking the oil with milk of lime and ferrous sulphate when the hydrocyanic acid is precipitated in the form of calcium ferrocyanide. The benzaldehyde remaining is rectified by means of water vapor. If the operation has been conducted carefully no trace of hydrogen cyanide remains. Its absence can be ascertained by means of the Prussian blue test recorded on p. 534 of volume I.
l) Liebig's Annalen 122 (1862), 81.
In place of the oil deprived of its hydrocyanic acid, the much cheaper artificial benzaldehyde is sometimes used. Because of the disagreeable odor and taste, due to the presence of chlorinated products, this artificial benzaldehyde can be used only in the production of cheaper soaps and not in the manufacture of liqueurs and perfumery. It is produced by boiling benzyl chloride with lead or copper nitrate or by heating the benzylidene chloride with sodium hydroxide or milk of lime.
Benzaldehyde prepared in this way can be recognized by its chlorine content which may be determined according to the combustion method described in vol. I, p. 609.
Properties. Bitter almond oil containing hydrocyanic acid is a strongly refractive liquid which is at first colorless but later turns yellow. Its odor is that of the chewed bitter almonds. On account of the poisonous hydrocyanic acid content, smelling the oil should be avoided or at least done cautiously! d16o1,045 to 1,070. A higher density may be attributed to a larger hydrogen cyanide content or more correctly benzaldehyde cyanhydrin (comp. p. 582 under Composition). Freshly prepared bitter almond oil is neutral. Later it acquires an acid reaction due to the oxidation of benzaldehyde to benzoic acid.
aD is mostly + 0°. In isolated instances the oil is slightly dextrogyrate. As high as + 0°9' has been observed; nD20o1,532 to 1,544. The magnitude of the index of refraction is inversely proportional to the hydrocyanic acid content. The oil is relatively soluble in water, one part of oil being soluble in somewhat more than 300 parts of water. However, the oil is much more soluble in 1 to 2 vol. or more of 70 p.c. alcohol, also in 2,5 vol. or more of 60 p.c. alcohol. In the course of time the solubility of the oil in 60 p.c. alcohol diminishes. The hydrocyanic acid content of the oil varies, that of crude oils being greater than that of the rectified oils. Thus in connection with a crude oil (d18o1,090; nD20o1,52986) 11 p.c. of Cnh, and in connection with a rectified oil (d15o1,053; nD20o1,54497) only 0,56 p.c. Cnh were observed. This demonstrates conclusively that the oil used for medicinal purposes should contain a definite amount of hydrocyanic acid. According to the U. S. Pharmacopoeia the oil should contain from 2 to 4 p.c. The method for its determination is described on p. 603 of vol. I.
If the oil is distilled over direct fire special precaution should be exercised because of the formation of hydrogen cyanide vapors. Under these conditions there passes over at first an oil richer in hydrogen cyanide, later one that is weaker. The residue contains benzoin, a polymerization product of benz-aldehyde resulting from the action of the hydrocyanic acid.
Bitter almond oil deprived of hydrocyanic acid, or natural benzaldehyde is a colorless, optically inactive liquid; b. p. 179°; d16o1,050 to 1,055 (a higher specific gravity may be due to a greater benzoic acid content); nD20o,542 to 1,546; soluble in 1 to 2 vol. and more of 70 p.c. alcohol. The pure benzaldehyde is much more prone to oxidation by atmospheric oxygen than is the oil containing hydrocyanic acid which acts as a preservative. The access of air readily converts the aldehyde into benzoic acid. For details see under Storage on p. 584.
Composition. Bitter almond oil consists of benzaldehyde, hydrocyanic acid and benzaldehyde cyanhydrin (phenylhydroxy-acetonitrile or mandelic acid nitrile). Upon the hydrolysis of the amygdalin in the process of fermentation, benzaldehyde and hydrocyanic acid result which upon prolonged contact combine to form benzaldehyde cyanhydrin.
C6H5Cho + Cnh = C6H5CH(OH)CN the hydrocyanic acid content. Whereas normal oils with a specific gravity of 1,052 to 1,058 contained from 1,6 to 4 p.c. of hydrocyanic acid, oils with a density of from 1,086 to 1,096 contained from 9 to 11,4 p.c. An experiment made to clear up this situation revealed that the specific gravity of a pure benz-aldehyde when allowed to stand over an aqueous 20 p.c. solution of hydrocyanic acid had increased from 1,054 to 1,0741).
Benzaldehyde Hydrocyanic acid Benzaldehyde cyanhydrin.
The proof that bitter almond oil really contains benzaldehyde cyanhydrin was supplied by Fileti1). Upon the reduction of bitter almond oil he obtained phenylethylamine, whereas a freshly prepared mixture of hydrocyanic acid and benzaldehyde yielded methylamine only.
Benzaldehyde cyanhydrin is readily decomposed. Even when boiled with water vapor, or in vacuo, it is resolved into its components. Hence it can be found only after the distillation of the oil. It is formed especially when the oil is allowed to remain in prolonged contact with water rich in hydrocyanic acid, a condition that obtains at times in the course of the production.
Inasmuch as benzaldehyde cyanhydrin has a relatively high specific gravity, viz., 1,124, the density of the oil increases with l) Gazz. chim. ital. 8 (1878), 446; Berl. Berichte 12 (1879), 296.
Concerning the equilibrium benzaldehyde + hydrocyanic acid benzaldehyde cyanhydrin see the paper by P. H.Wirth2).
Inasmuch as both odor and taste of benzaldehyde from bitter almonds are supposed to be better than those of the best artificial product, it may be assumed that the former contains some other unknown constituent. F. D. Dodge3) tried to isolate such a constituent by dissolving 60 g. of bitter almond oil deprived of hydrocyanic acid in 100 cc. of ether and adding 200 g. of a concentrated bisulphite solution. After 18 hours the resulting bisulphite addition product was removed by filtration, washed with ether and the ethereal solution evaporated in a vacuum desiccator after it had been shaken with some soda solution. Treated in this manner, a residue of 0,2 p.c. of a yellow oil was obtained. It was heavier than water and had a pleasant, characteristic odor, but could not be identified. Artificial benzaldehyde treated in like manner yielded traces of an oil without decided odor.
Adulteration and Examination. The qualitative detection of hydrocyanic acid has been described in vol. I, p. 533, the quantitative determination in vol. I, p. 603.
The detection of foreign oils in bitter almond oil is accomplished readily by converting the benzaldehyde into its bisulphite addition product and separating this from the non-aldehydic admixtures.
5 g. of the oil to be investigated and 45 g. of sodium bisulphite solution are transferred to a 100 cc. test tube and the mixture thoroughly shaken. Then 60 g. of water are added and the test tube is placed in hot water. If the oil is pure a clear solution results. Foreign substances collect at the surface and can be separated for the purpose of further investigation.
1) Report of Schimmel & Co. April 1893, 8.
2) Arch, der Pharm. 249 (1911), 382.
3) 8th International Congress of Applied Chemistry, Washington and New York, 1912, vol. 17, p. 15.
In order to test for nitrobenzene (oil of mirbane) either the original oil or the oil that floated on the bisulphite solution may be taken. It is dissolved in twenty times its weight of alcohol and the solution diluted with water until the turbidity remains permanent. Some zinc and dilute sulphuric acid are added and the mixture allowed to react for several hours. The solution is then filtered, freed from alcohol by evaporation and boiled for a short time with a drop of potassium bichromate solution. If any nitrobenzene was present it will have been reduced to aniline and this identified by its violet color after the oxidation with the bichromate.
The most common adulteration of bitter almond oil, whether deprived of its hydrogen cyanide content or not, consists in the addition or substitution of artificial benzaldehyde. Inasmuch as this frequently contains chlorinated products, the detection of chlorine may serve as a proof of the presence of artificial benz-aldehyde. For this purpose the combustion method described in vol. I, p. 609 is best employed.
However, inasmuch as benzaldehyde free from chlorine occurs in commerce, a negative chlorine test is by no means a sign of the purity of an oil. But in all cases in which chlorine is found, it may be regarded as a positive indication of adulteration with artificial benzaldehyde.
An addition of alcohol, which is said to occur not infrequently, may be ascertained in the ordinary manner.
As to the several methods for the determination of benz-aldehyde in bitter almond oil see Dodge, footnote 3, p. 583 and the Report of Schimmel & Co. April 1913, 23.
Storage. If a small amount of benzaldehyde be exposed to the air in a capsule, crystals of benzoic acid will soon separate and after a time the entire oil will have changed to a crystalline mass. The same change takes place when benzaldehyde is stored in bottles that are but partly filled. Hence to prevent such change, benzaldehyde should be kept in completely filled, well stoppered bottles. As has been shown by special experimerits1), an addition of 10 p.c. of alcohol acts as a preservative. If but 5 p.c. of alcohol be added the oxidation takes place more rapidly than in the undiluted oil.
Bitter almond oil containing hydrocyanic acid is much less readily oxidized than the oil freed from its acid content. The hydrocyanic acid acts as a preservative in like manner as do the 10 p.c. of added alcohol.