The odor of Tropaeolum ma/us, L (Ger. Kapuzinerkresse, family Tropaeolaceae), having an odor similar to that of the cress, induced A. W. Hofmann2) to examine the oil3). For the preparation of the oil 300 kg. of herb with flowers and immature seeds were distilled with water vapor. The aqueous distillate rich in oil was shaken out with benzene upon the evaporation of which 75 g. (= 0,025 p.c.) of oil were obtained.
1) Report of Schimmel & Co. October 1911, 53.
2) Berl. Berichte 7 (1874), 518.
3) The oil was first prepared by Muller, Liebig's Annalen 25 (1838), 207.
It distilled over between 160 and 300° but left a not inconsiderable residue. Only the first fractions, which had a disagreeable odor, contained traces of sulphur.
By far the largest fraction of the oil distilled at 231,9°. This fraction was a strongly refractive liquid with a specific gravity of 1,0146 at 18°. When treated with alkali it gave off ammonia abundantly. According to analysis the oil consists of the nitrile of phenylacetic acid. According to this investigation the garden cress oil contains as principal constituent the same constituent as the herb of Lepidium sativum, L.
In addition to small amounts of a hydrocarbon that was not investigated, the nitrile of phenylacetic acid was found in the lower as well as in the higher fractions.
Very different were the results obtained by Gadamer1) in a later investigation. He prepared the oil both by ether extraction of the juice, which had been obtained by expression of the comminuted herb, also by distillation of the carefully comminuted garden cress. In the latter instance the oil was obtained by shaking out the aqueous distillate with ether. 4 kg. of herb yielded 1,3 g. = 0,0325 p.c. of a brownish oil which had the pungent odor of cress which became especially noticeable when warmed. With ammonia the oil yields, well-nigh quantitatively, benzylthiourea melting at 162°. This reveals that the oil thus obtained, i. e. the normal garden cress oil, consists almost entirely of benzyl mustard oil.
This mustard oil owes its origin to the hydrolysis of a glucoside, the glucotropaeolin, C14H18Kns209 + Xh20, by means of an enzyme. Inasmuch as glucoside and enzyme exist in separate cells of the plant, the formation of benzyl mustard oil takes place only when the cell tissues have been ruptured and both substances can be brought into contact with each other. If the cell membranes are not ruptured previous to distillation, the enzyme is rendered inactive by the heat before it can hydrolyze the glucoside. Under the latter conditions the glucoside is decomposed into phenylacetic acid nitrile (benzyl cyanide) during the process of distillation.
1) Arch, der Pharm. 237 (1899), 111. - Bed. Berichte 32 (1899), 2336.
Thus the results of A. W. Hofmann can be explained when one assumes that the material used by him was not comminuted sufficiently previous to distillation.
The seeds of the garden cress likewise contain gluco-tropasolin 1). When treated in the appropriate manner they yield a volatile oil which contains sulphur2).