(2) It colored pine wood moistened with hydrochloric acid red.
(3) It gave a red color with vanillin and hydrochloric acid, and
(4) A deeper red color with oil of cloves and hydrochloric acid, becoming purple on standing.
(5) It gave a violet color with ferric chloride.
The substance is then, without doubt, fisetin. The formula[26] of fisetin is supposed to be
RHAMNOSE.
It was stated above that Schmid obtained a sugar solution by the decomposition of a fisetin-glucoside from Rhus cotinus, and Perkin obtained the same from a glucoside in Rhus rhodanthema. These investigators thought that the sugar was isodulcite or rhamnose, but they did not isolate it on account of the small quantities of material at their disposal. Moreover, the sugar is very hard to crystallize in the presence of other soluble substances and is not found in large quantity in plants. Maquenne[27] could obtain only 15 to 20 gm. of rhamnose by working up 1 kilogram of the berries of Rhamnus infectorius. Assuming that the free fisetin found in poison ivy leaves had its origin in the decomposition of a fisetin-glucoside by natural processes, it was reasonable to suppose that the sugar would also be found in the free state, although, according to Roscoe and Schorlemmer:[28] "Isodulcite does not occur in the free state in nature, but is found as a peculiar ethereal salt belonging to the class of glucosides. On boiling with dilute sulphuric acid, this splits up into isodulcite and other bodies...." The more recent works on the sugars and on plant chemistry[29] mention the occurrence of rhamnose only in the glucoside form, with one possible exception. The exception referred to is the occurrence of a free sugar, supposed to be rhamnose, in a certain palm-wine.[30] Czapek says:[31] "The well-known methyl pentoses do not occur in the free state in plant organisms so far as we know."
Since rhamnose forms a lead compound, the sugar, if present, should be found in the first lead precipitate, A, and also in filtrate A in case it is not completely precipitated in the presence of acetic acid and alcohol.