i86 GLUCOSIDES 



potassium bichromate and sulphuric acid yield vanillin, the 

 aromatic constituent of the fruits of Vanilla planifolia. 



The reaction was formerly employed for the preparation of 

 artificial vanillin, but has now been replaced by the oxidation of 

 isoeugenol, which is obtained by the action of dilute alkalis 

 upon eugenol, a substance contained in oil of cloves. 



The relationship between these three substances is as 

 follows : — 



CM = CI-ICH„OH CHO CH = CHCH, 



OCHy I JOCH, 



OH OH OH 



Conil'eryl alcohol Vanillin Isoeugenol 



SALICIN. 



Salicin, C^gH^gO^, occurs in the bark of Salix viminalis. 

 It has a bitter taste and crystallizes in colourless prisms and 

 scales. It is sparingly soluble in cold water but is more soluble 

 in hot alcohol, especially amyl alcohol, and may be extracted 

 from aqueous solutions by means of this solvent. Microscopic- 

 ally, salicin is indicated by the fact that it gives a bright red 

 colour with strong sulphuric acid, also with Frohde's reagent * 

 it yields a violet coloration. On steeping the section in a 

 solution of emulsin, saligenin is produced which gives a blue 

 colour with ferric chloride. 



Preparation. 



Salicin may be prepared by boiling the willow bark with 

 water which will extract a certain amount of tannin, colouring 

 and other matters together with the salicin. The greater part 

 of impurities may be precipitated by the addition of lead 

 acetate. The precipitate is then filtered and a stream of 

 sulphuretted hydrogen is passed through the filtrate in order 

 to remove the lead. The filtrate on evaporation yields crystals 

 of salicin which may be further purified by recrystallization 

 from alcohol. 



Another method is to treat the bark with benzene in order 



* Sodium molybdate dissolved in concentrated sulphuric acid. 



