190 GLUCOSIDES 



to remove resinous substances, colouring matters, etc., and then 

 digest with alcohol (sp. gr. -85). The solution thus obtained 

 will contain the glucoside and tannin ; the latter substance 

 may be removed by precipitation with hide powder or with 

 gelatine. The filtrate will contain the salicin, which on evapora- 

 tion and cooling will be deposited in the form of crystals. 



Salicin is hydrolysed by emulsin to glucose and the alcohol 

 saligenin according to the following equation : 



C 18 H 18 7 + H 2 = C 6 H 12 6 + C 6 H 4 OH CH 2 OH 



Salicin Saligenin 



By the action of sulphuric acid and potassium bichromate 

 salicin is oxidized to salicylic aldehyde C 6 H 4 OHCHO ; this 

 substance is a fragrant colourless liquid, b.p. 196, which 

 occurs in the essential oil of Spircea Ulmaria ; it is soluble in 

 water, the solution giving an intense violet coloration with ferric 

 chloride ; salicylic aldehyde stains the skin yellow. 



By employing dilute nitric acid as the oxidizing agent, 

 salicin is converted into helicin, a glucoside which on hydrolysis 

 yields glucose and salicylic aldehyde : 



C 13 Hi 8 7 + O = C 12 H 16 7 + H 2 

 Salicin Helicin 



C 12 H 1? O 7 + H 2 O = C 6 H ia O 6 + C ? H 4 OHCHO 

 Helicin Salicylic aldehyde 



The investigations of Weevers tend to show that ordinarily 

 in the decomposition of salicin, saligenin is really an inter- 

 mediate substance, the ultimate products being glucose and 

 catechol. Thus salicase splits salicin into glucose and sali- 

 genin, saligenase produces catechol from the saligenin, and 

 when the leaves decay a third enzyme, catecholase, produces 

 from the catechol an amorphous black pigment. He found 

 that in places where depletion of salicin was taking place 

 the saligenin appeared in quantities insufficient in amount to 

 account for the whole of the salicin ; also that catechol oc- 

 curred in such places after the glucoside had disappeared, in 

 a sufficiently large quantity to warrant the above conclusion. 

 Weevers considers that when glucose and catechol are pro- 

 duced the sugar is translocated, whilst the catechol remains 

 in situ, and combines with fresh glucose, and so reconstructs 

 salicin. 



