THE CARBOHYDRATES 215 



Amygdalin is a combination of 2 molecules of glucose, hydrogen 

 cyanide and benzaldehyde. 



Arbutin is a combination of glucose with hydroquinone (p. 261). 



The composition of glucosides is ascertained by identification of 

 their products of hydrolysis. 



Preparation. 



The quantity of glucoside present in plants is usually small. Since 

 enzymes are present which hydrolyse the glucoside, it is advantageous 

 to destroy the enzyme by heating with water or alcohol before extract- 

 ing the glucoside. The glucoside is usually isolated by extracting 

 the material with water, alcohol, ethyl acetate or other organic solvent, 

 concentrating the extract and crystallising out the glucoside. In 

 some cases the extract requires purification so that no general scheme 

 can be given for isolating glucosides. 



Properties. 



The glucosides are usually white crystalline substances, soluble in 

 water and having a bitter taste. They are soluble in some organic 

 solvents, but generally insoluble in ether. 



Reactions of Salicin. 



(1) Salicin does not reduce Fehling's solution. 



(2) Salicin solutions are hydrolysed by boiling with dilute sul- 

 phuric acid into glucose and salicylic alcohol. The solution, after 

 neutralisation with soda, reduces Fehling's solution. 



Reactions of Amygdalin. 



(1) Solutions of amygdalin do not reduce Fehling's solution. 



(2) Solutions of amygdalin are hydrolysed by boiling with dilute 

 nitric acid into benzaldehyde, hydrogen cyanide and glucose. The 

 solution smells of benzaldehyde and hydrogen cyanide. The presence 

 of hydrogen cyanide may be shown by testing with silver nitrate ; the 

 presence of glucose by neutralising with soda and testing with Fehling's 

 solution. 



Other glucosides are known which also contain hydrogen cyanide. 

 They are generally referred to as cyanogenetic glucosides. Their 

 presence in leaves may be detected by chewing a small piece of the 

 material, or better by introducing the bruised material and a drop of 

 chloroform into a small test tube, hanging a piece of picric acid test 

 paper l in it and closing it with a cork. Hydrogen cyanide is slowly 

 evolved and it colours the test paper orange red. 



1 This is prepared by dipping strips of filter paper into a i per cent, solution of picric 

 acid, drying them, wetting them with a 10 per cent, solution of sodium carbonate and 

 -again drying. 



