50 CARBOHYDRATES [ch. 



In solution, both the above forms pass by tautomerism into the 

 aldehyde form. 



In the plant there are, as will be described later (p. 157), many aromatic 

 and other compounds containing one or more hydroxy 1 groups. These 

 hydroxyl groups of the aromatic substances are frequently replaced by 

 a glucose (or other sugar) molecule, and such compounds are termed 

 glucosides, as, for instance, salicin, the glucoside of salicylic alcohol which 

 occurs in Willow bark (see p. 167) : 



HO 



CH2OH 

 Salicin 



These substances, moreover, may be classified either as a- or /8- 

 glucosides according to which of the above a or ^ forms of glucose has 

 combined with the residual part of the compound. Various glucosides 

 will be dealt with in Chaps, viii and X. 



Expt. 41. Tests for glucose. Before dealing with the sugars actually isolated from 

 the plant, it is advisable that the following tests and reactions should be performed 

 with pure glucose using a 0'2^/q solution. 



(a) Moore's test. Boil a little of the glucose solution with an equal volume of 

 caustic soda solution. A yellow colour is developed which is due to the formation of 

 a condensation product (caramel) of the sugar. 



(6) Trommer's test. Add a few drops of a 1 % copper sulphate solution to 2-3 c.c. 

 of 5 0/0 caustic soda solution. A blue precipitate of cupric hydroxide is formed. Add 

 now 2-3 c.c. of the glucose solution, and the precipitate will dissolve. On boiling, 

 the blue colour disappears, and a yellow or red precipitate of cuprous oxide is formed. 

 If only a little sugar is present the blue colour will disappear, but no oxide may be 

 formed. 



(c) Fehling's test. Boil a few c.c. of freshly made Fehling's solution in a test-tube 

 and note that it is unaltered. Then add an equal quantity of the glucose solution 

 and boil again. A red precipitate of cuprous oxide is formed. 



(d) Osazone test. Take 10 c.c. of a 0*5 ^Iq solution of glucose in a test-tube and 

 add as much solid phenylhydrazine hydrochloride as will lie on a sixpenny piece, 

 at least twice as much solid sodium acetate and also 1 c.c. of strong acetic acid. 



