48 



CARBOHYDRATES AND THEIR 



[CH. 



In the above state the carbon atom marked * is also asymmetric so 

 that two forms of glucose are possible, a- and /3-glucose : 



HO 



HO--C H 



H C 



CH 2 OH 



a-Glucose 



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

 aldehyde form. 



In the plant there are, as will be described later (p. 142), many aromatic 

 and other compounds containing one or more hydroxyl 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. 152) : 



H C 



CH 2 OH 



Salicin 



These substances, moreover, may be classified either as a- or /3- 

 glucosides according to which of the above a or /3 forms of glucose 

 combine with the residual part of the compound. Various glucosides will 

 be dealt with in Chaps, vn and IX. 



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 / solution. 



() 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. 



