THE CARBOHYDRATES 



18; 



Conditions, two compounds, one of which, a-glucose, has a high initial 

 rotatory power of 110, the other, /3-glucose, a low one of 19. On 

 being kept in solution both compounds give a solution of the same 

 rotatory power of 52*5. 



(2) Two isomeric methyl etc. derivatives (glucosides) are also ob- 

 tainable from glucose. 



(3) Glucose in its chemical behaviour is also less active than is 

 expected. 



These properties are most satisfactorily explained by assuming that 

 glucose has a 7-lactone formula, i.e. a formula in which four carbon 

 atoms and an oxygen atom are included in a ring. It is derived from 

 the hypothetical aldehydrol : 



c! 



HOH 

 CHOH 

 CHOH 



CHOH 

 I 

 CHO 



CHOH 

 CHOH 



CHOH 

 I 

 CHOH 



-OH 



CH 2 OH 



I 



[OH 



Aldehyde. Aldehydrol. -y-lactone. 



Under these conditions the carbon atom to which the aldehyde group 

 was attached becomes asymmetric. Two stereoisomeric forms are 

 therefore possible. These represent a- and /3-glucose : 



CH 2 OH 



OH 



CH 2 OH 

 H-C-OH 



H 



H C-OH HO-C-H 



Each form in solution will change into the aldehydrol form and 

 then into the other form. A solution of constant rotatory power will 

 contain an equilibrium mixture of a- and y3-glucose depending on the 

 concentration. The solution will give the reactions of an aldehyde as 

 it will contain a small quantity of aldehydrol (aldehyde). The two 

 derivatives are obtained from the a- and /3-forms. 



