54 THE CARBOHYDRATES 



CONSTITUTION AND ISOMERISM OF SUGARS. 



The analysis of any one of the hexose sugars, such as 

 dextrose, levulose, galactose or mannose, would yield the 

 same result; viz., 40 per cent of carbon, &6 per cent of 

 hydrogen, and 53*3 per cent of oxygen ; and this notwithstand- 

 ing the fact that these sugars are different substances. 



From the results of an analysis, it is possible to determine 

 the simplest ratio of the atoms to each other in the molecule 

 by dividing each percentage by the atomic weight of the 

 corresponding element, and then determining the simplest 

 numerical ratio between the resulting numbers : — 



C = 1^° = 3-3 ; H = ^ = 6-6; O = 53:3 ^ 3.3 

 12 I 16 



.•.C:H: = 3-3 : 6-6 : 3-3 



The formula CH^O thus arrived at, is known as the Empirical 

 Formula ; it indicates the fatio of the number of different 

 atoms in the molecule, but does not indicate their actual 

 number. The formula which, while maintaining the above 

 ratio, also shows the actual number of atoms present in the 

 molecule, is known as the Molecular Formula ; and it can only 

 be assigned correctly when the molecular weight is known. 

 Now the molecular weight of all these sugars is 180, hence 

 their molecular formula must be (CHaO)^ or C6Hi20g. 



Compounds such as the various hexoses which have the 

 same molecular formula and yet are not identical are said to 

 be isomers. 



The carbohydrates exhibit two kinds of isomerism, known 

 respectively as structural and stereo-isomerism. 



Structural isomerism is well illustrated by the two sugars 

 dextrose and levulose, A study of their reactions, which need 

 not here be detailed, leads to the conclusion that they both 

 contain five hydroxyl (OH) groups ; that dextrose belongs to 

 a class of compounds known as aldehydes, which are charac- 

 terized by the group— CHO; and that levulose is a ketone 

 and therefore contains the group = CO. These facts are all 

 explained by the following constitutional formulae : — 



