408 THE CHEMICAL COMPOSITION OF THE BODY [CH. XXVIII. 



are called Jcetoses. The only one of these which is of physiological 

 interest is laevulose. 



The alcohols of which we have already spoken are called monatomic, 

 because they contain only one OH group. Those which contain two 

 OH groups (such as glycol) are called diatomic ; those which contain 

 three OH groups (such as glycerin) are called triatomic ; and so on. 

 The hexatomic alcohols are those which contain six OH groups. Three 

 of these hexatomic alcohols with the formula C 6 H 8 (OH) 6 are of 

 physiological interest ; they are isomerides, and their names are sorbite, 

 mannite, and dulcite. By careful oxidation their aldehydes and 

 ketones can be obtained ; these are the simple sugars ; thus, dextrose 

 is the aldehyde of sorbite; mannose is the aldehyde of mannite; 

 laevulose is the ketone of mannite ; and galactose is the aldehyde of 

 dulcite. These sugars all have the empirical formula C 6 H 12 6 . They 

 furnish an excellent example of what is called stereochemical 

 isomerism ; that is, the position of the atoms or groups of atoms in 

 space within the sugar molecule varies. The constitutional formulae 

 of three important simple sugars are shown below. The six carbon 

 atoms in each case form an open chain, but the way in which the 

 hydrogen and hydroxyl atoms are linked to them differs. 



CH 2 OH CH 2 OH CH 2 OH 



H C OH H C OH H C OH 



HC OH H C OH OH C H 



I 1 i 



OH C H OH C H OH C H 



HC OH C-O HC OH 



I I I 



C OH CH 2 OH C OH 



[Dextrose.] [Laevulose.] [Galactose.] 



The aldehyde constitution of dextrose and galactose is at once 

 evident, the typical aldehyde group (COH) being at the end of the 

 chain, whereas the ketone constitution of laevulose is shown by the 

 typical ketone group (CO) not at the end of the chain. 



By further oxidation, the sugars yield acids with various names. 



If we take such sugars as typical specimens, we see that their 

 general formula is 



C n H 2m O m 



and as a general rule n = m ; that is, the number of oxygen and carbon 

 atoms are equal. This number in the case of the sugars already 

 mentioned is six. Hence they are called hexoses. 



Sugars are known to chemists, in which this number is 3, 4, 5, 7, etc., and 

 these are called trioses, tetroses, pentoses, heptoses, etc. The majority of these 



