THE CAKBOHYDRATES 15 



CH 3 .CHO + O=CH 3 .COOH. 



[acetic aldehyde] [acetic acid] 



The majority of the simple sugars are aldehydes of more complex 

 alcohols than this : they are spoken of as aldoses. The readiness with 

 which aldehydes are oxidisable renders them powerful reducing agents, 

 and this furnishes us with some of the tests for the sugars. 



Let us now turn to the case of the ketones. A secondary alcohol 

 is one in which the OH group is attached to a central carbon atom ; 

 thus secondary propyl alcohol has the formula 



CH 3 .CHOH.GH 3 . 



Its typical group is therefore CHOH. When this is oxidised, the 

 first oxidation product is called a ketone, thus : 



CH 3 .CHOH.GH 3 + 0=CH 3 .CO.CH 3 + H 2 O. 



[secondary propyl alcohol] [propyl ketone] 



It therefore contains the group CO in the middle of the chain. 

 Some of the sugars are ketones of more complex alcohols : these are 

 called ketoses. The only one of these which is of physiological interest 

 is levulose. 



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

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

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

 three OH groups (like 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) G 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 aldeliyde of mannite ; 

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

 dulcite. The sugars all have the empirical formula C 6 H 12 O 6 . The 

 constitutional formula for dextrose is : 



H H H H H H 



I I I I I I 

 H C C C C C C 



OH OH OH OH OH 



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

 If we take such a sugar as a typical specimen, we see that their general 

 formula is 



C B H 2m O OT 



