CH. XXVIII.] THE CARBOIIYDEATKS 407 



of these are glycogen, or animal starch ; dextrose ; and lactose, or milk 

 sugar. 



The carbohydrates may be conveniently defined as compounds of 

 carbon, hydrogen, and oxygen, the two last-named elements being in 

 the proportion in which they occur in water. But this definition is 

 only a rough one, and if pushed too far would include many substances 

 such as acetic acid, lactic acid, and inosite, which are not carbohydrates. 

 Kesearch has shown that the chemical constitution of the simplest 

 carbohydrates is that of an aldehyde, or a ketone, and that the more 

 complex carbohydrates are condensation products of the simple ones. 

 In order, therefore, that we may understand the constitution of these 

 substances, it is first necessary that we should understand what is 

 meant by the terms aldehyde and ketone. 



A primary alcohol is one in which the hydroxyl (OH) and two 

 hydrogen atoms are attached to the same carbon atom ; it, therefore, 

 contains the group CH 2 OFI. Thus the formula for common alcohol 

 (primary ethyl alcohol) is 



CH 3 .CH 2 OH. 



The formula for the next alcohol of the same series (primary 

 propyl alcohol) is 



CH 3 .CH 2 .CH 2 OH. 



If a primary alcohol is oxidised, the first oxidation product is 

 called an aldehyde ; thus ethyl alcohol yields acetic aldehyde : 

 CH 3 .CH 9 OH + O = CH 3 .COH + H 2 O. 



[Ethyl alcohol.] [Acetic aldehyde.] 



The typical group COH of the aldehyde is not stable, but is easily 

 oxidisable to form the group COOH (carboxyl), and the compound so 

 formed is called an acid ; in this way acetic aldehyde forms acetic 

 acid : 



CH 3 .COH + 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 and one hydrogen atom are attached to the 

 same carbon atom ; thus secondary propyl alcohol has the formula 



CH 3 .CHOH.CH 3 . 



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

 first oxidation product is called a ketone, thus : 



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



[Secondary propyl alcohol.] [Propyl ketone or acetone.] 



It therefore contains the group CO. 



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



