66 THE CARBOHYDRATES 



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

 the 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 : 



Dextrose. Levulose. 



CH a OH CH 2 OH 



CHOH CHOH 



CHOH CHOH 



CHOH CHOH 



CHOH CO 



CHO CH 2 OH 



Stereo-isomerism is the second type of isomerism, and is 

 exhibited by the three sugars dextrose, mannose and galac- 

 tose, all of which are aldehydes, and have therefore the same 

 structural formula. The possibility of isomerism in this case 

 is accounted for by the presence in these molecules of what 

 are known as asymmetric carbon atoms. Writing the formula 

 for dextrose once more in a slightly different way, it will be 

 seen that the carbon atom printed in " clarendon " (C) has its 

 four valencies attached respectively to the groups (CH 2 OH. 

 CHOH.CHOH.CHOH)-, H-, - OH, and -CHO: 



H 



CH 2 OH.CHOH.CHOH.CHOH C CHO 



OH 



Any carbon atom whose valency is satisfied by four different 

 groups or elements, whatever their nature may be, is said to be 

 asymmetric, since it is possible to represent it by either of two 

 solid models which are not super-imposable, the one being 

 the mirror image of the other ; there exists, therefore, between 

 two modifications of such an asymmetric carbon atom a dif- 

 ference due to the different spacial distribution of the four 

 substituting groups around it. Now the isomerism existing 

 between glucose and mannose is accounted for by their each 

 containing one of the two possible modifications of this same 

 asymmetric carbon atom. Similar considerations will show 



