32 CHEMISTRY OF PLANT LIFE 



ISOMERIC FORMS OF MONOSACCHARIDES 



Four sugars having the formula CoH^Oe, namely, glucose, 

 fructose, mannose, and galactose, occur very commonly and 

 widely distributed in plants. In addition to these, thirteen others 

 having the same percentage composition have been artificially 

 prepared, while seven additional forms are theoretically possible. 

 In other words, twenty-four different compounds, all having the 

 same empirical formula and similar sugar-like properties are 

 theoretically possible. In order to arrive at a conception of this 

 multiplicity of isomeric forms, it is necessary to understand the 

 two types of isomerism which are involved. One of these is 

 structural isomerism, and the other is space- or sfereo-isomerism. 



Structural Isomerism. This refers to an actual difference 

 in the characteristic groups which are present in the molecule. 

 As has been said, all carbohydrates, from the standpoint of the 

 characteristic groups which they contain, are aldehyde-alcohols. 

 The hexoses all contain five alcoholic groups and one primary 

 aldehyde, or one secondary aldehyde (ketone), group. If the 

 aldehyde oxygen is attached to the carbon atom which is at the 

 end of the six-membered chain, the structural arrangement is 



that of an aldehyde, C = 0, and the sugar is of the type known 



H 



as "aldoses"; whereas, if the oxygen is attached to any other 



carbon in the chain, the ketone arrangement, C = O, results and 



I 



the sugar is a " ketose." This difference is illustrated in the 

 Fischer open-chain formulas for glucose (an aldose) and fructose 

 (a ketose) as follows: 



Glucose Fructose 



CH 2 OH CH 2 OH 



CHOH CHOH 



CHOH CHOH 



CHOH CHOH 



CHOH C = 



CHO CH 2 OH 



