48 CHEMISTRY OF PLANT LIFE 



has a specific rotatory power of 19.4 at 20 C., becomes optically 

 inactive at 82 C. 



Sorbose is the only other ketohexose which has any importance 

 in plant chemistry. It does not occur free in plants, but is the 

 first oxidation product from the hexatomic alcohol, sorbitol, which 

 is present in the juice of the berries of the mountain-ash. Sorbose 

 is a crystalline solid, which is not fermentable by yeast, but which 

 otherwise closely resembles fructose. 



DISACCHARIDES 



The disaccharides, having the formula Ci2H220n, may be 

 regarded as derived from the monosaccharides by the linking 

 together of two hexose groups with the dropping out of a molecule 

 of water, in the same way that many other organic compounds 

 form such linkages. That this is a perfectly correct conception, is 

 shown by the fact that, when hydrolyzed, the disaccharides break 

 down into two hexose sugars, thus 



Ci2H220n +H20 = CeHi^Oe+CeH^Oe. 



With all known disaccharides, at least one of the hexoses obtained 

 by hydrolysis is glucose; hence all disaccharides may be regarded 

 as glucosides (CeH^Os'R) in which the R is another hexose 

 group. 



Since hexoses have both alcoholic and aldehyde groups, and 

 since either of these types of groups may function in the linkage 

 of the two hexoses to form a disaccharide, it is possible for two 

 hexoses, both of which are reducing sugars to be linked together 

 in three different ways: (1) through an alcoholic group of each 

 hexose, (2) through an alcoholic group of one and the aldehyde 

 group of the other, and (3) through the aldehyde group of each 

 hexose. Disaccharides linked in either of the first two ways will 

 be reducing sugars, since they still contain a potentially active 

 aldehyde group; but those of the third type will not be reducing 

 sugars, since the linkage through the aldehyde groups destroys 

 their power of acting as reducing agents. Examples of each of 

 these three types of linkage are found among the common disac- 

 charides, as will be pointed out below. 



The following table shows the general characteristics of the 

 common disaccharides. 



