GENERAL PROPERTIES OF THE SUGARS 369 



The dlsaccharides are formed by the condensation of two molecules of 

 monosaccharides. All of the important disaccharides are formed by con- 

 densation of two hexose molecules. The combining monosaccharide molecules 

 may both be the same kind or of different kinds. Trisaccharides are formed 

 by the condensation of three molecules of monosaccharides, and tetrasaccharides 

 by the condensation of four monosaccharide molecules. Upon hydrolysis these 

 tA'pes of carbohydrates produce two, three, or four monosaccharide molecules 

 respectively. The important tri- and tetrasaccharides are all formed by the 

 condensation of molecules of hexose sugars. 



The polysaccharides are produced by the condensation of a large number 

 of monosaccharide molecules. In most of the polysaccharides all of the con- 

 densing molecules are of the same kind, although some are formed by the 

 condensation of molecules of different kinds of monosaccharides. 



The compound carbohydrates are formed by the condensation of one or 

 more monosaccharide molecules with non-carbohydrate molecules. Some of 

 the groups listed under this heading, notably the tannins, can be considered 

 as only remotely related to the carbohydrates. 



General Properties of the Sugars. — The mono-, di-, tri-, and tetra- 

 saccharides are collectively called the sugars. All of these compounds possess 

 the property of sweetness and all of them are white, more or less crystalline 

 compounds which are soluble in water. 



1. Reducing and Non-reducing Sugars. — All of the monosaccharides and 

 some of the more complex sugars act as reducing agents. This action is due 

 to the presence of an aldehydic or ketonic group in the sugar molecule. Those 

 compound sugars in which the linking of the monosaccharides has occurred 

 in such a manner that the aldehydic or ketonic groups have lost their usual 

 reactivity are non-reducing. Sugars are commonly classified on this basis as 

 reducing sugars or non-reducing sugars. The reducing action of sugars is 

 most commonly determined by means of Fehling's or Benedict's solution, in 

 which, upon heating, a reducing sugar converts cupric hydroxide into cuprous 

 oxide. The latter compound separates from the solution in the form of a 

 reddish precipitate. These solutions are used not only for the qualitative 

 demonstration of reducing sugars but for their quantitative estimation, since 

 the quantity of precipitate formed, although not directly proportional, bears a 

 definite relation to the amount of sugar taking part in the reaction. 



2. Optical Activity. — Most of the soluble carbohydrates are, like many 

 other organic compounds, optically active when in solution. The optical 

 activity of a solution of carbohydrate refers to its property of rotating the 

 plane of polarized light. The specific rotatory power of a sugar is expressed 

 in terms of the number of degrees of angular rotation of the plane of polarized 



