2H A. I. RIXGER AXD EM1L J, BAUMANN 



2. Chemistry of the Carbohydrates 



Classification and Nomenclature. The carbohydrates, as has already 

 been indicated, are composed of carbon, hydrogen and oxygen, usually 

 having the formula CxII^nO* There are many substances having this 

 iienoric formula that are not carbohydrates, e. g., CH 3 -CIIOII-COOII 

 (lactic acid), but a more comprehensive definition, will develop as the 

 subject is presented. 



Carbohydrates may be divided into three great groups, according 

 to the number of saccharide groups (simple sugars) they contain: 

 monosaccharides, disaccharides, polysaccharides. Important monosac- 

 charides are d-glucose or grape sugar, d-fructose or levulose, d-mannose, 

 d-arabinoso and d-ribose. Common disaccharides are sucrose or cane 

 sugar (also known as saccharose), lactose or milk sugar, and maltose or 

 malt sugar. These comparatively simple carbohydrates are often called 

 sugars. Common polysaccharides are cellulose, starches, dextrin s, glyco- 

 gen and gums. 



The monosaccharides are further divided according to the number 

 of carbon atoms they contain trioses, pentoses, hexoses, octoses, nonoses, 

 etc. Those found occurring in nature are chiefly the tetroses, pentoses, 

 hexoses and a few heptoses. Some of the carbohydrates have the proper- 

 ties of an alcohol and aldehyde, others of -an alcohol and a ketone, and 

 these are known respectively as aldoses and ketoses. So an aldehyde sugar 

 having six carbon atoms would be called an aldo-hexose, and a ketone 

 sugar having six carbon atoms would be called a keto-hexose. 



Constitution. In the discussion of the structure of the carbohydrates, 

 d-glucose will be used as a typical example of the aldoses. The mariner 

 in which the elements carbon, hydrogen and oxygen are combined in 

 these compounds has been a problem which has gradually been elucidated 

 during the last century, although the last word on the subject has not yet 

 been written. The first step in the solution of the problem may be said 

 to have been devised by Liebig, when he gave forth his method for deter- 

 mining the percentages of carbon and hydrogen in organic matter. With 

 the development of definite concepts of valency by Kekule and others, 

 and of the asymmetric carbon atom by Le Bel and \ r an't Hoff in 1875, a 

 fairly definite idea of the structure of these substances became known. 



As shown by elementary analysis, glucose has the empirical formula 

 CII 2 O, and the molecular formula, C,jII 12 O , as shown by molecular 

 weight determinations, by the cryoscopic and ebullioscopic methods. When 

 treated with acids, acid anhydrids and acid chlorides, glucose forms ethe- 

 real salts or esters, 1 e.g., acetvl chloride will form a glucose pentacetate, 

 C 6 H 7 0(O.CO.CII :! ) 5 . 



1 Alcohols are compounds of carbon containing one or more hydroxyl groups, as 

 CHjOH, methyl alcohol. An organic acid is a compound containing a carboxyl group 



