528 CONSIDERATION OF CARBON COMPOUNDS. 



48. CARBOHYDRATES. 



General remarks. The name carbohydrates was originally given 

 to a class of compounds found chiefly in plants, and containing in 

 the molecule 6 atoms of carbon (or a multiple of 6) in combination 

 with hydrogen and oxygen in the proportion to form water, as shown 

 in the formula for grape-sugar, C 6 H 12 O 6 , cane-sugar, C 12 H 22 O U , etc. 

 While even formerly the name was not well chosen, because it 

 implies that these subfetances are carbon in combination with water, 

 to-day it is still less suitable, because members of the group have been 

 found which do not contain oxygen and hydrogen in the proportion 

 mentioned ; as, for instance, a sugar, termed rhamnose, having the 

 composition C 6 H 12 O 5 . We also know now carbohydrates containing 

 carbon atoms in numbers which have no relation to 6. While, there- 

 fore, the term carbohydrate no longer implies what it formerly 

 did, and no longer refers to the restricted number of compounds 

 which it formerly included, yet it is retained for the whole group of 

 compounds now to be considered. 



The group includes now, as heretofore, the different sugars, starches, 

 gums, etc., and also a number of compounds obtained by artificial or 

 synthetical processes. In order to show in its name that a substance 

 belongs to the carbohydrates, the ending ose is used to distinguish 

 these bodies from the members of other groups. 



Constitution. While the true atomic structure of many carbo- 

 hydrates is as yet not fully understood, the structure of others is 

 well known. It appears that some carbohydrates are true aldehydes, 

 while others are closely related to ketones, and yet others are the 

 anhydrides, or condensation products of the former. 



Thus, a sugar of the composition C 3 H 6 O 3 , termed glycerose, is obtained by the 

 action of mild oxidizing agents on glycerin, thus : 



C 3 H 8 3 + O = C 3 H 6 3 + H 2 0. 



If we bear in mind the fact that glycerin, C 3 H 5 (OH) 3 , is a triatomic alcohol, 

 and that alcohols by oxidation yield aldehydes, we realize the analogy existing 

 between the above reaction and that leading to the formation of the aldehydes, 

 previously considered. 



In a manner similar to the one producing glycerose from glycerin, a sugar 

 of the composition C 4 H 8 O 4 , and called erythrose, is obtained from the tetratomic 

 alcohol erythrite, C 4 H 6 (OH) 4 , while a sugar of the composition C 6 H 12 O 6 is ob- 

 tainable from the hexatomic alcohol mannite, C 6 H 8 (OH) 6 . In both cases two 

 atoms of hydrogen are split off from the alcohol molecules. 



The relationship existing between the sugars of the composition C 6 H ]2 O 6 



