THE CARBOHYDRATES AND THEIR METABOLISM 217 



pound.- By oxidizing glucose with bromine, gluconic acid is obtained. 

 This has the same number of carbon atoms as glucose, and in this way the 

 presence of an aldehyde is indicated, a fact which is confirmed by oxidizing 

 glucose with nitric acid to saccharic acid, a dicarboxylic acid, also con- 

 taining six carbon atoms. 



C 6 tf 12 + O C 6 H 12 7 



Glucose Gluconic Acid 



C 6 H 12 7 + >C G H 10 8 



Gluconic Acid Saccharic Acid 



Owing to the stability of glucose it may be assumed that each hydroxyl 

 group is attached to a different carbon atom, and as glucose is a derivative 

 of normal hexane, as shown above, its formula may be written 



CHO 



CH OH 

 CH OH 

 CH OH 



CH OH 



1 



CH 2 OH 



This formula was originally proposed by Baeyer (1) and Fittig (2) 2 

 But glucose is far less active than might be expected of a compound that 

 is an hydroxyaldehyde. Thus it does not react easily with sodium sulphite, 

 pyrotartaric acid, nor with phenylhydrazineparasulphonic acid as might 

 be expected of a substance having the formula shown. It does not 

 undergo Perkins' reaction for aldehydes with acetic anhydride and so- 

 dium acetate. Aldehydes are generally more volatile than the corre- 

 sponding alcohols. This is not true of glucose. Moreover, glucose and 

 many of its derivatives, as shall be seen presently, occur in two isomeric 

 forms which exhibit no aldehyde properties at all. This difficulty was 

 overcome by Tollens' (1883) suggestion of a ring (the y-oxide or y-lactone) 

 formula for glucose. This formula has now been generally adopted. On 



* The presence of a ketone group (CO) in carbohydrates was first demonstrated 

 by Kiliani in 1885 when he showed that, unlike glucose, which owing to its aldehydic 

 nature yields compounds with the same number of carbon atoms when oxidized, fructose, 

 under similar conditions, yeilds a number of products having less than the same 

 number of carbon atoms than the original substance, as, for instance, trihydroxy- 

 butyric acid. 



