186 HEXOSES 



Mannose, C H 12 6 , is the aldehyde of mannitol or mannite, 

 C 6 H 14 6 , a sweet substance occurring in many plants, especially in 

 the manna-ash (Fraxinus ornus), the dried sap of which constitutes 

 manna. The sugar is obtained by oxidising mannitol by means of 

 platinum black. It can also be obtained from the reserve cellulose 

 contained in many seeds, by the action of dilute sulphuric acid. 



Glucose, C 6 H 12 O 6 = CH 2 OH.(CHOH) 4 .CHO, also called dextrose 

 and grape sugar, occurs in many fruits, generally associated with 

 fructose. It is formed by the hydrolysis of poly saccharoses, e.g., 

 starch, dextrin and cellulose, of cane sugar, or of glucosides. 



Commercially, glucose is made by boiling starch with dilute sul- 

 phuric acid. So obtained, it is used in brewing and in the manufac- 

 ture of jams and sweetmeats. 



It crystallises with difficulty and is much less sweet than cane 

 sugar. It reduces copper solutions and easily undergoes fermentation. 



Its compounds with lime and baryta, C 6 H 12 6 .CaO and 

 C 6 H 12 O 6 .BaO, are insoluble in alcohol. 



Gulose, Galactose, Talose and Idose are isomeric sugars. All 

 these bodies are, like mannose and glucose, aldohexoses, i.e., contain 



the group HCO. 



The chief keto-hexoses are : 



Fructose, C 6 H 12 O 6 = CH 2 OH.(CHOH) 3 .CO.CH,OH, also called 

 levulose and fruit sugar, occurs in most sweet fruits. It is produced, 

 together with an equal amount of glucose, by the hydrolysis of cane 

 sugar : 



C ] . 2 H. 22 11 + H. 2 = C,H M 6 + C^O, 



Dextrose. Levulose. 



Since cane sugar is dextro-rotatory and levulose more laevo-rota- 

 tory than dextrose is dextro-rotatory, the mixture resulting from the 

 hydrolysis of cane sugar rotates the plane of polarisation to the left. 

 Hence the name invert sugar which is given to the mixture. Fructose 

 is also formed by the action of hot water upon inulin. 



Its constitution has already been given. 



Sorbinose or Sorbose occurs in the berries of the mountain ash, 

 probably as sorbitol, C (5 H 14 O 6 . It is a crystalline, very sweet sub- 

 stance, turned yellow by alkalies, and capable of reducing copper solu- 

 tions. It is not fermentable by yeast. 



The disaccharoses. These sugars consist of two molecules of 

 hexoses united together with the elimination of a molecule of water. 

 They, therefore, possess the composition C 12 H 22 O n . They are 

 probably to be regarded as ethers, i.e., the two hexose groups are 

 probably connected together through an atom of oxygen. Cane sugar 

 has no reducing power on copper salts and forms no osazone ; it, 

 therefore, probably contains no aldehyde nor ketone groups. Milk 

 sugar and maltose yield these reactions and probably contain the 

 aldehyde group CHO. 



The decomposition of the disaccharoses into hexoses by the 

 addition of a molecule of water (hydrolysis) can be brought about by 



