188 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Glucose, Grape Sugar, or Dextrose. 



Glucose occurs in the seeds, leaves and other parts of plants and together 

 with fructose in sweet fruits and honey. It is present to the extent of about 

 i per cent, in the blood of animals and in other organs of the animal body. 

 It is formed by the hydrolysis of cane sugar and other polysaccharides which 

 contain it. 



Small quantities of glucose are most conveniently prepared from cane 

 sugar as follows : 40 gm. of powdered cane sugar are added to a mixture of 

 5 c.c. of concentrated hydrochloric acid and 120 c.c. of 90 per cent, alcohol 

 heated to 45-50. The mixture is kept at this temperature for 2 hours with 

 occasional stirring and allowed to cool. Glucose crystallises out on cooling, 

 more rapidly after adding a crystal of anhydrous glucose which helps the 

 crystallisation. The crystals are filtered off, washed with alcohol and re- 

 crystallised from a mixture of 2 parts of alcohol and r of water. 



Commercially, glucose is prepared from the starch of potato, maize, 

 etc. The starch is hydrolysed by heating in copper vessels with dilute 

 sulphuric acid under 3 atmospheres pressure. The solution is neutralised 

 with chalk, the calcium sulphate filtered off and the filtrate heated with 

 animal charcoal to decolorise it. It is evaporated in vacuo to a syrup and 

 allowed to stand. The glucose crystallises in a cake of small crystals, which 

 are purified by crystallisation from dilute alcohol. 



Glucose crystallises from alcohol, or concentrated aqueous solutions at 

 30, in needles which are anhydrous. It crystallises from cold water in the 

 form of plates of the composition CjH 12 O<j . H 2 O. It is easily soluble in 

 water, very slightly soluble in absolute alcohol, but more soluble in methyl 

 alcohol. It is insoluble in ether. Glucose and other carbohydrates are difficult 

 to prepare free from moisture. This can only be effected by heating them 

 in vacuo at 70-110 in a vessel connected to phosphorus pentoxide (see p. 21). 



Fructose, Fruit Sugar, or Laevulose. 



Fructose occurs with glucose in fruits, honey, etc. It is most easily 

 prepared from the polysaccharide inulin by boiling it with 5 parts of '5 per 

 cent, sulphuric acid, or dilute oxalic acid, for i hour. The acid is removed 

 with barium carbonate and the solution is treated with charcoal and evaporated 

 to a syrup. The syrup is dissolved in alcohol from which fructose slowly 

 crystallises out. 



On a large scale fructose is made from cane sugar. The solution of cane 

 sugar which has been hydrolysed by dilute acid is neutralised and treated with 

 milk of lime. An insoluble calcium compound of fructose is formed; this is 

 filtered off, decomposed with carbon dioxide and the fructose obtained as above. 



Fructose crystallises from alcohol in the form of rhombic crystals ; it 

 crystallises from water in needles of the composition 2C 6 H l2 O6 H 2 O. It is 

 soluble in hot absolute alcohol and can thus be separated from other sugars. 



Mannose. 



Mannose does not occur as such in nature, but is widely distributed 

 as the polysaccharide mannan. It can be obtained by the oxidation of 

 mannitol, but is usually prepared by the hydrolysis of the mannan contained 

 in ivory-nut, which is used in making button-. The material is hydrolysed by 

 heating it on a water-bath for 6 hours with twice its weight of 6 per cent. 

 hydrochloric acid. The insoluble matter is removed by filtration and the 

 solution is decolorised by heating with animal charcoal, neutralised and 

 treated with phenylhydrazine acetate. Mannose phenylhydrazone is formed 

 from which the sugar is obtained by decomposing it with cold concentrated 

 hydrochloric acid. Mannose is a hard colourless solid, which deliquesces, is 

 easily soluble in water, slightly soluble in alcohol and insoluble in ether. 



