THE CARBOHYDRATES 19 



are known ; also that other chemical isomers, e.g. galactose 

 (which is an aldose like glucose) and sorbose (which is a 

 ketose like fructose), result from the rearrangement of the 

 groups in the molecule as a whole. 



Owing to the comparative readiness with which they 

 undergo oxidation, the hexose sugars exhibit the properties 

 of reducing agents. Thus, on warming, they reduce ammo- 

 nical solutions of silver salts, and alkaline cupric solutions. 

 Many different preparations of this kind, e.g. Tollen's, 

 Trommer's, Barfoed's, and Nylander's reagents, are used as 

 tests for the sugars. Fehling's solution is one of the oldest 

 and probably the best for most purposes. It is made by 

 adding strong caustic potash or soda to a solution of cupric 

 sulphate with which some alkaline tartrate has been mixed to 

 prevent precipitation of the cupric hydrate. On warming with 

 hexose sugars, this solution gives a bright red precipitate of 

 cuprous oxide, Cu 2 O. The amount of precipitate is proportional 

 to the quantity of sugar present; but when the reaction is 

 used quantitatively, the Fehling's l solution must be made and 

 used in the prescribed manner. 



The synthetical preparation of hexoses can be effected 

 in several ways. Two of these are of special interest, viz. 

 (i) by condensation of formic aldehyde, and (2) from acrolein 

 bromide. 



Formic aldehyde, CH 2 O, is known only in solution and 

 in the state of vapour. At ordinary temperatures it changes 

 rapidly into trioxymethylene (CH 2 O) 3 ; and this, in the 

 presence of lime, is further changed into a mixture of several 

 hexose sugars from which both glucose and fructose can be 

 isolated. 



Acrolein bromide is related to propylene as shown below : 



CH CH 3 CH 2 =CH CH 2 OH 



(Propylene.) (Allyl alcohol.) 



CH 2 =CH-CH.O CH 2 Br-CHBr CH.O 



(Allyl aldehyde or acrolein.) (Acrolein bromide.) 



1 Dissolve in separate portions of water 34*65 grains of crystallised 

 cupric sulphate (CuSO 4 + 5H 2 O)i 2O grams Rochelle salt, KNaT, and 

 1 80 grams of caustic soda ; then mix the solutions and make up to one litre. 



