50 CARBOHYDRATES AND THEIR [CH. 



Expt. 42. Preparation of galactose from agar-agar. Weigh out 50 gms. of agar- 

 agar. Put it into a round- bottomed flask fitted with an air condenser (see p. 45). 

 Add 500 c.c. of 2 % sulphuric acid and heat on a water-bath for 4 hrs. Neutralize 

 the solution with calcium carbonate and filter. Concentrate on a water-bath to a 

 syrup. On standing, crystals of galactose will separate out. Then add a little 50-75 / 

 alcohol and warm gently on a water-bath. By this means much of the dark-coloured 

 product will go into solution and can be poured off leaving the crystalline residue. 

 Take up this residue in a little hot water, boil well with animal charcoal to decolorize 

 the solution and filter. . Concentrate again on a water-bath. On cooling, colourless 

 prisms of galactose will separate out. 



Expt. 43. Oxidation of galactose to mucic acid. Heat the galactose obtained in 

 the last experiment with nitric acid (1 gm. galactose to 12 c.c. of nitric acid of sp. gr. 

 1*15, i.e. 5 pts. of concentrated acid and 12 pts. of water) on a water-bath, until the 

 liquid is reduced to one-third of its bulk. Then pour the product into excess of 

 distilled water. On standing (for a day or two), a white sandy microcrystalline preci- 

 pitate of mucic acid will separate out. 



Mannose. Mannose has not been detected free in many plants, but 

 is widely distributed as condensation products, the mannans, in certain 

 mucilages and in the cell-walls of the endosperm of various seeds 

 (see p. 59). From the mannans the sugar can be obtained by hydrolysis. 

 On adding phenylhydrazine hydrochloride and sodium acetate to a 

 solution of mannose, the phenylhydrazone, which is nearly insoluble in 

 water, is formed almost immediately and hence constitutes a ready 

 method for the detection of the sugar. 



Laevulose. This sugar, which is also termed fructose, is widely 

 distributed in plants, in the tissues of leaves, stems, fruits, etc. It is 

 formed, together with glucose, in the hydrolysis by acids of cane-sugar. 

 The original cane-sugar is dextro-rotatory, whereas laevulose is more 

 laevo-rotatory than glucose is dextro-rotatory ; hence the mixture from 

 the hydrolysis is laevo-rotatory and is known as invert sugar, the change 

 being termed inversion. The same hydrolysis is brought about by the 

 widely distributed enzyme, invertase. The polysaccharide, inulin, also 

 yields laevulose on acid hydrolysis. Laevulose is a white crystalline 

 substance, soluble in water and alcohol. Unlike glucose, it contains a 

 ketone instead of an aldehyde group : 



CH 2 OH 



C = 



I 

 HO C H 



H-C OH 

 H C -OH 



CH 2 OH 

 c?-Fructose 



