52 CARBOHYDRATES [ch. 



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. 61). From the mannans the sugar can be obtained by hydrolysis. 

 On adding phenylhydrazine hydrochloride and sodium acetate to a solu- 

 tion 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 dis- 

 tributed 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 sub- 

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

 instead of an aldehyde group : 



CH2OH 



I 

 c=o 



1 



HO— C— H 



I 

 H— C— OH 



I 

 H— C— OH 



! 

 CH2OH 



rf-Fructose 



Laevulose reduces Fehling's and other copper solutions. It yields 

 the same osazone as glucose with phenylhydrazine hydrochloride and 

 sodium acetate. It also forms an osazone with methylphenylhydrazine 

 (m.p. 158° C), a reaction which constitutes a distinction from glucose 

 since the latter gives no osazone with this substance. 



