PENTOSES 59 



PROPERTIES OF INDIVIDUAL PENTOSES. 



Arabinose. 



Arabinose is best obtained by the hydrolysis of cherry gum 

 with 4 per cent sulphuric acid ; it can also be obtained by the 

 hydrolysis of gum arabic and of peach gum. Arabinose has a 

 very sweet taste, is dextro-rotatory, a D in 10 per cent solu- 

 tion = +105, crystallizes in prisms, and melts at 160; it 

 reduces Fehling's solution, and yields with diphenylhydrazine 

 a characteristic diphenylhydrazone, melting at 218.* 



Xylose. 



Xylose may be obtained by the hydrolysis of xylane or 

 wood gum, and also from brewers' grains, maize fruits, straw, 

 and various forms of cellulose. It is optically inactive, and 

 crystallizes in prisms, melting at 144-145. When oxidized 

 with bromine it gives xylonic acid, which may be identified 

 by the fact that it forms an insoluble double salt with cadmium 

 bromide.f 



Methyl Pentoses (CH 3 C 5 H 9 O 5 ). 



(a) Rhamnose. Rhamnose, sometimes wrongly called iso- 

 dulcite, has the empirical formula C 6 H 12 O 5 , and is a pentose in 

 which one of the hydrogen atoms has been replaced by a 

 methyl group, its constitution being represented by the formula 



CH S CHOH CHOH CHOH CHOH CHO 



In common with other methyl pentoses it yields on distilla- 

 tion with hydrochloric acid, methylfurfural ; this latter may be 

 detected by warming a little of the distillate with an equal 

 volume of concentrated hydrochloric acid, when a yellow 

 colour is produced. 



Rhamnose has been obtained by the hydrolysis of a 

 number of glucosides, e.g., quercitrin, hesperidin, and xantho- 

 rhamnin, and also saponins. The substance forms glistening 

 crystals, m.p. 93 ; a D = + 8*07, and gives a phenylosazone 

 melting at 1 80. 



* Neuberg : " Ber. d. deut. chem. Ges.," 1900, 33, 2243. 

 f Widstoe and Tollens : id., igoo, 33, 136. 



