92 THE CARBOHYDRATES 



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 and mesquite gum * 

 (Prosopis jutiflora). Arabinose has a very sweet taste, is 

 dextro-rotatory, ac in lO per cent solution =+ 105°, crystal- 

 lizes in prisms, and melts at 160° ; it reduces Fehling's solution, 

 and yields with diphenyl hydrazine a characteristic diphenyl 

 hydrazone, melting at 204-205°. f 



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 a very sweet substance 

 and shows an optical activity of aj) =+ 19° i^i a 10 per cent 

 solution, it crystallizes in prisms, melting at 144-145°, and gives 

 a phenylosazone of melting-point, 161°. When oxidized with 

 bromine and boiled with cadmium carbonate it yields cadmium 

 xylonate, which with the cadmium bromide in solution forms 

 a sparingly soluble crystalline double salt 



(C5H906)2Cd . CdBrj . 2H2O.J 



Xylose may be conveniently obtained, in about a 12 per 

 cent yield, by boiling i kg. of corn cobs § (previously soaked 

 and washed in 2 per cent ammonia solution) for two hours under 

 a reflux condenser with 8 litres of 7 per cent sulphuric acid. 

 The solution is filtered on a Buchner funnel through cloth, and 

 is then carefully neutralized with precipitated chalk. After 

 filtering, the solution is treated with lead acetate, filtered, 

 freed from lead by hydrogen sulphide, again filtered and 



* Anderson and Sands : " Ind. Eng. Cliem.," 1925, 17, 1257. 



t Neuberg : " Ber. deut. chem. Gesells.," 1900, 33, 2243 ; ToUens and 

 Maurenbrecher : id., 1905, 38, 500. 



X Widstoe and ToUens : id., 1900, 33, 136. 



§ Hudson and Harding : " J. Amer. Chem. Soc," 1917, 39, 1038 ; 

 1918,40,1601. Ling and Nanji : " J. Chem. Soc," 1923, 123, 620. Irvine, 

 loc. cit. 



