72 THE CARBOHYDRATES 



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.* 



Xylose may be conveniently obtained, in about a 12 per 

 cent yield, by boiling I kg. of corn cobs f (previously soaked 

 and washed in 2 per cent ammonia solution) for 2 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 calcium hydroxide, avoiding 

 alkalinity. After filtering, the solution is mixed with 5 c.c. 

 of syrupy phosphoric acid, and decolorized with charcoal ; it 

 is then mixed with twice its volume of 95 per cent alcohol, 

 once more filtered, and evaporated in a vacuum to a thick syrup, 

 then mixed with 95 per cent alcohol, and allowed to crystallize. 

 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 which one of the hydrogen atoms has been replaced by a 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 3 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 180. 



* Widstoe and Tollens: ' Ber. deut. chem. Gesells.," 1900, 33, 136. 

 f Hudson and Harding: " J. Amer. Chem. Soc.," 1917, 39, 1038; 1918, 40, 

 1601. 



