94 THE CARBOHYDRATES 



obtained from pentoses under the same conditions gives a 

 pink colour. 



[a] 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^ =+ 8-07°, and gives a phenylosazone 

 melting at 180°, and a naphthyl hydrazone melting at 192°. 



[h) Fucose, which is isomeric with rhamnose, may be 

 obtained by the hydrolysis of sea-weeds by means of dilute 

 sulphuric acid ; it crystallizes in microscopic needles, and 

 yields a hydrazone, m.p. 172-173°. 



(c) Quinovose, another methyl pentose isomeric with rham- 

 nose, is produced by the hydrolysis of quinovite, a substance 

 formed by boiling quinovin contained in the bark of Cascarilla 

 hexandra with alcohol and hydrochloric acid. 



[d) Isorhamnose and Rhodeose are two methyl pentoses 

 obtained by the hydrolysis of the glucoside convolvulin. 



B. HEXOSES. 



Theory accounts for the existence of no less than thirty- 

 two sugars of the molecular formula CgHiaOe having a straight 

 six carbon atom chain. Of these sixteen are aldoses of the 



type 



CHgOH . CHOH . CHOH CHOH CHOH . CHO 



Aldohexose 



and the remaining sixteen are ketoses containing the ketonic 

 group attached either to the second or third carbon atom of 

 the chain — 



CH2OH CHOH . CHOH CHOH . CO . CHjOH or 



CH2OH CHOH CHOH CO . CHOH CH^OH 

 2 Ketohexose 3 Ketohexose 



The various possible isomeric aldoses and ketoses differ 

 only in the spatial relationships of the OH and H groups. 

 Although most of the aldoses and a few of the ketoses have 

 been synthesized, only three aldoses, glucose, mannose, and 

 galactose, and two ketoses, fructose and sorbose, have so 

 far been identified in nature. 



