REACTIONS OF THE CARBOHYDRATES 63 



To a few cubic centimeters of solution is added an equal volume of 

 twenty per cent, solution of hydrochloric acid containing a small pro- 

 portion of resorcinol. The liquid turns red on heating and a red sub- 

 stance is gradually deposited which is soluble in alcohol. This reaction 

 depends upon the formation of oxymethyl-f urf urol from the ketose by 

 heating with acids. If the acid be too concentrated or the mixture 

 boiled for more than about twenty seconds, the hexoses will also yield 

 a small proportion of oxymethyl-f urf urol and will in consequence yield 

 the same reaction. It is necessary therefore to avoid a higher con- 

 centration of hydrochloric acid in the final mixture than about twelve 

 per cent., and to boil only for a period of less than twenty seconds. 

 This danger of confusion with the aldoses may be avoided by employing 

 glacial acetic acid containing a small proportion of hydrochloric acid, 

 instead of concentrated hydrochloric acid, as the solvent for the 

 resorcinol. 



The Chemical Relationships of the Sugars. Each sugar being an 

 aldehyde or ketose, or at any rate potentially an aldehyde or ketose, 

 corresponds to an alcohol from which it is derived by oxidation. Re- 

 duction of the sugars, therefore, results in the formation of alcohols. 

 Glucose yields Sorbitol, mannose yields Mannitol and galactose yields 

 Dulcitol. The following are the formulae which illustrate the structure 

 of these alcohols and their derivation from the corresponding sugars. 



CHO CH 2 OH CHO CH 2 OH 



I I I I 



HC OH H C OH HO C H HO C H 



I II I 



HO C H HO C H HO C H HO C H 



+ H 2 = + H 2 = | 



H C OH H C OH H C OH H C OH 



1 II I 



H C OH H C OH H C OH H C OH 



I II I 



CH 2 OH CH 2 OH CH 2 OH CH 2 OH 



d-glucose. d-sorbitol. d-mannose. d-mannitol. 



CHO CH 2 OH 



H C OH H C OH 



HO C H HO C H 



I + H 2 = | 



HO C H HO C H 



H C OH H C OH 



CH 2 OH CH 2 OH! 



d-galactose. d-dulcitol. 



All of these alcohols occur in plants, mannitol especially being widely 

 distributed. They have a sweet taste, but they are not fermentable 

 by yeast. 



Just as the sugars, being potential aldehydes or .ketoses, are con- 



