58 CARBOHYDRATES MONOSACCHARIDES 



OHO CH 2 OH 



| I 



H C OH CO 



| I 



HO C H HO C H 



H C OH H C OH 

 | I 



H C OH H C OH 

 | I 



CH 2 OH CH 2 OH 



4-glucose d-fructose 



The mirror-image, which is in reality dextrorotatory, is therefore termed 

 l-fructose. The levorotation of d-fructose has led to its being very 

 generally designated levulose, by which name we will hereafter fre- 

 quently refer to it. 



Another ketose which occurs in nature is d-Sorbinose: 



HO 



which is formed when the juice of the mountain-ash is exposed to air, 

 by the oxidizing action of a ferment upon the alcohol sorbitol which is 

 contained in the juice. 



REACTIONS OF THE CARBOHYDRATES. 



The aldehyde and ketone, or potentially 1 aldehyde and ketone struc- 

 ture of the sugars renders them peculiarly liable to oxidation. Like 

 other aldehydes and ketones, they reduce metallic oxides in alkaline 

 solution; thus they reduce cupric to cuprous oxide, upon which fact 

 Fehling's method of sugar-estimation is based, and they reduce silver 

 salts in ammoniacal solution, leading to the formation of a silver mirror. 

 Other reactions which are characteristic of the sugar-group are the 

 following : 



On heating a solution of sugar in concentrated sodium or potassium 

 hydroxide, the liquid turns dark brown (Moore's test). 



If to about 0.5 c.c. of a dilute aqueous solution of glucose are 

 added a few drops of a ten per cent, alcoholic (acetone free) solution 

 of cHnaphthol, and 1 c.c. of concentrated sulphuric acid be cau- 

 tiously run into the lower part of the tube, so that the lighter solu- 

 tion floats upon the top of the heavy acid, the zone of contact becomes 

 reddish violet (Molisch's test). This reaction is due to the formation 



i Cf. below. 



