102 THE CARBOHYDRATES. [cH. V. 



It will be noticed that the first three are aldoses, 

 whilst fructose is a ketose. 



The first three are stereo-isomers, differing only in the 

 arrangement of the H and OH groups in space round the 

 four central carbon atoms, all of which are asymmetric. 

 (See p. 151.) It therefore follows that these compounds 

 are optically active, that is, their solutions can rotate the 

 plane of polarised light.* 



In the above formulae they are represented as being 

 aldehydes, but certain facts seem to indicate that they can 

 exist in another form. Thus, if glucose be dissolved in 

 water it is found that the solution at first has a much 

 higher rotatory power than when it has been kept for 

 some hours or has been boiled with a trace of alkali. This 

 phenomenon is known as mutarotation. Also it is very 

 much less active chemically than the above formulas 

 warrants. 



These properties are explained by assuming that when 

 first dissolved in water, glucose exists as a y-lactone, having 

 the formulae 



H*C.OH 



H.C 



H.C.OH 

 ,OH 



CH. 



* Ordinary glucose is dextro-rotatory. Fischer synthesised its laevo- 

 rotatory optical antipode which he called /-glucose. All those sugars which 

 are synthetically related to d-glucose or d-galactose he grouped into the d- class. 

 Thus ordinary fructose he named ^-fructose, though actually it is strongly 

 laevorotatory. 



