ISOMERISM 67 



that each of the three carbon atoms marked with a star is also 

 asymmetric, and it is therefore not surprising that it is possible 

 to account for no less than sixteen different isomeric aldehyde 

 sugars or aldoses ; of these, however, relatively few have been 

 found in nature. 



The constitution of glucose is ordinarily represented by 

 the formula CH 2 OH CHOH CHOH CHOH CHOH CHO, 

 which shows it to be a pentahydric alcohol and an aldehyde 

 at the same time. When dissolved in water, however, it 

 behaves in a peculiar manner, exhibiting the phenomenon 

 of muta-rotation, that is to say, the optical activity of the 

 resulting solution does not attain a steady value until some 

 time after the solution has been made up. 



The change is supposed to be connected with, some altera- 

 tion in its molecular configuration which may be explained by 

 assuming that the compound 



~ H \ 

 X OH 



is temporarily formed, but that water is thereupon split off 

 again between one of the hydroxyl groups of the terminal 

 carbon atom and the hydroxyl attached to the fourth carbon 

 atom as follows : 



OH 



I / OH 



CH 2 OH CHOH CH CHOH CHOH Cli( , 



\OH 



O 



CH 2 OH CHOH CH CHOH CHOH CHOH + H 2 O 



It will be seen that in this formula, sometimes known as 

 the 7 lactone or butylene oxide formula, the terminal carbon 

 atom (marked with an asterisk) has now become asymmetric, 

 whereas it was not so before ; this method of writing the for- 

 mula involves the possible existence of two optically isomeric 

 varieties of ordinary glucose, both of which are in fact known. ( 

 When glucose is crystallized from 70 per cent alcoholic solu- 

 tion at ordinary temperatures, a modification known as a 



Compare the formation of similar compounds from other aldehydes (p. 56). 

 fTanret: " Compt. rend.," 1895, 20. 1060; Lowry : "J. Chem. Soc.," 

 1899, 75. 2I 3 5 1903, 83, 1314. 



5 * 



