CARBOHYDRATES bl 



CH, O 



H / \ H 



C C ±=; HOCH2— CHOH— CHOH— CHOH— CHO ^ 



OH\ / OH 



CHOH— CHOH 



CH. O 



H // \ OH 



\/ \i 



c c 



1\ /\ 



OH\ / H 



CHOH— CHOH 



or 



CH2OH 



C O 



/\ \ H 



/ H \| 



HOHC C 



\ /OH 



CHOH— CHOH 



a-pyranoses 



HOCH2— CHOH— CHOH— CHOH— CHOH— CHO ;=± 



CH.OH 



C O OH 



HOHC 



\ / 



\ / H 

 CHOH— CHOH 



/3-ijyranoscs 



of the forms depends on the other groups present, these exerting a 

 directive effect favoring one over the other because the other groups 

 are themselves asymmetric. In other words, the a and /? isomers are 

 not mirror images but are typical examples of the diastereoisomers 

 discussed on page 54 and as such differ in many of their properties. 

 This equilibrium transition leading to the mixture of isomers is called 

 nuitarotation. This term reflects the discovery of the phenomenon by 

 way of changes in the optical rotations of certain carbohydrates. 



Mutarotation is quite characteristic of semiacetal but not of acetal 

 linkages. In aqueous solutions of the latter, the equilibrium, if there 

 is one, is so far toward the acetal form that changes are not observed. 

 Thus acetals of the carbohydrate type show no spontaneous tendency 

 to revert to semiacetals or to the acyclic forms. Nor, when acetals are 

 deliberately hydrolyzed to the semiacetals, is reversion to the acetals 

 spontaneous. 



