CARBOHYDRATES 



55 



GENERAL PROPERTIES OF CARBOHYDRATES 



Structures 



Carbohydrates are characterized by the presence of carbonyl groups, 

 often in acetal or semiacetal form, and hydroxyl groups. The carbonyl 

 groups are of tiie aldehyde or ketone type and typically react with 

 alcoholic hydroxyl groups to form semiacetals, also called hemi- 

 acetals. Since carbohydrates usually possess suitably located hydroxyl 

 groups on the same molecule as the carbonyl, the addition reactions 

 are internal to form cyclic derivatives. Natural carbohydrates are 

 largely found in these cyclic forms and might be represented thus: 



CH2- 

 CHOH 



-0 



CHOH 



HOCH. 



CHOH— CHOH 



semiacetal 



-CHOH— CHOH— CHOH— CHO 



aldehyde form 



O 



HOCH2— CH 



CHOH 



^CHOH— CHOH 



semiacetal 



Considerations of ring strain lead one to expect both of the systems 

 above, and semiacetals of both types can be made. Six-membered rings 

 of this type are derivatives of tetrahydropyran; five-membered types, of 



CHo 



CHo 



CH2 



CH2 





CH2 



CH2 



CH2 



CH. 



I. 



CHo 



O 



O 



tetrahydropyran tctrahydrofuran 



tetrahythofiaan. Carbohydrates are known correspondingly as py- 

 ranose or furanose forms. Many natural examples possess the first 

 ring system, a few the other, and some complex cases both, although 

 pyranose forms predominate. 



Semiacetals readily undergo reactions with alcohols to form acetals 



