22 CARBOHYDRATES 



B. Cyclic polyalcohols: inositol 



C. Sugar acids (other than uronic acids) 



1. Aldonic acids, C6H12O7: D-gluconic acid, n-mannonic acid, D-galactonic acid. 



2. Saccharic acids, CeHioOs : niucic acid, saccharic acid. 



3. Ketoaldonic acids, CgHiuOt: 2-Ketogluconic acid, 5-ketogluconic acid. 

 4. Ascorbic acids, CeHioOtt". ascorbic acid. 



MONOSACCHARIDES 



Simple sugars, or monosaccharides, containing 3, 4, 5, 6, and 7 carbon 

 atoms occur in nattire. They are called trioses, tetroses, 'pentoses, hexoses, 

 and heptoses, respectively. Those of the aldose type which contain 5 and 

 6 carbon atoms {aldopentoses and aldohexoses) are most common. 



Most of the monosaccharides may be represented by a formula of one 

 of the following types: 



CH2OH 



I y 



CHO CO 



I I 



(CHOH)„ (CHOH)„ 



CH2OH CH2OH 



Aldoses Ketoses 



(w = l,2,3,4, or 5) (n =0,1,2,3, or 4) 



Note that all the carbons are attached to each other and that each holds 

 an oxygen. Carbon 1 of the aldoses and 2 of the ketoses are especially 

 important because the most significant chemical reactions of the mono- 

 saccharides involve these points. 



Stereoisomerism of monosaccharides 



Organic molecules which contain an asymmetric carbon, i.e., a carbon 

 atom attached to four different atoms or groups, can exist in "right hand" 

 or ''left hand" forms. These are called dextro or d- and levo or L-forms, 

 respectively. The simple sugars all contain carbon atoms of this asym- 

 metric type, and hence can exist in the left- or right-handed patterns. 

 For example, the triose, giycer aldehyde, has one asymmetric carbon 

 (carbon 2, the center one) : 



(1) CHO CHO 



I I 



(2) H-C— OH HO-C— H 



I I 



(3) CH2OH CH2OH 



Carbon D-Form . L-Form 



number Glyceraldehyde 



