If 2 of the substituents of a carbon atom are the same (e.g., double bond to an oxygen or single bonds to 2 hydroxyls or to 2 hydrogens) that carbon is achiral.
Sugars contain chiral carbons which are those carbons having 4 different substituents. 6.17 in a simple stick model ( Fischer projection) and how it is closed into a ring structure ( Haworth projection). The structure of glucose is shown in Fig. Glucose is the most representative member of the hexoses. Six-carbon monosaccharides are prevalent but monosaccharides can have from 3 to 7 carbons in their structures (3 carbons, triose 4 carbons, tetrose 5 carbons, pentose, 6 carbons, hexose and 7 carbons, heptose). Sugars are made up of carbon, oxygen and hydrogen no nitrogen, except for amino sugars. Simple sugars are single molecules, whereas more complex sugars are more than one molecule joined together by chemical bonds.
