Aldohexoses stereogenic centers

The stereogenic center at C-2 in the D-aldose becomes. s/r-hybridized in the enediol. The other pairs of D-aldohexoses that form the same enediols are... 

For each D-aldohexose there will be, as one more stereogenic center is added, two aldoheptoses. Altogether, there are 16 D-aldoheptoses. 

The number of possible stereoisomers of a monosaccharide increases exponentially with the number of stereogenic centers present. An aldohexose has four stereogenic centers, and so it has 2 = 16 possible stereoisomers, or eight pairs of enantiomers. 

Fischer projection formulas are also used for compounds like aldohexoses that contain several stereogenic centers. In this case, the molecule is drawn with a vertical carbon skeleton and the stereogenic centers are stacked one above another. Using this convention, all horizontal bonds project forward (on wedges). 

Finally, to form the D-aldohexoses, we must add another carbon atom (bonded to H and OH) Just below the carbonyl of all the aldopentoses. Because there are four D-aldopentoses to begin with, and there are two ways to place the new OH (right or left), there are now eight D-aldohexoses. Each aldohexose now has four stereogenic centers, so there are 2 = 16 possible stereoisomers, or eight pairs of enantiomers. Only the D-enantiomer of each pair is shown in Figure 27.4. 

---