The Cyclic Hemiacetal Structures of Monosaccharides

The monosaccharide structures described so far are consistent with much of the known chemistry of these compounds, but they are oversimplified. We now examine the true structures of these compounds. 

We learned earlier that alcohols undergo rapid and reversible addition to the carbonyl group of aldehydes and ketones, to form hemiacetals (review Sec. 9.7). This can happen intramolecularly when the hydroxyl and carbonyl groups are properly located in the same molecule (eqs. 9.14 and 9.15), which is the situation in many monosaccharides. Monosaccharides exist mainly in cyclic, hemiacetal forms and not in the acyclic aldo- or keto-forms we have depicted so far. 

Manipulation of the Fischer projectioh formula of D-glucose to bring the C-5 hydroxyl group ih position for cyclization to the hemiacetal form. 

The carbons are arranged ciockwise numericaiiy, with C-1 at the right. 

Sometimes, as in the structure at the right, the ring hydrogens are omitted so that attention can be focused on the hydroxyl groups. 

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