Monosaccharides: Structures of Glucose and Fructose

Written as a simple open (straight) chain, D-glucose is $\text{CHO}{-}(\text{CHOH})_4{-}\text{CH}_2\text{OH}$: an aldehyde group at C1, four successive $-\text{OH}$-bearing carbons (C2 through C5), and a terminal $-\text{CH}_2\text{OH}$ at C6. This open-chain picture explains a great deal of glucose's chemistry -- for instance, why it is oxidised by Fehling's/Tollens' reagents (the free aldehyde group) and why it can be reduced to the sugar alcohol sorbitol. But some of glucose's real behaviour cannot be explained by this open-chain structure alone. Glucose fails to give the characteristic colour reaction of aldehydes with Schiff's reagent, and, strikingly, glucose pentaacetate (in which all five $-\text{OH}$ groups have been converted to acetate esters) does not react with hydroxylamine at all, even though hydroxylamine reacts readily with any genuinely free aldehyde group. Also, pure crystalline glucose exists in two distinct forms with two different specific rotations, which slowly interconvert in solution until a single constant rotation is reached -- a phenomenon called mutarotation.

All of this evidence points to the same conclusion: in solution (and in the solid state), glucose does not exist as the open chain at all, but predominantly as a six-membered cyclic structure, formed when the $-\text{OH}$ group on C5 attacks the aldehyde carbon C1 intramolecularly. This creates a cyclic hemiacetal, drawn conventionally as a Haworth structure: a six-membered ring containing five carbon atoms and one ring oxygen (this six-membered oxygen-containing ring is called a pyranose ring, so this form is named glucopyranose), with the $\text{CH}_2\text{OH}$ group at C5 sitting outside the ring. …