What Are the Cyclic Structures of Monosaccharides

They are only slightly soluble in ethanol and are insoluble in nonpolar solvents such as diethyl ether, dichloromethane, and benzene. 

In Section 12.6, we saw that aldehydes and ketones react with alcohols to form hemiacetals. We also saw that cyclic hemiacetals form very readily when hydroxyl and carbonyl groups are parts of the same molecule and their interaction can form a five- or six-membered ring. For example, 4-hydroxypentanal forms a five-membered cyclic hemiacetal. Note that 4-hydroxypentanal contains one stereocenter and that a second stereocenter is generated at carbon 1 as a result of hemiacetal formation  

Because monosaccharides have hydroxyl and carbonyl groups in the same molecule, they exist almost exclusively as five- and six-membered cyclic hemiacetals. 

As you study the open chain and cyclic hemiacetal forms of D-glucose, note that, in converting from a Fischer projection to a Haworth structure, 

A six-membered hemiacetal ring is shown by the infix -pyran-, and a five-membered hemiacetal ring is shown by the infix -furan-. The terms furanose and pyranose are used because monosaccharide five- and six-membered rings correspond to the heterocyclic compounds pyran and furan  

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The actual structure of monosaccharides such as o-glucose is not what has been shown up to this point. We previously learned that aldehydes and ketones react reversibly with alcohols to form hemiacetals and hemiketals, respectively. (Refer back to Sec. 14.7 to review this reaction.) This reaction takes place intramolecularly with monosacchardes since both OH and C = O are present in the same molecule. The structures of aldoses and ketoses are cyclic hemiacetals and hemiketals, respectively. 

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