Aldohexoses hemiacetal formation

FIGURE 22.12 In an aldohexose, intramolecular hemiacetal formation results in either a furanose (five-membered ring) or a pyranose (six-membered ring). 

One new twist to a known reaction is intramolecular hemiacetal formation. Carbonyl groups react with nucleophiles in the addition reaction (Chapter 16). When the nucleophile is an alcohol, hemi-acetals are formed, but generally they are not favored at equilibrium. However, when a relatively strain-free ring can be formed in an intramolecular hemiacetal formation, the cyclic form can be favored. Such is the case for aldohexoses (and many other sugars), which exist mainly in the srx-membered pyranose forms (Fig. 22.12). 

Figure 9.5 Cyclic, hemiacetal structures of D-glucose. The reaction between an alcohol and aldehyde group within an aldohexose results in the formation of a hemiacetal. The only stable ring structures are five- or six-membered. Ketohexoses and pentoses also exist as ring structures due to similar internal reactions.

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