Editing Monosaccharide (section)

=== {{vanchor|Cyclisation}} of monosaccharides (hemiacetal formation) ===
A monosaccharide often switches from the acyclic (open-chain) form to a [[cyclic compound|cyclic]] form, through a [[nucleophilic addition]] reaction between the carbonyl group and one of the hydroxyl groups of the same molecule.  The reaction creates a ring of carbon atoms closed by one bridging [[oxygen]] atom.  The resulting molecule has a [[hemiacetal]] or [[hemiketal]] group, depending on whether the linear form was an aldose or a ketose.  The reaction is easily reversed, yielding the original open-chain form.

In these cyclic forms, the ring usually has five or six atoms. These forms are called [[furanose]]s and [[pyranose]]s, respectively—by analogy with [[furan]] and [[pyran]], the simplest compounds with the same carbon-oxygen ring (although they lack the double bonds of these two molecules). For example, the aldohexose [[glucose]] may form a hemiacetal linkage between the aldehyde group on carbon 1 and the hydroxyl on carbon 4, yielding a molecule with a 5-membered ring, called [[glucofuranose]]. The same reaction can take place between carbons 1 and 5 to form a molecule with a {{Nowrap|6-membered}} ring, called [[glucopyranose]]. Cyclic forms with a seven-atom ring (the same of [[oxepane]]), rarely encountered, are called [[heptose]]s.

[[File:Existence in solution.png|center|thumb|500px|Conversion between the furanose, acyclic, and pyranose forms of {{sc|D}}-glucose]]
[[File:structure of D-pentoses.png|center|thumb|500px|Pyranose forms of some pentose sugars]]
[[File:structure of D-hexoses.png|center|thumb|500px|Pyranose forms of some hexose sugars]]

For many monosaccharides (including glucose), the cyclic forms predominate, in the solid state and in solutions, and therefore the same name commonly is used for the open- and closed-chain isomers.  Thus, for example, the term "glucose" may signify glucofuranose, glucopyranose, the open-chain form, or a mixture of the three.

Cyclization creates a new [[stereogenic]] center at the carbonyl-bearing carbon.  The −OH group that replaces the carbonyl's oxygen may end up in two distinct positions relative to the ring's midplane.  Thus each open-chain monosaccharide yields two cyclic isomers ([[anomer]]s), denoted by the prefixes α- and β-.  The molecule can change between these two forms by a process called [[mutarotation]], that consists in a reversal of the ring-forming reaction followed by another ring formation.<ref>{{cite book |last=Pigman |first=William Ward |author-link= William Ward Pigman |author2=Anet, E. F. L. J. |title=The Carbohydrates: Chemistry and Biochemistry |volume= 1A |editor=Pigman and Horton |edition=2nd|year=1972 |publisher=Academic Press |location=San Diego |pages=165–194 |chapter= Chapter 4: Mutarotations and Actions of Acids and Bases }}</ref>

====Haworth projection====
The stereochemical structure of a cyclic monosaccharide can be represented in a [[Haworth projection]]. In this diagram, the α-isomer for the pyranose form of a {{sc|D}}-aldohexose has the −OH of the [[anomeric carbon]] below the plane of the carbon atoms, while the β-isomer has the −OH of the anomeric carbon above the plane. Pyranoses typically adopt a chair conformation, similar to that of [[cyclohexane]]. In this conformation, the α-isomer has the −OH of the anomeric carbon in an axial position, whereas the β-isomer has the −OH of the anomeric carbon in equatorial position (considering {{sc|D}}-aldohexose sugars).<ref>{{GoldBookRef|title=Haworth representation|file=H02749}}</ref>

<div class=skin-invert-image><gallery>
Alpha-D-Glucopyranose.svg|α-{{sc|D}}-Glucopyranose
Beta-D-Glucopyranose.svg|β-{{sc|D}}-Glucopyranose
</gallery></div>