Editing Hexose (section)

==Cyclic forms==
Like most monosaccharides with five or more carbons, each aldohexose or 2-ketohexose also exists in one or more cyclic (closed-chain) forms, derived from the open-chain form by an [[intramolecular reaction|internal rearrangement]] between the carbonyl group and one of the hydroxyl groups.

The reaction turns the {{chem2|\dO}} group into a hydroxyl, and the hydroxyl into an [[ether group|ether bridge]] ({{chem2|\sO\s}}) between the two carbon atoms, thus creating a ring with one [[oxygen]] atom and four or five carbons.

If the cycle has five carbon atoms (six atoms in total), the closed form is called a [[pyranose]], after the [[ether|cyclic ether]] [[tetrahydropyran]], that has the same ring.  If the cycle has four carbon atoms (five in total), the form is called [[furanose]] after the compound [[tetrahydrofuran]].<ref name=morr1998>Robert Thornton Morrison and Robert Neilson Boyd (1998): ''Organic Chemistry'', 6th edition. {{isbn|9780138924645}}</ref>  The conventional numbering of the carbons in the closed form is the same as in the open-chain form.

If the sugar is an aldohexose, with the carbonyl in position 1, the reaction may involve the hydroxyl on carbon 4 or carbon 5, creating a [[hemiacetal]] with five- or six-membered ring, respectively. If the sugar is a 2-ketohexose, it can only involve the hydroxyl in carbon 5, and will create a [[hemiketal]] with a five-membered ring.

The closure turns the carboxyl carbon into a [[stereocenter|chiral center]], which may have either of two configurations, depending on the position of the new hydroxyl.  Therefore, each hexose in linear form can produce two distinct closed forms, identified by prefixes "α" and "β".

{{multiple image
|align = center
|total_width = 600
|image1 = Alpha-D-Glucopyranose-with-H.png
|caption1 = α-{{sc|D}}-Glucopyranose.
|image2 = Beta-D-Glucopyranose-with-H.png
|caption2 = β-{{sc|D}}-Glucopyranose.
|image3 = Alpha-D-Fructofuranose-with-H.png
|caption3 = α-{{sc|D}}-Fructofuranose.
|image4 = Beta-D-Fructofuranose-with-H.png
|caption4 = β-{{sc|D}}-Fructofuranose.
|footer = Closed forms of {{sc|D}}-glucose and {{sc|D}}-fructose, in the [[Haworth projection]].
}}

It has been known since 1926 that hexoses in the crystalline solid state assume the cyclic form.  The "α" and "β" forms, which are not enantiomers, will usually crystallize separately as distinct species. For example, {{sc|D}}-glucose forms an α crystal that has [[specific rotation]] of +112° and melting point of 146&nbsp;°C, as well as a β crystal that has specific rotation of +19° and melting point of 150&nbsp;°C.<ref name=morr1998/>

The linear form does not crystallize, and exists only in small amounts in water solutions, where it is in equilibrium with the closed forms.<ref name=morr1998/> Nevertheless, it plays an essential role as the intermediate stage between those closed forms.

In particular, the "α" and "β" forms can convert to into each other by returning to the open-chain form and then closing in the opposite configuration.  This process is called [[mutarotation]].