Editing Hexose (section)

{{Short description|6-carbon simple sugar}}
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|caption1 = {{sc|D}}-[[Glucose]].
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|image2 = D-Fructose.svg
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|caption2 = {{sc|D}}-[[Fructose]].
|footer = Two important hexoses, in the [[Fischer projection]].
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In [[chemistry]], a '''hexose''' is a [[monosaccharide]] (simple sugar) with six [[carbon]] atoms.<ref name=lind2007/><ref name=robyt1997/>  The chemical formula for all hexoses is {{chem2|C6H12O6}}, and their [[molecular weight]] is 180.156 g/mol.<ref name=":0">{{Cite web|url=https://pubchem.ncbi.nlm.nih.gov/compound/D-Psicose#section=Names-and-Identifiers|title=<small>D</small>-Psicose|last=Pubchem|website=pubchem.ncbi.nlm.nih.gov|language=en|access-date=2018-04-26}}</ref>

Hexoses exist in two forms, open-chain or cyclic, that easily convert into each other in aqueous solutions.<ref name=morr1998/> The open-chain form of a hexose, which usually is favored in solutions, has the general structure {{chem2|H\s(CHOH)_{''n''−1}\sC(\dO)\s(CHOH)_{6−''n''}\sH}}, where ''n'' is 1, 2, 3, 4, 5.  Namely, five of the carbons have one [[hydroxyl]] functional group ({{chem2|\sOH}}) each, connected by a  [[single bond]], and one has an oxo group ({{chem2|\dO}}), forming a [[carbonyl]] group ({{chem2|C\dO}}). The remaining bonds of the carbon atoms are satisfied by seven [[hydrogen]] atoms.  The carbons are commonly numbered 1 to 6 starting at the end closest to the carbonyl.

Hexoses are extremely important in [[biochemistry]], both as isolated molecules (such as  [[glucose]] and [[fructose]]) and as building blocks of other compounds such as [[starch]], [[cellulose]], and [[glycoside]]s. Hexoses  can form [[disaccharide|dihexose]] (like [[sucrose]]) by a condensation reaction that makes 1,6-[[glycosidic bond]].

When the carbonyl is in position 1, forming a [[formyl group]] ({{chem2|\sCH\dO}}), the sugar is called an '''aldohexose''', a special case of [[aldose]]. Otherwise, if the carbonyl position is 2 or 3, the sugar is a derivative of a [[ketone]], and is called a '''ketohexose''', a special case of [[ketose]]; specifically, an '''''n''-ketohexose'''.<ref name=lind2007/><ref name=robyt1997/> However, the 3-ketohexoses have not been observed in nature, and are difficult to synthesize;<ref name=yuen1961/> so the term "ketohexose" usually means 2-ketohexose.

In the linear form, there are 16 aldohexoses and eight 2-ketohexoses, [[stereoisomer]]s that differ in the spatial position of the hydroxyl groups. These species occur in pairs of [[optical isomer]]s.  Each pair has a conventional name (like "glucose" or "fructose"), and the two members are labeled "{{sc|D}}-" or "{{sc|L}}-", depending on whether the hydroxyl in position 5, in the [[Fischer projection]] of the molecule, is to the right or to the left of the axis, respectively.  These labels are independent of the [[optical activity]] of the isomers. In general, only one of the two enantiomers occurs naturally (for example, {{sc|D}}-glucose) and can be [[metabolism|metabolized]] by animals or [[fermentation|fermented]] by [[yeast]]s.

The term "hexose" sometimes is assumed to include '''deoxyhexoses''', such as [[fucose]] and [[rhamnose]]: compounds with general formula {{chem2|C6H12O_{6-''y''}|}} that can be described as derived from hexoses by replacement of one or more hydroxyl groups with hydrogen atoms.