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Hexose
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In organic chemistry, a hexose is a monosaccharide with six carbon atoms having the chemical formula C6H12O6. Hexoses are classified by functional group, with aldohexoses having an aldehyde at position 1, and ketohexoses having a ketone at position 2.
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The aldohexoses have four chiral centres for a total of 16 possible aldohexose stereoisomers (24). Of these, only three occur in nature: D-glucose, D-galactose, and D-mannose. The D/L configuration is based on the orientation of the hydroxyl at position 5, and does not refer to the direction of optical activity. The eight D-aldohexoses are:
CH=O CH=O CH=O CH=O | | | | HC-OH HO-CH HC-OH HO-CH | | | | HC-OH HC-OH HO-CH HO-CH | | | | HC-OH HC-OH HC-OH HC-OH | | | | HC-OH HC-OH HC-OH HC-OH | | | | CH2OH CH2OH CH2OH CH2OH D-Allose D-Altrose D-Glucose D-Mannose
CH=O CH=O CH=O CH=O | | | | HC-OH HO-CH HC-OH HO-CH | | | | HC-OH HC-OH HO-CH HO-CH | | | | HO-CH HO-CH HO-CH HO-CH | | | | HC-OH HC-OH HC-OH HC-OH | | | | CH2OH CH2OH CH2OH CH2OH D-Gulose D-Idose D-Galactose D-Talose
Of these D isomers all, except altrose, are naturally occurring. No L isomers are found naturally.
A mnemonic (attributed to Louis Fieser) often employed to remember the eight aldohexoses is "all altruists gladly make gum in gallon tanks".
The ketohexoses have 3 chiral centres and therefore eight possible stereoisomers (23). Of these, only the four D-isomers are known to occur naturally:
CH2OH CH2OH CH2OH CH2OH | | | | C=O C=O C=O C=O | | | | HC-OH HO-CH HC-OH HO-CH | | | | HC-OH HC-OH HO-CH HO-CH | | | | HC-OH HC-OH HC-OH HC-OH | | | | CH2OH CH2OH CH2OH CH2OH D-psicose D-fructose D-sorbose D-tagatose
Only the naturally occurring hexoses are capable of being fermented by yeasts.
The aldehyde and ketone functional groups in these carbohydrates react with neighbouring hydroxyl functional groups to form intramolecular hemiacetals or hemiketals, respectively. The resulting ring structure is related to pyran, and is termed a pyranose. The ring spontaneously opens and closes, allowing rotation to occur about the bond between the carbonyl group and the neighbouring carbon atom, yielding two distinct configurations (α and β). This process is termed mutarotation. Hexose sugars can form dihexose sugars with a condensation reaction to form a 1,6-glycosidic bond.
It is a weak acid and a weak electrolyte.[citation needed]