Discover

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.

Contents

Aldohexoses

Ketohexoses

Mutarotation

Other Information

See also

Aldohexoses

The aldohexoses have four chiral centres for a total of 16 possible aldohexose stereoisomers (2⁴). 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' 'altr'uists 'gl'adly 'ma'ke 'gu'm 'i'n 'gal'lon 'ta'nks".

Ketohexoses

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.

Mutarotation

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.

Other Information

It is a weak acid and a weak electrolyte.

See also

★ Triose

★ Tetrose

★ Pentose

★ Heptose

★ Glucose