Hexose

Hexose

Overview
In organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

, a hexose is a monosaccharide
Monosaccharide
Monosaccharides are the most basic units of biologically important carbohydrates. They are the simplest form of sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose , fructose , galactose, xylose...

 with six carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 atoms, having the chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 C6H12O6. Hexoses are classified by functional group
Functional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...

, with aldohexoses having an aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....

 at position 1, and ketohexose
Ketohexose
A ketohexose is a ketone-containing hexose . The most common ketohexoses, each of which represents a pair of enantiomers , include fructose, psicose, sorbose, and tagatose...

s having a ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...

 at position 2.

The aldohexose
Aldohexose
An aldohexose is a hexose with an aldehyde group on one end.The aldohexoses have four chiral centres for a total of 16 possible aldohexose stereoisomers . Of these, only three commonly occur in nature: D-glucose, D-galactose, and D-mannose...

s 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
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...

 at position 5, and does not refer to the direction of optical activity.
The eight D-aldohexoses are:
Of these D-isomers, all except D-altrose
Altrose
Altrose is an aldohexose sugar. D-Altrose is an unnatural monosaccharide. It is soluble in water and practically insoluble in methanol. However, L-altrose has been isolated from strains of the bacterium Butyrivibrio fibrisolvens....

 are naturally occurring.
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Encyclopedia
In organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

, a hexose is a monosaccharide
Monosaccharide
Monosaccharides are the most basic units of biologically important carbohydrates. They are the simplest form of sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose , fructose , galactose, xylose...

 with six carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 atoms, having the chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 C6H12O6. Hexoses are classified by functional group
Functional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...

, with aldohexoses having an aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....

 at position 1, and ketohexose
Ketohexose
A ketohexose is a ketone-containing hexose . The most common ketohexoses, each of which represents a pair of enantiomers , include fructose, psicose, sorbose, and tagatose...

s having a ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...

 at position 2.

Aldohexoses


The aldohexose
Aldohexose
An aldohexose is a hexose with an aldehyde group on one end.The aldohexoses have four chiral centres for a total of 16 possible aldohexose stereoisomers . Of these, only three commonly occur in nature: D-glucose, D-galactose, and D-mannose...

s 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
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...

 at position 5, and does not refer to the direction of optical activity.
The eight D-aldohexoses are:
Of these D-isomers, all except D-altrose
Altrose
Altrose is an aldohexose sugar. D-Altrose is an unnatural monosaccharide. It is soluble in water and practically insoluble in methanol. However, L-altrose has been isolated from strains of the bacterium Butyrivibrio fibrisolvens....

 are naturally occurring. L-Altrose, however, has been isolated from strains of the bacterium Butyrivibrio fibrisolvens.

A mnemonic for the aldohexoses is "All Altruists Gladly Make Gum in Gallon Tanks": allose, altrose, glucose, mannose, gulose, idose, galactose, talose. When drawn in this order, the Fischer projections of the D-aldohexoses follow a pattern. Allose has all four hydroxyl groups on the right. At carbon 2, the hydroxyl groups alternate right-left. At carbon 3, the first two are on the right, the next two are on the left, and so on. At carbon 4, the first four are on the right and the rest are on the left. At carbon 5, all eight D-aldohexoses have the hydroxyl group on the right.

Cyclic hemiacetals


It has been known since 1926 that 6-carbon aldose sugars form cyclic hemiacetal
Hemiacetal
Hemiacetals and hemiketals are compounds that are derived from aldehydes and ketones respectively. The Greek word hèmi means half...

s. The diagram below shows the hemiacetal forms for D-glucose and D-mannose.


The numbered carbons in the open-chain forms correspond to the same numbered carbons in the hemiacetal forms. The formation of the hemiacetal causes carbon number 1, which is symmetric in the open-chain form, to become asymmetric in the cyclic version. This means that both glucose and mannose (as well as all the other aldohexoses) each have two cyclic forms. In solution, both of these exist in equilibrium with the open-chain form. The open-chain form, however, does not crystallize. Hence the two cyclic forms become separable when they are crystallized. For example, D-glucose forms an alpha crystal that has specific rotation
Specific rotation
In stereochemistry, the specific rotation of a chemical compound [α] is defined as the observed angle of optical rotation α when plane-polarized light is passed through a sample with a path length of 1 decimeter and a sample concentration of 1 gram per 1 millilitre. It is the main property used to...

 of +112° and melting point of 146 °C, as well as a beta crystal that has specific rotation of +19° and melting point of 150 °C.

Ketohexoses


The ketohexose
Ketohexose
A ketohexose is a ketone-containing hexose . The most common ketohexoses, each of which represents a pair of enantiomers , include fructose, psicose, sorbose, and tagatose...

s have 3 chiral centres and therefore eight possible stereoisomers (23). Of these, only the four D-isomers are known to occur naturally:
Only the naturally occurring hexoses are capable of being fermented by yeast
Yeast
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...

s.

Mutarotation


The aldehyde and ketone functional groups in these carbohydrates react with neighbouring hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...

 functional groups to form intramolecular
Intramolecular
Intramolecular in chemistry describes a process or characteristic limited within the structure of a single molecule, a property or phenomenon limited to the extent of a single molecule.- Examples :...

 hemiacetal
Hemiacetal
Hemiacetals and hemiketals are compounds that are derived from aldehydes and ketones respectively. The Greek word hèmi means half...

s and hemiketals, respectively. The resulting ring structure is related to pyran
Pyran
In chemistry, a pyran, or oxine, is a six-membered heterocyclic, non-aromatic ring, consisting of five carbon atoms and one oxygen atom and containing two double bonds. The molecular formula is C5H6O. There are two isomers of pyran that differ by the location of the double bonds...

, and is termed a pyranose
Pyranose
Pyranose is a collective term for carbohydrates that have a chemical structure that includes a six-membered ring consisting of five carbon atoms and one oxygen atom. The name derives from its similarity to the oxygen heterocycle pyran, but the pyranose ring does not have double bonds...

. The ring spontaneously opens and closes, allowing rotation to occur about the bond between the carbonyl
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....

 group and the neighbouring carbon atom, yielding two distinct configurations (α and β). This process is termed mutarotation
Mutarotation
Mutarotation is the change in the optical rotation that occurs by epimerization...

. Hexose sugars can form dihexose
Disaccharide
A disaccharide or biose is the carbohydrate formed when two monosaccharides undergo a condensation reaction which involves the elimination of a small molecule, such as water, from the functional groups only. Like monosaccharides, disaccharides form an aqueous solution when dissolved in water...

 sugars with a condensation reaction to form a 1,6-glycosidic bond
Glycosidic bond
In chemistry, a glycosidic bond is a type of covalent bond that joins a carbohydrate molecule to another group, which may or may not be another carbohydrate....

.