Aldose
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An aldose is a
monosaccharide Monosaccharides (from Greek '' monos'': single, '' sacchar'': sugar), also called simple sugars, are the simplest forms of sugar and the most basic units (monomers) from which all carbohydrates are built. Chemically, monosaccharides are polyhy ...
(a simple sugar) with a carbon backbone chain with a
carbonyl group In organic chemistry, a carbonyl group is a functional group with the formula , composed of a carbon atom double-bonded to an oxygen atom, and it is divalent at the C atom. It is common to several classes of organic compounds (such as aldehydes ...
on the endmost carbon atom, making it an
aldehyde In organic chemistry, an aldehyde () (lat. ''al''cohol ''dehyd''rogenatum, dehydrogenated alcohol) is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred ...
, and hydroxyl groups connected to all the other carbon atoms. Aldoses can be distinguished from
ketose In organic chemistry, a ketose is a monosaccharide containing one ketone () group per molecule. The simplest ketose is dihydroxyacetone (), which has only three carbon atoms. It is the only ketose with no optical activity. All monosaccharide keto ...
s, which have the carbonyl group away from the end of the molecule, and are therefore
ketone In organic chemistry, a ketone is an organic compound with the structure , where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group (a carbon-oxygen double bond C=O). The simplest ketone is acetone ( ...
s.


Structure

Like most carbohydrates, simple aldoses have the general
chemical formula A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, such as pare ...
C''n''(H2O)''n''. Because
formaldehyde Formaldehyde ( , ) (systematic name methanal) is an organic compound with the chemical formula and structure , more precisely . The compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde. It is stored as ...
(n=1) and
glycolaldehyde Glycolaldehyde is the organic compound with the formula . It is the smallest possible molecule that contains both an aldehyde group () and a hydroxyl, hydroxyl group (). It is a highly Reactivity (chemistry), reactive molecule that occurs both ...
(n=2) are not generally considered to be carbohydrates, the simplest possible aldose is the triose glyceraldehyde, which only contains three
carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
s. Because they have at least one asymmetric carbon center, all aldoses exhibit
stereoisomerism In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in ...
. Aldoses can exist in either a - form or - form. The determination is made based on the chirality of the asymmetric carbon furthest from the aldehyde end, namely the second-last carbon in the chain. Aldoses with alcohol groups on the right of the Fischer projection are -aldoses, and those with alcohols on the left are -aldoses. -aldoses are more common than -aldoses in nature. Examples of aldoses include glyceraldehyde, erythrose,
ribose Ribose is a simple sugar and carbohydrate with molecular formula C5H10O5 and the linear-form composition H−(C=O)−(CHOH)4−H. The naturally occurring form, , is a component of the ribonucleotides from which RNA is built, and so this comp ...
,
glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
and
galactose Galactose (, ''wikt:galacto-, galacto-'' + ''wikt:-ose#Suffix 2, -ose'', ), sometimes abbreviated Gal, is a monosaccharide sugar that is about as sweetness, sweet as glucose, and about 65% as sweet as sucrose. It is an aldohexose and a C-4 epime ...
. Ketoses and aldoses can be chemically differentiated through
Seliwanoff's test Seliwanoff’s test is a chemical test which distinguishes between aldose and ketose sugars. If the sugar contains a ketone group, it is a ketose. If a sugar contains an aldehyde group, it is an aldose. This test relies on the principle that, whe ...
, where the sample is heated with acid and
resorcinol Resorcinol (or resorcin) is a phenolic compound. It is an organic compound with the formula C6H4(OH)2. It is one of three isomeric benzenediols, the 1,3-isomer (or ''meta- (chemistry), meta''-isomer). Resorcinol crystallizes from benzene as co ...
. The test relies on the
dehydration reaction In chemistry, a dehydration reaction is a chemical reaction that involves the loss of an H2O from the reacting molecule(s) or ion(s). This reaction results in the release of the H2O as water. When the reaction involves the coupling of two molecu ...
which occurs more quickly in ketoses, so that while aldoses react slowly, producing a light pink color, ketoses react more quickly and strongly to produce a dark red color. Aldoses can
isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the exi ...
ize to ketoses through the Lobry-de Bruyn-van Ekenstein transformation.


Nomenclature and common aldoses

Aldoses are differentiated by the number of carbon atoms in the main chain. The minimum number of carbon atoms in a backbone needed to form a molecule that is still considered a carbohydrate is 3, and carbohydrates with three carbon atoms are called trioses. The only aldotriose is glyceraldehyde, which has one chiral stereocenter with 2 possible enantiomers, - and -glyceraldehyde. Some common aldoses are: * Triose: glyceraldehyde * Tetroses: erythrose, threose * Pentoses:
ribose Ribose is a simple sugar and carbohydrate with molecular formula C5H10O5 and the linear-form composition H−(C=O)−(CHOH)4−H. The naturally occurring form, , is a component of the ribonucleotides from which RNA is built, and so this comp ...
, arabinose, xylose, lyxose *
Hexose In chemistry, a hexose is a monosaccharide (simple sugar) with six carbon atoms. The chemical formula for all hexoses is , and their molecular weight is 180.156 g/mol. Hexoses exist in two forms, open-chain or cyclic, that easily convert into ...
s:
glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
The most commonly discussed category of aldoses are those with six carbon atoms, aldohexoses. Some aldohexoses that are widely called by common names are: * -(+)- Allose * -(+)- Altrose * -(+)-
Glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
* -(+)- Mannose * -(−)- Gulose * -(+)- Idose * -(+)-
Galactose Galactose (, ''wikt:galacto-, galacto-'' + ''wikt:-ose#Suffix 2, -ose'', ), sometimes abbreviated Gal, is a monosaccharide sugar that is about as sweetness, sweet as glucose, and about 65% as sweet as sucrose. It is an aldohexose and a C-4 epime ...
* -(+)- Talose


Stereochemistry

Aldoses are commonly referred to by names specific to one stereoisomer of the compound. This distinction is especially vital in biochemistry, as many systems can only use one enantiomer of the carbohydrate and not the other. However, aldoses are not locked into any one conformation: they can and do fluctuate between different forms. Aldoses can tautomerize to ketoses in a dynamic process with an
enol In organic chemistry, enols are a type of functional group or intermediate in organic chemistry containing a group with the formula (R = many substituents). The term ''enol'' is an abbreviation of ''alkenol'', a portmanteau deriving from "-ene ...
intermediate (more specifically, an enediol). This process is reversible, so aldoses and ketoses can be thought of as being in equilibrium with each other. However, aldehydes and ketones are almost always more stable than the corresponding enol forms, so the aldo- and keto- forms normally predominate. This process, with its enol intermediate, also allows stereoisomerization. Basic solutions accelerate the interconversion of isomers. Carbohydrates with more than four carbon atoms exist in an equilibrium between the closed ring, or cyclic form, and the open-chain form. Cyclic aldoses are usually drawn as Haworth projections, and open chain forms are commonly drawn as Fischer projections, both of which represent important stereochemical information about the forms they depict.


References

{{Authority control Aldols