CARBOHIDRATOS

Carbohydrates
Carbohydrates are the most abundant class of organic compounds found in living organisms. They originate as products of photosynthesis, an endothermic reductive condensation of carbon dioxide requiring light energy and the pigment chlorophyll.
n CO2   +   n H2O   +   energy      CnH2nOn   +   n O2
As noted here, the formulas of many carbohydrates can be written as carbonhydrates, Cn(H2O)n, hence their name. The carbohydrates are a major source of metabolic energy, both for plants and for animals that depend on plants for food. Aside from the sugars and starches that meet this vital nutritional role, carbohydrates also serve as a structural material (cellulose), a component of the energy transport compound ATP, recognition sites on cell surfaces, and one of threeessential components of DNA and RNA.
Carbohydrates are called saccharides or, if they are relatively small, sugars. Several classifications of carbohydrates have proven useful, and are outlined in the following table.
Complexity
Simple Carbohydrates
monosaccharides
Complex Carbohydrates
disaccharides, oligosaccharides
& polysaccharides

Size
Tetrose
C4 sugars
Pentose
C5 sugars
HexoseC6 sugars
Heptose
C7 sugars

etc.

C=O Function
Aldose 
sugars having an aldehyde function or an acetal equivalent.
Ketose 
sugars having a ketone function or an acetal equivalent.

Reactivity
Reducing 
sugars oxidized by Tollens' reagent (or Benedict's or Fehling's reagents).
Non-reducing 
sugars not oxidized by Tollens' or other reagents.

1. Glucose
Carbohydrates have beengiven non-systematic names, although the suffix ose is generally used. The most common carbohydrate is glucose (C6H12O6). Applying the terms defined above, glucose is a monosaccharide, an aldohexose (note that the function and size classifications are combined in one word) and a reducing sugar. The general structure of glucose and many other aldohexoses was established by simple chemical reactions.The following diagram illustrates the kind of evidence considered, although some of the reagents shown here are different from those used by the original scientists.

Hot hydriodic acid (HI) was often used to reductively remove oxygen functional groups from a molecule, and in the case of glucose this treatment gave hexane (in low yield). From this it was concluded that the six carbons are in anunbranched chain. The presence of an aldehyde carbonyl group was deduced from cyanohydrin formation, its reduction to the hexa-alcohol sorbitol, also called glucitol, and mild oxidation to the mono-carboxylic acid, glucuronic acid. Somewhat stronger oxidation by dilute nitric acid gave the diacid, glucaric acid, supporting the proposal of a six-carbon chain. The five oxygens remaining in glucoseafter the aldehyde was accounted for were thought to be in hydroxyl groups, since a penta-acetate derivative could be made. These hydroxyl groups were assigned, one each, to the last five carbon atoms, because geminal hydroxyl groups are normally unstable relative to the carbonyl compound formed by loss of water. By clicking on the above diagram, it will change to display the suggested productsand the gross structure of glucose. The four middle carbon atoms in the glucose chain are centers of chirality and are colored red.
Glucose and other saccharides are extensively cleaved by periodic acid, thanks to the abundance of vicinal diol moieties in their structure. This oxidative cleavage, known as the Malaprade reaction is particularly useful for the analysis of selective O-substitutedderivatives of saccharides, since ether functions do not react. The stoichiometry of aldohexose cleavage is shown in the following equation.
HOCH2(CHOH)4CHO + 5 HIO4
  ——>  
H2C=O + 5 HCO2H + 5 HIO3


            The Configuration of Glucose


http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/carbhyd.htm
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