Carbohydrates | Organic Chemistry 3

The Carbohydrates are studied in this chapter: the naming and structures of carbohydrates, the Fischer projection, the conformations and cyclic forms of sugars, the Haworth projection, the oxidation and reduction of sugars, the ester and ether formations, the polysaccharides.

Names and Structures of Carbohydrates

Simplest carbohydrates = sugars (polyhydroxy aldehydes and ketones).
Monosaccharide = simple sugar = aldehyde or ketone containing at least 2 additional hydroxy groups.


Naming:

- polyhydroxy aldehydes ⇒ aldoses; polyhydroxy ketones ⇒ ketoses
- sugars are called according to their chain length:
3 carbons ⇒ trioses; 4 carbons ⇒ tetroses; 5 carbons ⇒ pentoses; etc.
- D and L configuration: sugars are classified as D when the highest-numbered stereocenter (= the one farthest from the carbonyl group) is R, L when it is S. Configuration D is the most common in nature
- 2 diastereomers that differ only at one stereocenter are called epimers

 

Monosaccharides:

 

 

Sugars:
 

 

Fischer Projection

Fischer projection is used for open-chain carbohydrates.
In a Fischer projection: horizontal lines = atoms above the plane of the page

 

 

Convert a skeletal structure to a Fischer projection:

 

 

Conformations and Cyclic forms of Sugars

Hexoses and pentoses can take the form of 5- or 6-membered cyclic hemiacetals.
The 5-membered ring is called furanose whereas the 6-membered ring pyranose.
During the cyclization process, the carbonyl carbon turns into a new stereocenter called anomeric carbon.
In the D-series sugar: if anomeric carbon is S ⇒ α anomer; if it is R ⇒ β anomer.

 

 

Mechanism: Hemiacetal formation

Haworth Projection

Haworth projection is used for cyclic forms of carbohydrates.
It represents the real 3D structure of the sugar.

 

 

Convert a Fischer projection to a Haworth projection:
Groups on the right in the Fischer projection = down in the Haworth projection.
 

Oxidation of Sugars

Oxidation to mono- or dicarboxylic acids:
 

Reduction of Sugars

Ester and Ether Formation

Ester formation:
 


Mechanism: Addition-Elimination

 

Ether formation:
 



Mechanism: Williamson ether synthesis

Polysaccharides

Polysaccharides are the polymers of monosaccharides.
Nature is remarkably conservative in the construction of such macromolecules: the three most abundant natural polysaccharides (cellulose, starch and glycogen) are derived from the glucose.

 

Cellulose is a polysaccharide:
 

 

Many sugars exist in nature in modified form or as simple appendage to other structures.
Modified sugars may contain nitrogen: