Carbohydrates | Organic Chemistry 3
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.
- 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
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 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
Mechanism: Williamson ether synthesis
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: