Carbohydrates | Organic Chemistry 3
Names and Structures of Carbohydrates
The simplest carbohydrates are sugars (polyhydroxy aldehydes and ketones)
Monosaccharides, also called simple sugars, are the simplest form of sugar and the most basic units of carbohydrates: they are aldehydes or ketones that contain at least 2 additional hydroxy groups
Nomenclature:
- Polyhydroxy aldehydes are aldoses; polyhydroxy ketones are 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
The Fischer projection is used for open-chain carbohydrates. In a Fischer projection: horizontal lines correspond to atoms above the plane of the page
How to convert a skeletal structure to a Fischer projection:
Conversion of (1S,2R,3R)-2-amino-1-bromobutane-1,3-diol 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 while the 6-membered ring is called 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
The Haworth projection is used for cyclic forms of carbohydrates. It represents the real 3D structure of sugar
How to convert a Fischer projection to a Haworth projection:
Groups on the right in the Fischer projection asre 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:
- Formation of alkoxides (RO-) by deprotonation of alcohols with Ag2O
- SN2 reaction between alkoxides and CH3I
Polysaccharides
Polysaccharides are polymers of monosaccharides. Nature is remarkably conservative in the construction of such macromolecules: the 3 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: