Carbohydrates

Carbohydrates

1. Classification according to

   • Functional Group
     • Aldoses - Carbohydrates containing aldehyde group
       • • Glyceraldehyde is the simplest carbohydrate
     • Ketoses - Carbohydrates containing ketone group
       • Dihydroxy acetone ketotriose
     • Aldotetroses
       • The carbohydrate after aldotriose are called aldotetroses
     • Aldopentoses and Aldohexoses
       • Aldopentoses: C5, three chiral carbons Aldohexoses: C6, four chiral carbons
   • Size of molecules
     • Disaccharides - Sucrose
       • A disaccharide is formed when a hydroxyl group on one monosaccharide reacts with the anomeric carbon of another monosaccharide to form a glycosidic bond
         • The three most common disaccharides are maltose, lactose and sucrose
     • Polysaccharides - trisaccharide raffinose
     • Monosaccharides - Glucose
       • Six-carbon sugars (hexoses) and five-carbon sugars (pentoses) are the most frequently encountered monosaccharide carbohydrate units in nature.
         • Hexoses - glucose and fructose
         • Pentose - ribose
     • Oligosaccharides - trisaccharide raffinose
   • Number of carbons
     • Carbons of Ketone
       • 4- Tetrose 5- Pentose 6- Hexose 7- Heptose 8- Octose
     • Carbons of Aldehyde
       • 4- Tetrulose 5- Pentulose 6- Hexulose 7-Heptulose 8- Ovtulose

2. Nomenating

   • The carbonyl group in glucose and ribose is an aldehyde, such as Aldoses. Fructose, has a ketone group and is therefore classified as a ketose.
   • Glucose termed as an aldohexose, fructose a ketohexose, ribose an aldopentose, names which indicate both the number of carbons and the nature of the carbonyl group.
   • use of the suffix - ulose to indicate a ketose.

3. Sugar predominate in either two forms

   • 1- Enantiomer: pairs of D sugars and L sugars
   • 2- Epimers: sugars that differ only at one of several chiral centers

4. Preparation

   • Kiliani-Fischer synthesis
   • Ruff degradation – a series of reactions that removes the reducing carbon ( C=O ) from a sugar and decreases the number of chiral centers by one used to relate configuration
   • Chain Shortening and Lengthening

5. Reaction

   • I. Oxidation Aldehydes can be oxidized to corresponding carboxylic acids
   • Oxidation reactions of monosaccharides
     • 1- Aldonic acids aldehyde group is converted to a carboxyl group ( glucose – gluconic acid)
     • 2- Uronic acids: aldehyde is left intact and primary alcohol at the other end is oxidized to COOH. This reaction can not be done in laboratory, but it is done enzymatically • Glucose --- glucuronic acid • Galactose --- galacturonic acid
     • 3- aldaric acids: – oxidation at both ends of monosaccharide) • Glucose ---- saccharic acid • Galactose --- mucic acid • Mannose --- mannaric acid
   • III. Esterification: An acid chloride or acid anhydride can add to an alcohol to form an ester
   • IV.ALKYLATION: ETHER FORMATION Treatment of monosaccharide with an alkyl halide in the presence of base result in the formation of ether linkage. Use silver oxide as a catalyst
   • II. Reduction: Carbonyl groups can be reduced to alcohols (catalytic hydrogenation)
   • V. Osazone formation Aldoses and ketoses react with three equivalents of phenylhydrazine

6. convert Fischer projection to Haworth projection:

   • 1- number the ring clockwise starting next to the oxygen 2- Put the hydroxyl groups attached to the right of the chiral centres in Fischer projection directed below the plane of the ring in Haworth projection. And the hydroxyl group to the left in Fischer projection should be directed above the ring in Haworth projection. for D-sugars, the OH group at the anomeric position is drawn down for a and up for b. For Lsugars a is up and b is down 3- The terminal CH2OH in Ficher projection should be projected above the plane of the ring in Haworth projection in sugars of the D family. In sugars of L family the terminal CH2OH in Ficher projection should be projected below the plane of the ring in Haworth projection
     • Both six-membered pyranose and five-membered furanose structures are encountered, a particular ring size usually being characteristic for any one sugar. Thus, although glucose has the potential to form both sixmembered and five-membered rings, an aqueous solution consists almost completely of the six membered hemiacetal form; five-membered rings are usually formed more rapidly, but six-membered rings are generally more stable and predominate at equilibrium

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