A. Isomerism
- In organic chemistry, isomerism is defined as the existence of more than one compound with the same molecular formula. A close observation of the structure of monosaccharides (hexoses) indicate that they possess same molecular formula (C6H12O6) but with different physical and chemical properties. There are different types of isomerism. D-glucose and D-fructose differ in the position of carbonyl group (aldehyde and ketone group). These two compounds are functional isomers. Another type of isomerism exhibited by compounds possessing asymmetric carbon atom like monosaccharides, is stereoisomerism. These stereoisomers differ in the spatial arrangement of atoms or groups.
There are two types of stereoisomerisms - geometrical and optical isomerism
- Geometrical isomers (cis-trans) differ in the spatial arrangement of atoms across a double bond. Geometrical isomerism is not noticed among carbohydrates
- Optical isomers differ in the arrangement of atoms around an asymmetric carbon atom. The number of possible optical isomers can be calculated using the formula 2n where n=number of asymmetric carbon atoms. For example, glucose contains four asymmetric carbon atoms and the possible optical isomers of glucose are 24 = 16
Epimers, enantiomers and diastereomers
- Epimers are monosaccharides differing in configuration around a single carbon atom other than the carbonyl carbon. e.g. Mannose and glucose are epimers with respect to carbon 2. Galactose and glucose are epimers with respect to carbon 4. Enantiomers are non- superimposable mirror images of each other. They differ in the ability to rotate the plane polarized light. A solution of one enantiomer rotates the plane of such light to the right, and a solution of the other to the left. D-glucose and L-glucose are examples of enantiomers.Diastereomers are stereoisomers that are not mirror images of each other. D-glucose, Dmannose, D-galactose and other members of aldohexose are diastereoisomers.
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Last modified: Tuesday, 27 March 2012, 6:58 PM
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