Stereoisomers are the compounds which havethe same molecular formula and same functional group but having a different configuration (arrangement) in the space.These compoundscontain one or more than one asymmetric centrewhich can bechiral or achiral.
A chiral centre in amolecule is SP3 carbon centrecarrying 4 different functional groups. In a chiral molecule mirror image of the molecule used to be different from its original vision. Polarimeter can be used to measures the optical rotation of the chiral compound.
Compound1 containing asymmetric centre is a chiral molecule.
Chiral molecules contain one or more than one chiral centre, and it can rotate plane polarised light in a specific direction. The molecule in which images are non-superimposable on each other in the 3-dimensional space and does not contain any plane of symmetry or centre of symmetry is called a chiral molecule.
The molecule in which mirror images can be superposed on each other is called as achiralmolecule.
Stereoisomers can be of two types:-
- Enantiomers are the compounds having same molecular formula and functional group which contain one or one than one chiral center in the molecule.
- Any molecule having a chiral centre must have enantiomers.
- Enantiomers show same physical properties but shows different specific rotation with plane polarized light (optical rotation can be either + or - are direction but having same magnitude but opposite in direction).
- Enantiomers show same chemical properties except for their reaction with optically active compounds.
- A mixture of 2 enantiomers in equal amount is called a racemic mixture.
- Enantiomers show different interaction with other chiral molecules: Mostly in enzymes active site binds with specific enantiomer.
- Diastereomersare having the same molecular formula and the functional group which may contain one or multiple chiral centres in the molecule. These molecules are non- mirror image of each other.
- Diastereomers show different physical properties, chemical properties and specific rotation towards plane polarised light.
Example of Diastereomers.
Cis and Trans isomers used to be Diastereomers which are not mirror images of each other.
- Molecules with two or more chiral carbons which are not mirror images of eacho ther used to Diastereomers.
The configurational isomers can be assigned by two methods
- Absolute configuration (R and S system)
- Relative configuration (D and L system)
Absolute configuration (R and S system)
Cahn-Ingold-Prelog has given the R and S system convention for the stereoisomers. It is the most commonly used for naming the configurations of chirality centres.(R)Or(S)convention is based three-dimensional configuration of four functional groups in the space.
Steps to obtain R-S Configuration of a molecule
- Assign a relative priority to each functional group bonded to the asymmetric center. Atoms with higher atomic numbers receive higher priorities.Group will be assigned from 1 to 4 in the following order
Atomic number I > Br > Cl > S > F > O > N > C > H
In case of ties between atoms, the next atoms along the chain of each group can be used to give priority number.
- In case of multiple bonds,double and triple bonds will be treated as if each were a bond to a separate atom.
- Make an arrow from the highest priorityto the next lower the priority group (1 to 4).If arrow rotates in Clockwise direction, the molecule will be assigned as R conformerwhile if the arrow rotates in Counterclockwise direction it will be designated as S conformer.
Relative configuration (D and L system)-
Fisher and Rosanoff have given D and L representation of isomers. This method is used for the assignment in carbohydrates (Flat representation of a 3-D molecule). In fisher representation chiral carbon is at the intersection of horizontal and vertical lines.
Number of Stereoisomers
The total number of isomers can be calculated by 2n rule where n is the number of asymmetric centre in the molecule (having none of the symmetry in the molecule).
Racemic Mixtures contain equal quantities of enantiomers which does not show any optical activity. The Racemic mixture may have a different boiling point (b. p.) and melting point (m. p.) from the enantiomers.
Resolution of isomers
It is the process of separation of individual isomers from the racemic mixture. Following methods can be used to separate isomers.
- Mechanical methods
- Biochemical methods
- Chemical methods
- Chromatographic methods
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