12th chemistry Stereoisomerism - Organic Chemistry
Stereoisomerism - Organic Chemistry
It is due to the difference in arrangement of atoms or group in space.
It is classified into two types
- Configurational Isomerism
- Conformational Isomerism
Configurational Isomerism is further classified into (I) Geometrical Isomerism (II) Optical Isomerism.
- Geometrical Isomerism
In this, the isomer have different spatial arrangement of atoms or group around the double bonded carbon atom.
- Carbon-carbon rotation must be restricted either double bond or by cyclic structure.
- The group attached to doubled bonded carbon atom must be different.
- Triple bonded carbon does not have stereogenic centre so it does not exhibit Geometrical isomerism.
The isomers which have similar group on the same side of the doubly bonded carbon are called 'cis' isomers and the isomers which have similar group on the opposite sides are called 'trans' isomers.
- In aldoxime and ketoxime the prefix 'syn' and 'anti' indicates 'cis' and 'trans' isomer respectively.
- If four different groups are attached to carbon atom then 'E' and 'Z' system is used if higher groups are on same side then 'Z' system is used and if higher groups are on opposite side then 'E' system is used.
- Geometrical isomer shows different physical properties. There chemical properties are similar but not identical.
- Cis-isomer is more polar and have higher boiling point that trans-isomer.
- Trans-isomer are more stable and have higher melting point then cis-isomer.
- Optical Isomerism
These isomers resembles in their chemical properties but shows different behavior towards plane polarized light.
- Organic molecules must have chiral centre when all the group bonded to carbon are different then the carbon is called as chiral carbon or assymetric carbon.
- Organic molecules must show enantiomerism i.e. non-super imposable mirror images. For this, molecules must have chiral centre and must not have plane symmetry.
Optical isomer may be dextrorotatory or laevo rotatory depending upon the rotation of plane polarized light.
Representation of the structure (Fischer Projection)
- Most oxidised carbon is arranged vertically at the top.
- Group attached to carbon atom are assigned a priority which is divided on the basis of atomic number.
- Rotate the molecule such that the group of lowest priority is directed away. If rotation is clockwise the configuration is 'R' and if the rotation is anticlockwise then configuration is specified as 'S'.
- If group of lowest priority is at the top of vertical line then rotate the molecule through180 to bring atom of lowest priority at the bottom.
- If group of lowest priority is on the lefts hand side, then position of atoms are group are changed in anticlockwise direction to bring the group of lowest priority at the bottom except changing the position of the group present at the top and vice versa, if the group of lowest priority is present on the right hand side.
Properties of Enantiomers
- They have almost identical physical properties.
- They rotate the plane polarized light in opposite direction to same extent.
- They have identical chemical properties.
They have different physical properties, they may or may not be optically active.
Meso Compounds: These compounds are optically inactive inspite of the presence of chiral carbon atoms .
This is due to presence of plane symmetry.
Racemic mixture: It is equimolar mixture of (+) and (-) enantiomers. This mixture is optically inactive due to external compensation.
Resolution: It is the mtehod of separation of Racemic mixture. The mixture is treated with a suitable optically active reagent which give a mixture of two diastereomers which are separated by fractional crystallization.
Number of possible Stereoisomers in a compound.
- If molecule can not be divided into two equal halves
No. of 'd' and 'l' form = 2n
No. of meso form = 0
Where 'n' is number of assymetric carbon.
- When molecule can be divided into equal halves
n = even
No. of d and l form = 2(n-1)
No. of meso form =
- If n = odd
No. of 'd' and 'l' form = 2n-1 -
No. of meso form =
It arises due to the rotation through single bond. This results in staggered and eclipsed structures. e.g., ethane
All other conformation between eclipsed and staggered form may be obtained by rotating the two adjacent carbon to 60C. Staggered > Gauche > Partially eclipsed > Fully eclipsed.