Those compounds having same molecular formula and similar or different physical, chemical properties are called isomers and this phenomenon is called isomerism. This phenomenon is very important as it is one of the most important reason for a vast variety of organic compounds. Even one molecular formula corresponds to large number of of organic compounds and it is due to isomerism.
Types of isomerism:
There are two major types of isomerism in organic chemistry:
- Structural isomerism
- Stereo isomerism
Compounds having same molecular formula but different connectivity of atoms and thus, different structural formula show structural isomerism and such compounds are called structural/skeletal isomers.
For example: 1,2-dichlorobenzene and 1,4- dichlorobenzene are structural isomers of each other. i.e.
Compounds having same molecular formula, same structural formula but different spatial arrangement of atoms or group of atoms show stereo isomerism and these compounds are called stereo isomers.
For example: cis-2-butene and trans-2-butene are stereo isomers of each other. i.e.
Classification of structural isomers:
There are different classes of structural isomers:
- Chain Isomers
- Functional group isomers
- Position isomers
Those compounds which have same molecular formula but different structural formula due to difference in nature of carbon chain (which may be branched, linear, zigzag etc.) are known as chain isomers.
For example: n-pentane and 2-methyl butane are chain isomers of each other. i.e.
Functional group isomers:
Those compounds having same number of atoms but different connectivity of atoms or group of atoms due to different functional group are called functional group isomers.
For example: dimethyl ether and ethanol are functional group isomers of each other. i.e.
Those Compounds which has same molecular formula but different structural formula due to change in position of same functional group on carbon chain are called position isomers.
For example: 2-pentanone and 3-pentanone are position isomers of each other. i.e.
Structural isomerism arised due to unequal distribution of carbon atoms around functional group is called metamerism and such isomers are called metamers. This isomerism is most common in ethers.
For example: Diethyl ether and methyl n-propyl ether are metamers of each other. i.e.
Compounds that show structural isomerism due to transfer of hydrogen atom from one atom to other within the molecule are called tautomers.
Any carbonyl compound having alpha hydrogen atom show tautomerism.
For example: acetophenone show tautomerism via hydrogen transfer. i.e.
Classification of Stereo isomers
There are two major types of stereo isomers:
- Configurational isomers
- Conformational isomers
Those stereo isomers that are interconvertible only through bond cleavage and then bond formation are called configurational isomers.
For example: cis trans isomers of alkenes. i.e.
Types of Configurational Isomers:
- Geometrical isomers
- Optical isomers
These are cis-trans isomers
Cis isomers: If similar groups are on same side of a double bond is called cis isomers.
For example: cis-2-butene.
Trans isomers: If similar groups are on the opposite side of a double bond is called trans isomers. For example: trans-2-butene
Those configurational isomers which are optically active are called optical isomers. If one of the carbon atoms in a compound is chiral, (lack of symmetry) such compound shows optical activity and thus optical isomer. A carbon is chiral if all the groups attached to it are different. e.g.
Types of optical isomers
There are two types of optical isomers.
Enantiomers: Those optical isomers which are mirror images of each other and are non-superimposable with each other are called enantiomers. Such isomers have same optical activity but in opposite direction.
Diastereomers: Those optical isomers which are not enantiomers are called diastereomers. Such isomers have different degree of rotation. These are not mirror images of each other.
Those stereoisomers which are interconvertible through rotation of any of the groups around the axis of single bond are called conformational isomers.
Types of conformational isomers:
In eclipsed confirmation there is minimum distance between bulky groups in a compound. e.g.
In staggered confirmation there is maximum distance between bulky groups. e.g.
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