Contents
What is isomerism ?
Different compounds having same molecular formula but different properties are called isomers. The phenomenon of existence of such compounds is called isomerism. Eg.
Types of isomerism
Isomerism can be divided into two broad classes:
- Structural isomerism.
- Stereoisomerism.
Structural isomerism
Compounds having same molecular formula but different structural formula are called structural isomers. Phenomenon of existence of such compounds is called structural isomerism.
Structural isomerism can be categorized into following types:
- Chain isomerism
- Position isomerism
- Functional isomerism
- Metamerism
- Tautomerism
- Ring – chain isomerism.
1. Chain isomerism
Compounds having same molecular formula and same functional group but different in their carbon chain are called chain isomers. The phenomenon of existence of such compounds is called chain isomerism. Examples:
2. Position isomerism
The isomers having same carbon chains but different positions of functional group, substituent or multiple bonds ( i.e. double or triple bond) are called position isomers . Examples :
3. Functional isomerism
Compounds having same molecular formula but different functional groups are called functional isomers. Phenomenon of existence of such compounds is called functional isomerism. Examples:
Note : : Functional isomers-
Alcohol
Ether Aldehyde
Ketone Carboxylic acid
Ester
4. Metamerism
Compounds having same molecular formula but different in alkyl groups present in the either side of the same functional group are called metamers. The phenomenon is called metamerism. Example :
Note : ethers, ketones, 20 amine, 30 amine, esters etc. show metamerism.
5. Tautomerism
It is a special type of functional isomerism in which there is migration of α- hydrogen from carbon to electronegative atom and vice-versa with necessary rearrangement of covalent bonds. Eg . keto – enol tautomerism.
- In this example, there is migration of α- H from carbon to electronegative oxygen atom and vice-versa.
6. Ring – chain isomerism
In ring-chain isomerism, one of the isomers has an open-chain structure whereas the other has a ring structure.
- They generally contain a different number of Π – bonds. Eg.
Stereoisomerism (Space isomerism)
- Compounds having same molecular formula but different arrangement of atoms or groups in space are called stereo isomers and the phenomenon of existence of such compounds is called stereoisomerism.
- It is also known as space isomerism.
Stereoisomerism can be categorized into following types:
- Geometrical isomerism
- Optical isomerism
1. Geometrical isomerism
- Stereoisomerism which arises due to different geometrical arrangement of atoms or group of atoms around rigid bond is called geometrical isomerism.
- It is also known as cis-trans isomerism. Eg.
Note : If the similar groups are present on the same side of a rigid bond it is cis- form and if it is present on opposite side then it is trans form.
2. Optical isomerism
- Compounds having same molecular and structural formula but differ in the way they interect with plane-polarized light (ppl) are called optical isomers.
- Optical isomers are also known as enantiomers.
- These are non- superimposable mirror images of each other.
- Optical isomers rotate plane polarized light in opposite directions.One isomer rotates it clockwise (Dextrorotatory, lebeled as D or +) and other rotates it antilockwise (Levorotatory, lebeled as L or -).
Example :
- Optical isomerism is possible in compounds having at least one chiral centre i.e. a carbon atom bonded with four diferent groups.
See the IUPAC nomenclature of Organic compounds ….
References
- Finar, I. L., Organic Chemistry, Vol. I and Vol. II, Prentice Hall, London, 1995.
- Morrison, R.T. , Boyd, R.N., Organic Chemistry, Sixth edition, Prentice-Hall of India Pvt. Ltd., 2008.
- March, j., Advanced Organic Chemistry, Fourth edition, Wiley Eastern Ltd. India, 2005.
- https://www.differencebetween.com/difference-between-structural-and-optical-isomers-in-carbohydrates/
- https://www.toppr.com/content/story/amp/finding-optical-isomers-53824/