Ethyl alcohol Dimethyl ether
As we can see above, ethyl alcohol and dimethyl ether have different structural formula. But their molecular formula is same which is C2H6O. These types of compounds have not only different structural formulas but shows distinct properties as well. Thus, we can say a molecular formula can represent more than one compound. This phenomenon is known as isomerism. The word isomerism is made up of two words iso & meros, iso means same and meros means parts. So, word isomerism means same parts. Ethyl alcohol and dimethyl ether are called isomers of each other.
What do you mean by Isomerism? & What is an Isomer?
“Compounds that are represented by the same molecular formulae, but different structural formulae are called isomers and this phenomenon is known as isomerism.”
Isomers not only show different structural formula but different physical and chemical properties as well.
Examples of Isomers-
1. Ethyl alcohol and dimethyl ether are isomers of each other as both the compounds have the same molecular formula – C2H6O while different structural formulae.
2. Compounds such as pentane, iso-pentane and neopentane are isomers of each other. These all three compounds have the same molecular formula- C5H12. But different structural formulae.
Pentane
Isopentane
Neopentane
Types of Isomers or Isomerism –
Isomerism can be divided into mainly two types- Structural isomerism & Stereoisomerism. These can be further subdivided into different subtypes, which is illustrated in the diagram above.
Structural Isomerism - Compounds which show isomerism due to difference in their structures are known as structural isomers. This phenomenon is known as structural isomerism.
Examples of structural isomers – n-Butane and isobutane.
n-butane and isobutane have the same molecular formula- C4H10 but different structural formulae.
Butane Isobutane
Structural isomerism can be further divided into following subtypes-
1. Chain isomerism
2. Functional isomerism
3. positional isomerism
4. Metamerism isomerism
5. Tautomerism isomerism
6. Ring-chain isomerism
1.Chain isomerism – Those structural isomers which differ in chain of carbon atoms are known as chain isomers and the phenomenon is termed as chain isomerism. Thus, chain isomers differ in the arrangement of C-atoms in straight or branched chain of compounds.
Examples of chain isomers – Hexane – C6H14
Hexane
2-methyl pentane
3-methyl pentane
2,2-dimethyl butane
2,3- dimethyl butane
Important note – To show chain isomerism, organic compound or carbon compound should have minimum ‘4’ carbon atoms.
2.Functional Isomerism – Those compounds which have same molecular formula, but different functional groups are called functional isomers and this phenomenon is known as functional isomerism or functional group isomerism.
Example – Alcohol and ethers
Ethanol (functional group-OH) and dimethyl ether(functional group- R-O-R’)- Molecular formula – C2H6O
Structural formula of ethanol – Structural formula of dimethyl ether-
3.Positional isomerism – Those structural isomers which differ in position of substituents or functional group or multiple bonds, are known as positional isomers and this phenomenon is known as positional isomerism.
Positional isomerism example –
1. But-2-ene & But-1-ene (differ in position of double bond)
But-2-ene But-1-ene
2. 1-butanol & 2-butanol (differ in position of functional group -OH)
1-butanol 2-butanol
3. 1-chlorobutane & 2-chlorobutane (differ in position of substituent group -Cl)
1-chlorobutane 2-chlorobutane
4.Metamerism Isomerism – Those structural isomers which differ in their alkyl groups which are attached to their functional groups, are known as metamerism isomers and the phenomenon is known as metamerism isomerism.
Example- diethyl ether & methyl propyl ether
Diethyl ether [on both sides alkyl group -ethyl(C2H5) is attached]
Methyl propyl ether [one side alkyl group-methyl(CH3) is attached while on another side alkyl group- propyl(C3H7) is attached]
5.Tautomerism isomerism – Those structural isomers which differ in the position of hydrogen atoms or protons or electrons, are known as tautomers and this phenomenon is known as tautomerism. It is also known as desmotropism (desmos- bond & tropism-turn). Tautomers occur in equilibrium state with each other as they are easily inter-convertible.
To show tautomerism, compounds must contain highly electronegative elements with multiple bonds.
Example – Keto-enol tautomerism in acetone
Acetone (in keto form) Acetone(in enol form)
Ketone Enol
6.Ring-chain isomerism – Those structural isomers in which one isomer has open chain structure while another one has closed chain or ring structure, are known as ring-chain isomers and this phenomenon is called ring-chain isomerism.
Examples- Butene & cyclobutane
Butene
Cyclobutane
Stereoisomerism – Those compounds which have same molecular formula but show different spatial arrangement of atoms in them are known as stereoisomers and the phenomenon is known as stereoisomerism.
Example – 1,2-dichloroethene (C2H2Cl2)
Trans-1,2-dichloroethene Cis-1,2-dichloroetene
Stereoisomerism can be subdivided into following subtypes-
1. Geometrical isomerism – It is also known as Cis-Trans isomerism. Those stereoisomers in which isomerism arises due to restricted rotation of carbon-carbon double bond are known as geometrical isomers and this phenomenon is known as geometrical isomerism. In cis-trans isomerism it should be noted that the groups attached to the double bonded carbon atom should be different.
Cis Trans
Geometric Isomers Example – cis-2-butene & trans-2-butene
Cis-2-butene Trans-2-butene
2.Optical isomerism – Those stereoisomers which are mirror images of each other or differ in optical activity are known as optical isomers and this phenomenon is known as optical isomerism.
Conditions for optical isomerism – 1. In optical isomers carbon atoms are attached to four different atoms or groups.
2. Chiral center – Optical isomers have chiral centers, it means a carbon atom which is attached to the 4 different groups or atoms.
3. Optical isomer which rotates plane polarized light towards right side is called dextro (d) while the optical isomer which rotates towards left side is called laevo(l). A mixture containing equal amounts of dextro & laevo isomers is called racemic mixture. Racemic mixtures show no effect on plane polarized light as rotations of d & l cancel out each other.
Optical Isomers example or Example of d & l isomers
D-mannose L-mannose
Those optical isomers which are mirror images of each other are called enantiomers. While which are non-mirror images of each other are called diastereomers.
Examples – Enantiomers –
Diastereomers –
Cis Trans
FAQs on Isomerism
1. What is isomerism in simple terms?
Isomerism is a phenomenon in chemistry where two or more compounds have the exact same molecular formula (the same number of atoms of each element) but differ in their physical and chemical properties. This difference arises because the atoms are arranged differently in space.
2. What are isomers? Can you give an easy example?
Isomers are the molecules that share the same molecular formula but have different structures. For example, both butane and isobutane have the molecular formula C₄H₁₀. However, butane has a straight chain of four carbon atoms, while isobutane has a branched chain, giving them different boiling points and properties.
3. What are the two main categories of isomerism?
Isomerism is broadly divided into two main types:
- Structural Isomerism: This occurs when isomers have the same molecular formula but a different connectivity or arrangement of atoms. The atoms are bonded together in a completely different order.
- Stereoisomerism: This occurs when isomers have the same molecular formula and the same connectivity of atoms, but the atoms are arranged differently in three-dimensional space.
4. What is structural isomerism and what are its different types?
Structural isomerism involves compounds with the same molecular formula but different structural formulas. The main types based on the Class 11 syllabus are:
- Chain Isomerism: Isomers have different arrangements of the carbon skeleton (e.g., a straight chain vs. a branched chain).
- Position Isomerism: Isomers have the same carbon skeleton and functional group, but the position of the functional group or substituent on the chain is different.
- Functional Isomerism: Isomers have the same molecular formula but possess completely different functional groups (e.g., an alcohol and an ether).
- Metamerism: This is a special type where isomers have the same functional group but differ in the nature of the alkyl or aryl groups attached to either side of it.
5. How can you tell the difference between chain and position isomers?
The key difference lies in the carbon backbone. In chain isomerism, the fundamental structure of the carbon chain itself changes. For example, pentane (a 5-carbon straight chain) and 2-methylbutane (a 4-carbon chain with a methyl branch) are chain isomers. In position isomerism, the carbon backbone remains the same, but a functional group or substituent atom moves to a different position. For example, propan-1-ol and propan-2-ol are position isomers.
6. Why is isomerism so important in organic chemistry?
Isomerism is a fundamental concept because it explains the immense diversity of organic compounds. A single molecular formula can represent multiple distinct substances with unique properties. This is critical in many fields:
- In medicine, one isomer of a drug might be a life-saving medicine, while its stereoisomer could be inactive or even harmful.
- In industry, different isomers can have different boiling points, solubilities, and reactivities, making them suitable for different applications.
7. What is the process of isomerization?
Isomerization is a chemical process where one molecule is converted into another molecule that has the exact same atoms, but arranged differently. It's essentially the transformation of one isomer into another. This process doesn't happen on its own; it often requires energy (like heat or light) and a catalyst to rearrange the molecular structure.
