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Isomerism – Definition, Types, Structure, Examples

  • Last Updated : 16 Dec, 2021

Organic chemistry is a discipline of chemistry concerned with the structure, characteristics, and interactions of organic molecules containing carbon in covalent bonds. Their structural formula is determined by studying their structure. Physical and chemical qualities are studied, as well as chemical reactivity, to better understand their behavior. The chemical synthesis of natural products, medicines, and polymers, as well as the laboratory and theoretical (in silico) research of individual organic molecules, are all part of the study of organic processes.

Hydrocarbons (compounds containing solely carbon and hydrogen) and compounds based on carbon but also including other elements, such as oxygen, nitrogen, sulphur, phosphorus (found in many biochemicals), and halogens, are among the substances studied in organic chemistry. The study of carbon–metal bonding is known as organometallic chemistry.

Isomerism

One of the most essential features of organic molecules is their propensity to show isomerism. Two or more compounds with the same molecular formula but different physical and chemical properties are known as isomers. Isomerism is the term for this phenomenon.

Types of Isomerism

Isomerism can be divided into two categories:

  1. Structural or constitutional isomerism
  2. Stereoisomerism.

Structural Isomerism or Constitutional Isomerism

The constitution of a molecule refers to the quantity, kind, and connection of its atoms. The structural isomers or constitutional isomers differ in the connection of their atoms, and this phenomenon is known as constitutional isomerism or structural isomerism.

Because they have the same molecular formula, ethanol and dimethyl ether are isomers. C2H6O. Because the atoms in each chemical are bonded differently, they are structural isomers. The following are some examples of structural isomerism in various forms:

  • Chain isomerism
  • Position isomerism
  • Functional isomerism
  • Metamerism
  • Tautomerism
  • Ring chain isomerism

Chain Isomerism

Chain isomers are isomeric substances that differ solely in the arrangement of carbon atoms in the base chain, and chain isomerism is isomerism between them. The carbon atom chain or skeleton differs amongst isomers of this type. 

Carbon atoms can be arranged in a straight or branched chain. Chain isomerism does not exist in molecules comprising up to three carbon atoms since they are a continuous chain with no chance of branching. It is demonstrated for compounds with four or more carbon atoms. 

  • Chain isomerism in alkanes: Alkanes containing four or more carbon atoms. For example, Two chain isomers are represented by the chemical formula C4H10.

  • Chain isomerism in alkenes: When writing chain isomers for alkenes, the position of the double bond should not be modified. For example, Two chain isomers are represented by the chemical formula C4H8.

  • Chain isomerism in alkynes: Chain isomerism occurs in alkynes, which are compounds having five or more carbon atoms. The two chain isomers are represented by the chemical formula C5H8.

Position Isomerism

Position isomerization is a phenomenon that occurs when two or more compounds differ in the position of a component atom or functional group on the carbon skeleton.

C3H8O, for example, is made up of two alcohols:

Functional Isomerism

Two or more molecules with the same chemical formula but distinct functional groups are called functional isomers. Functional isomerism is the term for this occurrence. 

For example, the chemical formula C3H6O denotes both an aldehyde and a ketone:

Metamerism

Metamers are molecules that have the same chemical formula but differ in atomic or group locations on both sides of bridge functional groups. Metamers are distinguished by the nature of the alkyl groups surrounding a certain functional group. 

As a result, they have a connection. For example, the chemical formula C4H10O accommodates three metameric isomeric ethers.

Tautomerism

Tautomers are functional isomers that exist in dynamic equilibrium at the same time. The most prevalent kind is keto-enol tautomerism, which is caused by the 1,3 movement of the hydrogen atom from carbon to oxygen and vice versa.

For example,

Tautomerism in an aldehyde or ketone requires the existence of a – hydrogen, which is involved in 1,3 – migration. In the lack of a – hydrogen, tautomerism is absent. Acetone, acetaldehyde, and acetophenone, for example, might display tautomerism due to the existence of a—hydrogen, whereas benzaldehyde and benzophenone do not due to the absence of a – hydrogen.

Ring-chain Isomerism

Ring chain isomerism is structural isomerism that differs from the other structural isomers discussed previously. It is present in isomeric open-chain and cyclic molecules. Compounds with the same chemical formula but open-chain or cyclic structures are known as ring chain isomers. And this phenomenon is known as ring chain isomerism.

The ring chain isomers are represented by the chemical formula C3H6.

Stereoisomerism

Stereoisomers are compounds that have the same chemical and structural formulae but differ in the relative arrangement of the atoms or groups in space, a phenomenon known as stereoisomerism.

Stereoisomerism can be divided into two categories:

  1. Conformational isomers: Conformational isomers are stereoisomers having different relative positions of atoms inside the molecule that can be interconverted by rotating around sigma links. In other words, there is no need to break and re-make covalent bonds in order to interconvert these isomers.
  2. Configurational isomers: Stereoisomers that can only be interconverted by breaking and reconstructing covalent bonds rather than rotating around sigma bonds are known as configurational isomers. Geometrical isomerism and optical isomerism are the two types of configurational isomerism.
    • Geometrical isomerism or cis-trans isomerism: Isomerism generated by constrained rotation around a link in a molecule is known as geometrical or cis-trans isomerism. A wide range of substances exhibit geometrical isomerism, which can be characterized as follows:
      • C=C, C=N, N=N are examples of compounds with a double bond.
      • A cyclic molecule, including homocyclic, heterocyclic, and fused ring systems.
      • Steric hindrance compounds that prevent rotation around a single bond. Biphenyls, for example.
    • Optical isomerism: Certain chemicals can exist in two stereoisomeric forms that are incompatible with one another. These isomers have equal physical properties, such as melting point, boiling point, refractive index, density, solubility, and so on, despite the change in atom arrangement. They are also similar in most of their chemical interactions. They only differ in their behaviour when it comes to plane polarised light; one rotates it to the right, while the other rotates it to the left, but to the same degree. An optical isomer is a pair of chemicals that have the same optical properties. Optical isomers are two compounds that contain the same number and type of atoms, bonds, and spatial configurations of the atoms but are not superimposable mirror images of each other. Enantiomers are isomers that are non-superimposable mirror reflections of each other.

Sample Questions

Question 1: What is isomerism?

Answer:

Two or more compounds with the same molecular formula but distinct physical and chemical properties are known as isomers. Isomerism is the term for this phenomenon.

Question 2: What are the types of isomerism?

Answer

Isomerism can be divided into two categories. Stereoisomerism and structural isomerism are the two types.

  • Structural isomerism
    1. Chain isomerism
    2. Positional isomerism
    3. Functional isomerism
    4. Metamerism
    5. Tautomerism
    6. Ring chain isomerism
  • Stereoisomerism
    1. Constitutional isomerism
    2. Conformational isomerism
      • Optical isomerism
      • Geometrical isomerism

Question 3: What is Ring-chain isomerism?

Answer:

Compounds with the same chemical formula but open-chain or cyclic structures are known as ring chain isomers. Ring-chain isomerism is the term for this phenomena.

For example,  the molecular formula C3H6 represents the ring chain isomers

Question 4: What is Structural isomerism?

Answer:

The structural isomers or constitutional isomers differ in the connection of their atoms, and this phenomenon is known as constitutional isomerism. There are six types of structural isomerism.

  1. Chain isomerism
  2. Positional isomerism
  3. Functional isomerism
  4. Metamerism
  5. Tautomerism
  6. Ring chain isomerism

Question 5: What is Positional isomerism?

Answer:

Position isomers are two or more compounds that differ in the position of a constituent atom or functional group on the carbon skeleton, and the phenomenon is known as position.

 For example,C3H8O, represents two alcohols:


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