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Isomerism: Examples, Types - Structural isomerism Types and Stereoisomerism

 

 The phenomenon where chemical compounds having the same molecular formula exhibit different chemical structures is called isomerism. Chemical compounds having the same molecular formula have different shapes are termed isomers.

Types of isomerism

Isomerism is primarily classified into two types - Structural isomerism and stereoisomerism.

Structural isomerism

Structural isomerism is the phenomenon where chemical compounds having the same molecular formula have different chemical structures. Structural isomers differ in the way functional groups and other atoms are linked to the central atom. Structural isomerism is also called constitutional isomerism. Structural isomers may or may not have the same functional groups, however, the molecular formula remains the same.

Structural isomerism is further classified into various types and are explained below.

  1. Chain isomerism

    The phenomenon where structural isomers differ in the chain of carbon atoms is called chain isomerism. Chain isomerism is also called skeletal isomerism. Consider the example of pentane and its structural isomers to understand chain isomerism better.

    In the given example, we can see that the pentane molecule exists in 3 different forms - one linear and 2 branched.

  2. Position isomerism

    The phenomenon where structural isomers differ in the position of functional groups or substituent atoms or multiple bonds is called positional isomerism.

    This example illustrates positional isomerism where the double bond is present in two different position i.e. C1 and C2.

    This example illustrates positional isomerism where the isomers differ in the position of the bromine atom i.e. C1 and C2.

  3. Functional isomerism

    In functional isomerism, isomers have the same molecular formula but different functional groups.
     
    • Alcohols and ethers having the same molecular formula form functional isomers.
    • Ketones and aldehydes having the same molecular formula form functional isomers.
    • Carboxylic acids and ethers having the same molecular formula form functional isomers.

    In this example, we can see that the isomers are having the same molecular formula but different functional groups. The functional isomer of propanone (consists of a keto functional group) is propanal (consists of an aldehyde functional group).

  4. Metamerism

    Metamerism is a phenomenon where structural isomers differ in the arrangement of alkyl groups present around the functional group. Metamerism is a rare type of isomerism and is mostly seen in chemical molecules containing divalent attached by alkyl groups.

    Check out the isomers of C4H10O. The first isomer has two ethyl groups present on either side of the functional group. The second isomer has one ethyl and one propyl group present on either side of the functional group.

  5. Tautomerism

    The phenomenon where the position of protons or electrons or hydrogen is different in each isomer. Tautomerism is seen in very few chemical compounds and takes place through intramolecular proton transfer. Tautomers exist in equilibrium and are interchangeable.

    This example illustrates keto-enol tautomerism where one isomerism contains an alcohol group with a double bond (enol form) and the other contains a ketone functional group (keto form).

  6. Ring-chain isomerism

    In the ring-chain isomerism, one of the structural isomers exists in chain form whereas the other one has a ring in it. This phenomenon is widely seen in chemical entities containing more than two pi bonds.

    Propene exists in two forms - linear and ring.

Stereoisomerism

Isomers having the same chemical formula but differing in the spatial arrangement of atoms is called stereoisomerism. Isomers exhibiting stereoisomerism are called stereoisomers. This phenomenon is seen in compounds existing in a 3D space.

Stereoisomerism is further classified into types and are explained below.

  1. Geometric isomerism

    Geometric isomerism is popularly called cis-trans isomerism. In geometric isomerism, chemical compounds differ in the arrangement of atoms or a group of atoms around the carbon-carbon double bond. This isomerism is mainly due to the restricted rotation of atoms present around the carbon-carbon double bond.

    An isomer containing similar groups on the same side of the double bond is called the cis isomer whereas the one containing similar groups on opposite sides is called the trans isomer.

    Consider the example of but-2-ene. Cis isomer has similar atoms i.e. CH₃ and H present on the same side whereas trans isomer has similar atoms present on the opposite sides.

  2. Optical isomerism

    Optical isomerism is characterized by the formation of non-superimposable mirror images. This is a phenomenon where chemical structures with the same molecular formula differ in the spatial arrangement of atoms, resulting in the formation of non-superimposable mirror images. These non-superimposable mirror images are called enantiomers.

    The two types of enantiomers are dextro and laevo. The enantiomer that rotates plane polarized light in clockwise direction is called dextro form whereas the one rotating plane polarized light in anticlockwise direction is called laevo form.

    Consider the example of alanine. It has two optical isomers - dextro and laevo.
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