Constitutional Isomers
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Constitutional Isomers and Stereoisomers

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Isomerism is one of the most important topics covered in organic chemistry. In most organic reactions, the products are formed based on the concept of isomerism. Students may land up predicting the wrong product of a given reaction if they do not have a clear idea of the concept of isomerism. By the virtue of isomerism, compounds have the same molecular formula but differ in physical and chemical properties. Such compounds are referred to as Isomers. The classification of isomers is based on connectivity. Diastereomers, and enantiomers are also stereoisomers. Isomers are broadly classified as Constitutional Isomers and Stereoisomers.


What Are Constitutional Isomers?

These are also known as structural isomers. The formula of these isomers are the same but the arrangement of atoms in the compounds are different.


Types of Constitutional Isomers

Constitutional isomers are classified into the following types.

  • Position Isomers

These isomers have a different position of the functional groups in them and the carbon skeleton chain remains the same. These isomers are also known as regioisomers.

For example 1-propanol and 2-propanol, ortho dichlorobenzene, and meta-dichlorobenzene. This phenomenon is Position isomerism.

  • Functional Isomers

Compounds that have the same molecular formula but different functional groups are said to be functional isomers. For example alkene and cycloalkene. This phenomenon is the functional group isomerism.

For example, C3H8O can be presented as:

CH3OCH2CH3

Or

CH3CH2CH2OH

  • Chain Isomers

When compounds have the same molecular formula but the carbon chain of their structure is different then they are called chain isomers. These isomers are also known as skeletal isomers. This phenomenon is also known as chain or nuclear isomerism.

As seen in the figure the carbon chain is different but the number of carbon atoms is the same.

  • Metamers

These isomers are formed when different groups are attached to the polyvalent atom of the functional group and thus formula remains the same but the groups attached to the polyvalent atom become different. This phenomenon is known as Metamerism. It is exhibited by ketones and ethers.

  • Tautomers

These are the constitutional isomers that undergo rapid interconversion with each other. Tautomers cannot exist independently of each other. Tautomers exist in dynamic equilibrium.


Stereoisomerism 

Compounds that have the same chemical formula, as well as the same structure but different spatial arrangement or configuration, are known as  Stereoisomers. This phenomenon is known as Stereoisomerism.


Types of Stereoisomers

  • Geometrical isomers

  • Optical isomers


Geometrical isomers

These isomers are also known as cis, trans-isomers. This phenomenon is geometrical isomerism and is exhibited by alkenes. In alkenes, there is a restricted rotation of groups about the double bonds between the two carbon atoms. Geometrical isomerism can also be seen in compounds having groups C=N or N=N bond-like in aldoximes. When similar groups are present on the same side of the double bond then it is called the cis isomer. When similar groups are on the opposite side of the double bond then it is called the trans isomer.

(Image to be added soon)

cis and trans stereoisomers can be interconverted under extreme conditions like very high temperatures.


Optical Isomerism

Only those compounds with a chiral carbon can exhibit optical isomerism. Chiral carbon is that carbon that is attached to all the four different functional groups. The compound with a chiral carbon is said to have chirality. If chirality is present then the compound becomes optically active. A compound must be asymmetric to exhibit optical isomerism.

FAQ (Frequently Asked Questions)

What is R and S Configuration?

R and S nomenclature system was given by Cahn, Ingold, and Prelog. This nomenclature tells the configuration of the chiral center. In a pair of enantiomers having a chiral center one out of two will have R configuration and the other will have S configuration. If the sequence of groups attached to the chiral center is left to right then it is R configuration and if the sequence is from right to left then it is S configuration.

This system is based on the 3-Dimensional formula and not on the 2 -D Projection. The priority of the groups attached to the chiral carbon is determined by various priority sequence rules.

2. What are enantiomers and diastereomers?

Enantiomers are optical isomers that have mirror images non-superimposable to each other. An optically active substance can rotate the plane polarised light in either left or right direction. The one which rotates toward the right is dextrorotatory represented by d/+ and if it rotated towards the left then its laevorotatory which is represented by l/-. A compound and its enantiomer can be distinguished using a plane polarised light. For example, Lactic acid has a chiral carbon and has two enantiomers d and l form. The d and l forms are non-superimposable mirror images.

Diastereomers are not mirrored images but have different physical and chemical properties. Many compounds have more than one chiral center. In such compounds, diastereomers are found.

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