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Organic chemistry - the basics

Structural isomerism

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Explainer Video

Tutor: Alisha

Summary

Structural isomerism

In a nutshell

Atoms in a given molecule may be arranged differently giving rise to structural isomers. There are three types of structural isomers which are chain, positional and functional group isomers.

Isomers

Molecules with the same molecular formula as one another but with a different arrangement of atoms are known as isomers. There are two types of isomers:

Structural isomers
Stereoisomers

Stereoisomers are molecules with the same molecular and structural formula but with a different arrangement of atoms in space.

Structural isomers

There are three types of structural isomers:

Chain isomers
Positional isomers
Functional group isomers

Chain isomers

The carbon skeleton of each molecule differs from one another. One molecule may have a straight chain whilst the other is branched. These isomers have the same chemical properties but have different physical properties.

Example

Molecular formula: $C_4H_{10}$

Chemistry; Organic chemistry I; KS5 Year 12; Structural isomerism — A. Butane; B. $2$ -methylpropane

Positional isomers

In positional isomers the isomers have the same functional group however they are positioned in a different part of the carbon chain. These isomers have different physical properties and may have different chemical properties.

Example

Molecular formula: $C_3H_8O$ 

Functional group isomers

In this type, the same atoms are arranged into different functional groups. These isomers have different chemical and physical properties.

Example

Molecular formula: $C_2H_6O$ 

Important: There is a degree of flexibility around $C-C$ bonds. This means a molecule may seem to have an isomer but in fact it is the same molecule rotated differently.

Example

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