Organic Chemistry Tutorials

Structural Isomers

Core Concepts

In this tutorial you will learn how to define structural isomers, and how they differ from stereoisomers. You will also learn how to categorize different types of structural isomers.

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What is Structural Isomerism?

Structural Isomerism, also called compositional isomerism, describes a set of molecules that are made with the same atoms in the same amounts, but are arranged differently.

2-butanol (left) and 1-butanol (right) are structural isomers because they have the same amount and quantity of molecules, but differ in the position of the alcohol (-OH) group.

Structural vs. Stereoisomerism

Structural isomers differ specifically in the connections between atoms, and not the arrangement of atoms in space, like in the Butanol example above, where the only difference between atoms is the position of the alcohol. Stereoisomers like cis trans isomers have the same bonds between the same atoms, but differ in the conformation of those bonds. For example, trans-2-butene and cis-2-butene differ in their conformation, but the bonds between atoms do not change from one molecule to the next. This means that while they look different, they are stereoisomers, and not structural isomers.

trans-2-butene cis-2-butene 

Types of Structural Isomers 

Structural isomers can be further categorized by their specific structural differences.

Positional Isomerism

Position isomerism, sometimes called regioisomerism, describes when molecules differ only in the placement of an atom, or group of atoms, that is not included in that molecule’s backbone. In the example given for 2-butanol (left) and 1-butanol (right), only the OH group moves, and so it is considered a positional isomer.

Functional Isomerism

Functional Isomerism describes when molecules differ in the functional group that is present. This is significant because molecules with the same chemical composition, but different functional groups can have drastically different reactivity. Below, dimethyl ether (left), and ethanol both have the chemical formula C2H6O. Dimethyl ether is an ether, meaning it had a oxygen in between two carbons, and Ethanol is a alcohol, meaning it has a terminal -OH group. Ethers are generally less reactive than alcohols.

Here are some examples of functional groups.

Skeletal Isomerism

Skeletal Isomerism, also referred to as chain isomerism, describes when molecules differ in the arrangement of their carbon backbone, or skeleton. In the example below, isobutane (left) and butane (right) are skeletal isomers, because they differ in how their carbon backbone is ordered. While butane is linear, isobutane is not, with three of its carbons branching off of the a central carbon atom.

Further Reading

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