3.1.4 Structural Isomerism

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Isomers are molecules that have the same molecular formula (same types and numbers of atoms) but are arranged differently. Structural isomerism is one of the main types of isomerism and occurs when the atoms in the molecules are connected in different ways.

There are three main types of structural isomerism that you need to understand:

1. Chain Isomerism

Chain isomerism occurs when the carbon atoms in the molecule are arranged in different ways to form different carbon skeletons. The molecules still have the same molecular formula, but the structure is different due to branching.

Properties

Chain isomers often have similar chemical properties but slightly different physical properties such as boiling points.
More branched isomers tend to have lower boiling points because the molecules pack less efficiently, leading to weaker van der Waals forces between them.

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Example: Butane ( $C_4H_{10}$ ) can exist as n-butane (a straight chain) or 2-methylpropane (a branched chain).

2. Position Isomerism

Position isomerism occurs when the functional group of a molecule is attached to different positions on the carbon chain. The molecular formula and functional group remain the same, but the functional group's location varies.

Properties

Position isomers often have similar physical properties, but their chemical properties can vary slightly depending on the position of the functional group.

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Example: In butan-1-ol and butan-2-ol, both molecules have the same molecular formula ( $C_4H_{10}O$ ) and an alcohol ( $-OH$ ) group, but the position of the $-OH$ group is different.

3. Functional Group Isomerism

Functional group isomerism occurs when compounds have the same molecular formula but contain different functional groups. This leads to significant differences in both their chemical and physical properties.

Properties

Because these isomers have different functional groups, they exhibit completely different chemical properties and often have very different physical properties.

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Example: Ethanol ( $C_2H_6O$ ) and dimethyl ether ( $C_2H_6O$ ) have the same molecular formula, but ethanol contains an alcohol group ( $-OH$ ), while dimethyl ether has an ether group ( $C-O-C$ ).

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Summary

Understanding these types of structural isomerism is key to recognising how molecules can differ even when they have the same molecular formula. This concept is important in predicting the properties and reactivities of organic compounds.

Structural Isomerism Simplified Revision Notes for A-Level AQA Chemistry

3.1.4 Structural Isomerism

1. Chain Isomerism

Properties

2. Position Isomerism

Properties

3. Functional Group Isomerism

Properties

Summary

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