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Positional Isomerism

What is it?

Positional isomerism, an example of structural isomerism, occurs when functional groups are in different positions on the same carbon chain. In GCSE Chemistry, you usually meet positional isomers of alcohols and alkenes. At A Level, you find out about positional isomers involving other functional groups in aliphatic (straight- or open-chain) compounds, and in aromatic compounds.

What is here?

I have put models of the positional isomers of butanol, butene and methylphenol here. The first two are typical of the sort of molecules you meet in GCSE and at AS Level; you will meet aromatic compounds like methylphenol at A2 level. You should be prepared to work out positional isomers for simple organic compounds like these for your examinations, and you also need to be able to name the them at AS/A Level. You can see:

butan-1-ol

butan-2-ol

 

butan-1-ol

butan-2-ol

 

 

but-1-ene

but-2-ene

Note: this is cis-but-2-ene, which has a geometric isomer called trans-but-2-ene (select here to find out more)

but-1-ene

but-2-ene

 

2-methylphenol

3-methylphenol

4-methylphenol

2-methylphenol

3-methylphenol

4-methylphenol

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The models are ball-and-stick, but you can right click to get a menu of options, such as animations and changing to space-filling models. You can also move the models using the left mouse button.

Molecular modelling applet courtesy of ChemAxon Ltd

 

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