3.1.5 Introduction to Organic Chemistry - Isomerism

Specification

Students should:
  • know and understand the meaning of the term structural isomerism
  • be able to draw the structures of chain, position and functional group isomers

Isomerism and branching

Any structure that can be drawn can exist providing the fundamental rules have been fulfilled.

This means that one molecular formula can have other possible structures. This is called isomerism. The chains formed by the alkanes and other organic molecules do not have to be straight (actually zigzag) but may be branched, i.e. having "branches" of carbon atoms attached to the main unbroken chain.

This particular form of isomerism is called chain isomerism.

Example: The alkane C4H10 exists in two isomeric forms - a straight chain form and a branched form

butane methylpropane

Isomerism


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Positional isomerism

This is when atoms, or groups attached to a hydrocarbon chain change their position:

1-bromopropane 2-bromopropane
1-bromopropane structure 2-bromopropane structure

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Functional group isomerism

This is when the atoms of a structure are rearranged to such an extent that the actual functional group changes:

ethanoic acid ethene-1,2-diol
ethanoic acid structure ethene-1,2-diol structure

The molecular formula of both structures above is C2H4O2, but it is clear that they are completely different molecules, one a carboxylic acid and the other an unsaturated (double bond) diol (two alcohol groups).


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