Organic Chemistry

Isomerism

Isomerism

Isomers are molecules which share the same molecular formula however their structures are different. There are a number of different type of isomerism but for your exam you will just need to be aware of structural isomerism.

Structural isomers are molecules which share the same molecular formula but their covalent bonds are arranged differently. This can be due to:

  • Positional isomerism: the functional group is in different positions.
  • Chain isomerism: the carbon skeleton is arranged differently.
  • Functional isomerism: they have different functional groups.

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Positional isomerism

Due to the fact alkanes have no functional group they do not show positional isomerism.

Alkenes which consist of four or more carbon atoms do show positional isomerism. For example, but-1-ene and but-2-ene.

Haloalkanes which consist of three or more carbon atoms also show positional isomerism. For example, 1-chloromethylpropane and 2-chloromethylpropane.

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Chain isomerism

The way in which carbon atoms are arranged is called the carbon skeleton. If the carbon skeleton contains four or more carbon atoms it can show chain isomerism. For example, methyl-propane or butane.

A carbon skeleton which contains no more than three carbon atoms can only be arranged as a straight line with no branches.

A carbon skeleton with four carbon atoms can be arranged in one of two ways.

A carbon skeleton with five carbon atoms can be arranged in one of three ways.

A carbon skeleton with six carbon atoms can be arranged in one of five ways.

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Functional isomerism

Alkanes which are arranged in a ring formation are known as cycloalkanes. Like alkenes, they have the general formula CnH2n. Therefore, cycloalkanes and alkenes are said to show functional isomerism. For example, cyclohexane and hex-1-ene.

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Distinguishing between isomers

Isomers usually differ in terms of their melting and boiling points. The more branching an isomer has, the lower its boiling point tends to be. This is due to surface area: molecules with no branching have a larger surface area meaning they pack together more tightly and so possess stronger Van der Waal’s forces.