Structural isomerism

Structural isomerism is the formation of organic compounds that have the same molecular formula but different structural formulae.


The molecular formula tells us the number and type of each atom in one molecule of a compound. However, for a given molecular formula
there may be different ways in which these atoms are arranged.

These different molecules, with different structures, are actually different compounds, even though they have the same molecular formula. Compounds that have the same molecular formula but different structural formulae are known as structural isomers.

There are three types of structural isomerism:

  • position isomerism
  • functional group isomerism
  • chain isomerism.

Position isomerism

In position isomerism, it is the position of the functional group that differs in each isomer. For example, the compound with the molecular formula C3H6Br2 has four possible position isomer.

For example, the molecular formula C3H7CI has two possible isomers, 1-chloropropane and 2-chloropropane.

1-chloropropane

For 1-chloropropane, the chloro functional group is on carbon 1.

1-chloropropane

2-chloropropane

For 2-chloropropane, the chloro functional group is on carbon 2.

2-chloropropane

Take extra care when drawing the structural or displayed formula of different isomers not to repeat the same structure. For example, the following are all 1-chloropropane. They are all the same because the carbon atoms are all counted from the end that gives the lowest possible numbers in the name.

1-chloropropane

1-chloropropane

1-chloropropane

Functional group isomerism

In functional group isomerism there are different functional groups present for the same molecular formula. For example, the molecular formula C3H8O can either be an alcohol or an ether.

propan-1-ol

propan-1-ol

methoxyethane

methoxyethane

Propan-1-ol and methoxyethane have the same molecular formula but they are two isomers with different functional groups and so their chemical properties differ.


Chain isomerism

Chain isomers differ in the structure of their carbon chain skeleton. For example, the molecular formula C4H90H can either represent butan-1-ol or 2-methylpropan-1-ol.

butan-1-ol

butan-1-ol

2-methylpropan-1-ol

2-methylpropan-1-ol