Structural Isomerism (AQA AS Chemistry): Revision Note

Molecules: Straight, Branched & Cyclic

• Straight, branched, and cyclic organic molecules are also called aliphatic compounds as long as they do not contain a benzene ring

Straight-chain

• Straight-chain organic molecules are those in which the carbon atoms are connected in one continuous chain

Pentane is a straight-chain organic molecule, as the carbon atoms are connected in one continuous chain

Branched

• Branched organic molecules have side groups attached to the main chain of carbon atoms

2-methylbutane is a branched organic molecule, as the main chain (consisting of 4 carbon atoms) has a methyl branch

Cyclic

• Cyclic organic molecules are those in which the carbon atoms are connected in a ring shape

Cyclopentane is a cyclic organic molecule, as the carbons are attached in a ring structure

Structural Isomerism

• Structural isomers are compounds that have the same molecular formula but different structural formulae

  • E.g., propene and cyclopropane

Both propene and cyclopropane are made up of 3 carbon atoms and 6 hydrogen atoms, but the structure of the two molecules differs

• There are three different types of structural isomerism:

  • Chain isomerism

  • Positional isomerism

  • Functional group isomerism

Chain isomerism

• Chain isomerism is when compounds have the same molecular formula, but their longest hydrocarbon chain is not the same

• This is caused by branching

  • E.g., pentane and 2,2-dimethylpropane 

Both compounds are made up of the same atoms; however, the longest carbon chain in pentane is 5, and in 2,2-dimethylpropane it is 3 (with two methyl branches)

Positional isomerism

• Positional isomers arise from differences in the position of a functional group in each isomer

  • The functional group can be located on different carbons

  • For example, butan-1-ol and butan-2-ol 

Both compounds have an alcohol group and are made up of 4 carbons, 10 hydrogens, and one oxygen; however, in butanol, the functional group is located on the first carbon, and in 2-butanol, on the second carbon

Functional group isomerism

• When different functional groups result in the same molecular formula, functional group isomers arise

• The isomers have very different chemical properties, as they have different functional groups

  • For example, butanol and ethoxyethane

Both compounds have the same molecular formula, but butanol contains an alcohol functional group and ethoxyethane an ether functional group

Deducing Isomers

• You should be able to deduce all possible isomers for organic compounds, knowing their molecular formula