Structural Isomers (DP IB Chemistry): Revision Note

Structural Isomers

What are isomers?

• One group of isomers is the structural isomers

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

Isomers of C₃H₆ 

• There are three different types of structural isomerism:

  1. Functional group isomerism

  2. Positional isomerism

  3. Branched chain isomerism

Functional group isomerism

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

• These isomers have very different chemical properties as they have different functional groups

Functional group isomers of C₄H₁₀O

• It can help to be aware of which homologous series can be functional group isomers of each other:

  • Alkenes and cycloalkanes

  • Alcohols and ethers

  • Aldehydes and ketones

Positional isomerism

• Positional isomers have the same molecular formula and functional group, but the functional group is attached to different carbon atoms in the chain

Position isomers of butanol, C₄H₉OH, diagram

• Both butan-1-ol and butan-2-ol contain:

  • 4 carbon atoms

  • 10 hydrogen atoms

  • 1 oxygen atom

• The difference lies in the position of the –OH (alcohol) group on the carbon chain:

  • In butan-1-ol, the OH is attached to carbon-1.

  • In butan-2-ol, the OH is attached to carbon-2.

• The molecule may appear reversed depending on how it's drawn, but the carbon numbering still starts from the end closest to the functional group

• Some organic compounds that can be described as having primary, secondary or tertiary structures will exhibit isomerism

  • The terms primary, secondary and tertiary relate to the number of carbon atoms that the functional group carbon is attached to

Demonstrating primary, secondary and tertiary structures in alcohols  

• Alcohols, e.g. propanol

  • Propan-1-ol (a primary alcohol) and propan-2-ol (a secondary alcohol) are position isomers:

    • They have the same molecular formula, but the –OH group is attached to different carbon atoms

  • 2-methylpropan-2-ol is a tertiary alcohol and an isomer of propanol, but it is a branched-chain isomer, not a positional isomer

• Halogenoalkanes, e.g. C₄H₁₁Br

  • 1-bromobutane (primary) and 2-bromobutane (secondary) are position isomers:

    • The Br atom is attached to different carbons in a straight chain

  • 2-bromo-2-methylpropane (tertiary halogenoalkane) is also an isomer of C₄H₁₁Br, but it is a branched isomer, not a positional one

Branched chain isomerism

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

• This is caused by branching, where the longest hydrocarbon is broken into smaller pieces and some of these smaller pieces are added as side-chains / branches

Isomers of C₅H₁₂ 

Isomerism in amines

• Amines use the terms primary, secondary, and tertiary, similar to alcohols and halogenoalkanes, but the classification is based on the number of carbon atoms (or alkyl groups) attached to the nitrogen atom

• Primary amines (1°):

  • The nitrogen is bonded to one carbon atom (or alkyl group)

• Secondary amines (2°):

  • The nitrogen is bonded to two carbon atoms (or alkyl groups)

• Tertiary amines (3°):

  • The nitrogen is bonded to three carbon atoms (or alkyl groups)

Examples of primary, secondary and tertiary amines

The number of carbons attached to the nitrogen atom indicate if an amine is primary (1 carbon), secondary (2 carbons) or tertiary (3 carbons)

• This means that amines do show isomerism

  • It is ambiguous whether isomerism in amines is position or branched chain 

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