Structural Isomers (SL IB Chemistry)

What are isomers?

• Isomers are compounds that have the same molecular formula but a different arrangement of atoms
• One group of isomers is the structural isomers
  • These are compounds that have the same molecular formula but different structural formulae

Isomers of C₃H₆ 

Propene and cyclopropane are both made of 3 carbon and 6 hydrogen atoms but the structure of the two molecules differs

• 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

Both compounds have the same molecular formula however butan-1-ol contains an alcohol functional group and ethoxyethane an ether functional group

• 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 arise from differences in the position of a functional group in each isomer
  • This literally means that the functional group is located on different carbon atoms

Position isomers of butanol, C₄H₉OH, diagram

Both compounds are made up of 4 carbon, 10 hydrogen and one oxygen atom. However, the alcohol / OH group is located on different carbon atoms

• In the example above, the functional group is the alcohol / OH group
  • The OH group can be attached to carbon-1, which gives rise to butan-1-ol
  • The OH group can be attached to carbon-2, which gives rise to butan-2-ol
  • Careful: In the butan-1-ol diagram, it appears that the OH group is attached to carbon-4 
    • If you imagine looking at the molecule from the other side, you would see that the OH group is attached to carbon-1
    • The same is true of butan-2-ol, where the OH group appear to be attached to carbon-3 
• 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  

Classifying primary, secondary and tertiary alcohols and alcohols with more than one alcohol group

• Alcohols, e.g. propanol
  • The primary alcohol propan-1-ol and the secondary alcohol propan-2-ol are position isomers of each other
  • Careful: The tertiary alcohol 2-methylpropan-2-ol is another isomer but it is branched chain not position isomerism 
• Halogenoalkanes, e.g. C₄H₁₁Br
  • The primary halogenoalkane 1-bromobutane and the secondary halogenoalkane 2-bromobutane are position isomers of each other
  • Careful: The tertiary halogenoalkane 2-bromo-2-methylpropane is another isomer but, again, it is branched chain not position isomerism 

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, i.e. where the longest hydrocarbon is broken into smaller pieces and some of these smaller pieces are added as side-chains / branches

Isomers of C₅H₁₂ 

Both compounds contain 5 carbon and 12 hydrogen atoms. However, the longest carbon chain in pentane is 5 and in 2,2-dimethylpropane it is 3 (with two methyl branches)

Isomerism in amines

• Amines follow a slightly different classification system to alcohols and halogenoalkanes, although the terms primary, secondary and tertiary are still used
• The classification is based on the number of alkyl groups attached to the nitrogen in the amine
  • Primary amines are those in which the nitrogen is attached to one other carbon atom (or alkyl group)
  • In secondary amines, the nitrogen atom is attached to two other carbon atoms (or alkyl groups)
  • In tertiary amines, the nitrogen is attached to three other 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