Alkanes and structural isomerismEdexcel International A Level Chemistry: Revision notes
Section 1
Alkanes
Alkanes are hydrocarbons: they contain carbon and hydrogen only. They are saturated: every carbon–carbon bond is a single bond, so each carbon atom holds the maximum number of hydrogen atoms.
Their general formula is CₙH₂ₙ₊₂. The first six are methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀, pentane C₅H₁₂ and hexane C₆H₁₄. Octane is C₈H₁₈ and decane is C₁₀H₂₂.
Alkanes are named with a stem for the longest chain and the ending -ane; branches are named as prefixes such as methyl.
'Saturated' means only single bonds, not 'no double bonds'. A compound with a triple bond is also unsaturated.
Section 2
Cycloalkanes
Cycloalkanes are saturated hydrocarbons in which the carbon atoms form a ring. Their general formula is CₙH₂ₙ, which is two hydrogen atoms fewer than the alkane, because the two ends of the chain are joined by an extra C–C bond.
Examples: cyclopropane C₃H₆, cyclobutane C₄H₈, cyclopentane C₅H₁₀ and cyclohexane C₆H₁₂. Side chains are named as prefixes: methylcyclopropane is C₄H₈.
Cycloalkanes with CₙH₂ₙ are not isomers of alkanes, because they have a different molecular formula.
Section 3
Structural isomerism
Structural isomers are compounds with the same molecular formula but different structural formulae, meaning a different arrangement of atoms.
In alkanes, structural isomers arise from chain branching:
- C₄H₁₀: butane and 2-methylpropane (2 isomers)
- C₅H₁₂: pentane, 2-methylbutane and 2,2-dimethylpropane (3 isomers)
- C₆H₁₄: hexane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane and 2,2-dimethylbutane (5 isomers)
Cycloalkanes also have structural isomers: C₄H₈ gives cyclobutane and methylcyclopropane; C₅H₁₀ gives cyclopentane, methylcyclobutane, ethylcyclopropane, 1,1-dimethylcyclopropane and 1,2-dimethylcyclopropane.
Rotating a chain on paper does not give a new isomer. If the longest chain and the branches are the same, it is the same compound.
Section 4
Finding and naming isomers
To find every isomer, work systematically:
- Draw the longest chain (the straight-chain isomer).
- Shorten the chain by one carbon and put the carbon on every different position (a branch on carbon 1 just gives a longer chain).
- Shorten again and place two branches, on different carbons and then on the same carbon.
- Check each by naming it: if two drawings have the same name, they are the same compound.
To name: choose the longest chain, number from the end that gives the lowest numbers to the branches, and list the branches in alphabetical order. Number each branch, so write 2,3-dimethylbutane, not 2-3-dimethylbutane or dimethylbutane.
Check your count: C₄H₁₀ has 2 isomers, C₅H₁₂ has 3 and C₆H₁₄ has 5.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Alkanes and structural isomerism
- Petrol is a mixture of hydrocarbons including pentane, octane and cyclohexane. A refinery chemist classifies each of these compounds as an alkane or a cycloalkane.Explain what is meant by the terms 'hydrocarbon' and 'saturated' when they are applied to octane.2 marks
- Pentane, C₅H₁₂, is a liquid in petrol that boils at 36 °C. A refinery sample of C₅H₁₂ is found to contain two other compounds with the same molecular formula, which boil at 28 °C and 10 °C.Explain why the compound that boils at 28 °C, 2-methylbutane, is a structural isomer of pentane.2 marks
- A hydrocarbon X is found in petrol. Analysis shows that it contains 83.3% carbon and 16.7% hydrogen by mass, and its relative molecular mass is 72.0. X does not decolourise bromine water and contains no carbon–carbon double bonds.Use the data to deduce the empirical formula and the molecular formula of X, showing your working.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).