What is a homologous series? Write its characteristics with an example.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Define in one line. A homologous series is a group of organic compounds with the same functional group, where each member differs from the next by a -CH₂- group.
2 · Give an example series. Alkanes: CH₄, C₂H₆, C₃H₈, C₄H₁₀ (general formula CₙH₂ₙ₊₂).
3 · Write the structures. [CH₄, CH₃-CH₃ and CH₃-CH₂-CH₃ written one below the other, with the extra -CH₂- circled each time]
4 · Write the characteristics.
Same general formula.
Successive members differ by -CH₂-, that is 14 u.
Same functional group, so similar chemical properties.
Physical properties such as boiling point change gradually.
Prepared by similar general methods.
5 · Real life. LPG is a mixture of two neighbours in the alkane series, propane and butane.
6 · Close in one line. So the members of a homologous series behave alike and change step by step.
Scoring tip: Write at least four characteristics. A quick mass check (16, 30, 44 u) shows the 14 u difference and impresses the examiner.
