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11.1.2Organic Chemistry

Homologous Series — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 4 parts Lesson 11 of 12 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 2Alkanes, alkenes and cycloalkanes

Homologous SeriesGeneral FormulaSaturationFunctional FeatureFirst 3 Members
AlkanesCnH2n+2Saturated (single C-C bonds only)Unreactive, undergo combustionMethane (CH4), Ethane (C2H6), Propane (C3H8)
AlkenesCnH2nUnsaturated (contains C=C double bond)Reactive, undergo addition reactionsEthene (C2H4), Propene (C3H6), Butene (C4H8)
CycloalkanesCnH2nSaturated (closed carbon ring, single bonds)Ring structureCyclopropane (C3H6), Cyclobutane (C4H8), Cyclopentane (C5H10)
1Trends in physical properties
As carbon chain length increases, melting and boiling points increase because intermolecular forces between larger molecules become stronger.
2Bromine water test for unsaturation
  • Alkenes immediately decolourise bromine solution from brown-orange to colourless by an addition reaction: C2H4 + Br2 → C2H4Br2 (1,2-dibromoethane).
  • Saturated hydrocarbons (alkanes and cycloalkanes) do not react; bromine remains brown-orange.

Part 2 of 2Isomers and systematic naming

Isomers and systematic naming
Isomers
  • Compounds with the same molecular formula but different structural formulae.
  • May have different physical properties (e.g. branched isomers have lower boiling points due to less surface contact between molecules).
  • Alkenes and cycloalkanes with the same number of carbon atoms are isomers of each other (e.g. propene and cyclopropane both have formula C3H6).
IUPAC naming rules for branched hydrocarbons
  1. Find and name the longest continuous carbon chain (stem name).
  2. Number the carbon chain from the end that gives branches (or double bonds) the lowest possible numbers.
  3. Identify branch groupsmethyl (-CH3), ethyl (-C2H5).
  4. Use prefixes di-, tri-, tetra- if multiple identical branches are present.

Key terms

TermMeaning
Homologous seriesA family of hydrocarbons with the same general formula and similar chemical properties.
IsomerCompounds with identical molecular formulae but different structural arrangements of atoms.
SaturatedA hydrocarbon containing only single covalent carbon-carbon bonds.
UnsaturatedA hydrocarbon containing one or more double carbon-carbon covalent bonds.

Avoid these mistakes

Avoid these mistakes
Confusing alkenes and cycloalkanes — both share the general formula CnH2n, but alkenes are unsaturated while cycloalkanes are saturated.
Writing "clear" instead of "colourless" when bromine water decolourises.

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