Homologous Series
Section: 11. Organic Chemistry | Syllabus: Cambridge IGCSE Chemistry (0620)
What is a Homologous Series?
A homologous series is a family of similar compounds with similar chemical properties, due to the presence of the same functional group.
The general characteristics of a homologous series are that its members:
- have the same functional group
- have the same general formula
- differ from one member to the next by a -CH₂- unit (one extra carbon and two extra hydrogens)
- display a trend in physical properties (e.g. boiling point increases as chain length increases)
- share similar chemical properties
The Four Main Homologous Series
| Homologous series | General formula | Functional group | First member |
|---|---|---|---|
| Alkanes | CnH2n+2 | none (saturated) | methane, CH₄ |
| Alkenes | CnH2n | C=C double bond | ethene, C₂H₄ |
| Alcohols | CnH2n+1OH | -OH (hydroxyl) | methanol, CH₃OH |
| Carboxylic acids | CnH2n+1COOH | -COOH (carboxyl) | methanoic acid, HCOOH |
Within one series, successive members are built up by adding one -CH₂- unit at a time: methane (CH₄) → ethane (C₂H₆) → propane (C₃H₈), each step adding one carbon and two hydrogens.
Worked Example
- Question (based on a real exam question): A, B, C and D are the first four members of a family of organic compounds with similar chemical properties: A is CH₂O₂, B is ethanoic acid, C is propanoic acid, D is butanoic acid. (i) State the term for this family. (ii) State its general formula.
- Answer: (i) homologous series; (ii) general formula CnH2n+1COOH - found by comparing successive members: methanoic acid HCOOH (n=0), ethanoic acid CH₃COOH (n=1).
Common Mistakes
- ❌ Confusing a general formula (e.g. CnH2n+2) with a molecular formula (e.g. C₃H₈)
- ✓ the general formula uses "n" and applies to every member of the series at once; the molecular formula is only for one specific compound, found by substituting a number for n
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