Carbon and its Compounds · Lesson 6 of 13
Homologous Series
“Every new family member arrives carrying one extra CH₂ unit.”
• Define a homologous series. • Explain why successive members differ by a —CH₂— unit. • Calculate the molecular-mass difference between consecutive homologues. • Explain why chemical properties remain similar while physical properties change gradually. • Recognise general formula patterns for alkanes, alkenes and alkynes.
Carbon chains can grow one carbon at a time while retaining the same functional group or family pattern. This creates regular families in which structure, formula and properties change in a predictable way.
A Homologous Series is a family of carbon compounds with the same functional group and similar chemical properties in which successive members differ by a —CH₂— unit.
Successive Members Differ By A Methylene Unit
Methane and ethane differ by one carbon and two hydrogens. Ethane and propane differ in the same way. The repeating difference is therefore —CH₂—.
Problem
Find the molecular-mass difference between CH₄ and C₂H₆.
- 1.CH₄ has mass 12 + 4 = 16 u.
- 2.C₂H₆ has mass 24 + 6 = 30 u.
- 3.The difference is 30 − 16 = 14 u.
- 4.This equals the mass of one CH₂ unit.
Alcohol Homologous Series
Methanol, ethanol, propanol and butanol have carbon chains of increasing length but all contain the alcohol functional group. Their chemical properties are therefore similar in important ways.
Physical Properties Show Gradation
As molecular mass increases across a homologous series, melting and boiling points generally increase. Solubility in a given solvent may also change gradually. These physical changes occur even though the functional group remains unchanged.
Chemical Properties Remain Similar
Chemical reactions are strongly controlled by the functional group. Because every member of a homologous series contains the same functional group, the series shows similar chemical behaviour.
General Formula Patterns
| Family | General Formula |
|---|---|
| Alkanes | CₙH₂ₙ₊₂ |
| Alkenes | CₙH₂ₙ |
| Alkynes | CₙH₂ₙ₋₂ |
The chapter explicitly gives the alkene pattern CₙH₂ₙ and asks students to generate corresponding patterns for alkanes and alkynes. These formulae are useful when checking whether a molecular formula fits a hydrocarbon family.
Problem
Check whether C₄H₈ fits the alkene series.
- 1.For alkenes, use CₙH₂ₙ.
- 2.Set n = 4.
- 3.Then H = 2 × 4 = 8.
- 4.The predicted formula is C₄H₈, so it fits the alkene pattern.
What Homologous Does Not Mean
Members of a homologous series are not identical compounds. Their molecular formulae, molecular masses and physical properties differ. What remains regular is the functional group, the type of chemical behaviour and the CH₂ progression.
Quiz
By what unit do successive members of a homologous series differ?
What is the molecular-mass difference associated with one CH₂ unit?
Which property usually remains similar within a homologous series?
What is the general formula for alkenes?
Which statement is correct about physical properties across a homologous series?
Practice Problems
- Define a homologous series and give an example using alcohols.
- Calculate the molecular masses of CH₃OH and C₂H₅OH and show the difference.
- Explain why successive homologues differ by 14 u.
- Explain why chemical properties remain similar while boiling points can change.
- Use the formula CₙH₂ₙ to generate the next three alkenes after ethene.
- Use CₙH₂ₙ₊₂ and CₙH₂ₙ₋₂ to check whether C₅H₁₂ and C₄H₆ fit alkane or alkyne patterns.
Key Takeaways
• A homologous series contains compounds with the same functional group and similar chemical behaviour. • Successive members differ by one —CH₂— unit and therefore by 14 u in molecular mass. • Physical properties show gradual changes as molecular mass increases. • Chemical properties remain similar because the functional group remains the same. • Alkanes, alkenes and alkynes follow regular general formula patterns.