Beginner Topic 733 of 1102 Organic Chemistry
Homologous Series
A family of compounds with the same functional group differing by a CH2 unit.
In-depth explanation of Homologous Series
A homologous series is a family of organic compounds that share the same functional group and the same general formula, with successive members differing by one CH2 unit. Within a series, physical properties change in a regular way: boiling point typically rises by about 20 to 30 degrees Celsius for each added CH2 because London dispersion forces increase with molecular size. Chemical behaviour stays broadly similar across the family, so knowing one member helps predict the others. The general formula, such as CnH2n+2 for alkanes, allows the members to be generated and compared systematically.
Going deeper: Homologous Series
Key ideas: Each member differs from the next by CH2, a mass difference of 14 u. General formulas: alkanes CnH2n+2, alkenes and cycloalkanes CnH2n, alkynes CnH2n-2, saturated alcohols CnH2n+2O. Boiling point rises about 20 to 30 degrees per added CH2 because of stronger London forces, while chemical properties stay almost the same within the series.
Common mistakes and connections: Mixing up general formulas is common: CnH2n fits both alkenes and cycloalkanes, which are therefore isomers of each other. Students also assume solubility changes smoothly, but in alcohols it drops sharply beyond about four carbons. Connects to isomerism, degree of unsaturation and mass spectrometry.
Visualization of Homologous Series
Each alkane differs from the next by one CH2 unit (14 g/mol), so molar mass rises in equal steps along a homologous series. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Methanol, ethanol and propanol form the homologous series of primary alcohols with the general formula CnH2n+1OH.
Frequently asked questions about Homologous Series
What is Homologous Series?
A family of compounds with the same functional group differing by a CH2 unit.
Is Homologous Series a beginner, intermediate or advanced topic?
Homologous Series is a beginner topic in Organic Chemistry. It needs no earlier chemistry lessons and is a good starting point.
What should I learn before Homologous Series?
Study these earlier topics in Organic Chemistry first: Tetravalency of Carbon, Catenation, Functional Groups.
What comes after Homologous Series?
The next topics in Organic Chemistry are Alkanes and Alkenes.
Can I practise Homologous Series with quizzes?
Yes. The interactive lesson includes a self-check question on Homologous Series, and the Organic Chemistry quiz draws on every topic in this area.
Where Homologous Series fits in Organic Chemistry
Homologous Series is topic 5 of 148 in Organic Chemistry.