Chapter 8 · Chemistry
Homologous Series
Video not playing? YouTube
Methane, ethane, propane, butane: four different compounds, one family. How each member differs from the next by one CH₂ unit, and what that means for their properties.
Download the short notesPDF, 236 KB · solved examples, practice set and answer key Download the workbook
PDF, 188 KB · fill in the blanks, true/false, assertion-reason, MCQ, numericals, short answer, match the columns; detailed solutions at the end Up next in this chapter
IUPAC Naming: Root, Suffix and PrefixHow a chemical name is built from a molecule: the root counts the parent chain's carbons, the suffix gives its bonds and principal group, and the prefix names what is attached.Chemistry · simulation
Key points
- A homologous series is a series of organic compounds with the same functional group and the same general formula, in which successive members differ by –CH₂–.
- Each member is a homologue. Successive members differ in molar mass by 14 u (CH₂ = 12 + 2).
- Alkanes CₙH₂ₙ₊₂: CH₄ → C₂H₆ → C₃H₈ → C₄H₁₀ → C₅H₁₂, each one CH₂ more.
- Chemical properties are similar (not identical), because the members share the same functional group.
- Physical properties change gradually with chain length: boiling point rises, for example.
Three common series
| Series | General formula | First three members |
|---|---|---|
| Alkanes | CₙH₂ₙ₊₂ | CH₄, C₂H₆, C₃H₈ |
| Alcohols | CₙH₂ₙ₊₁OH | CH₃OH, C₂H₅OH, C₃H₇OH |
| Carboxylic acids | CₙH₂ₙ₊₁COOH (n = 0, 1, 2 …) | HCOOH, CH₃COOH, C₂H₅COOH |
Similar chemistry, gradual physical change
| Alkane | Formula | Molar mass (u) | Boiling point (°C) |
|---|---|---|---|
| Methane | CH₄ | 16 | −161.5 |
| Ethane | C₂H₆ | 30 | −88.6 |
| Propane | C₃H₈ | 44 | −42.1 |
| Butane | C₄H₁₀ | 58 | −0.5 |
| Pentane | C₅H₁₂ | 72 | 36.1 |
All of them burn in oxygen to give CO₂ and H₂O, and all undergo substitution with chlorine in light: similar chemistry. The boiling point rises steadily as the chain grows, because larger molecules attract one another more strongly. Boiling points at 1 atm (CRC Handbook).
Try it yourself
Goal: Predict each next member of a series, see the +CH₂, +14 u pattern and the rising boiling point, and decide whether two compounds are homologues.
- Choose a family: alkanes, alcohols or carboxylic acids.
- Predict the formula of the next member.
- Add CH₂ and check your prediction.
- Read the table: formula, molar mass and boiling point.
- In part 4, decide whether each pair is in the same series.
Quick quiz
5 questions. Choose an answer for each, then submit to see your score and the explanations.
More from Organic Chemistry – Some Basic Principles and Techniques
Functional GroupsTwo molecules can share the same carbon chain and still behave differently. How one small group of atoms gives a compound its chemical identity: –OH, –CHO, C=O, –COOH and –NH₂.Chemistry · simulation
Classifying Organic CompoundsMillions of organic compounds, one first question: is the carbon chain open, or a ring? How to sort compounds into acyclic, alicyclic, aromatic and heterocyclic.Chemistry · simulation
Organic Chemistry: Chapter IntroductionParacetamol, plastic, cotton and DNA share one element: carbon. Why carbon forms millions of compounds, and what this chapter teaches.Chemistry
Wöhler's Urea and the End of Vital ForceChemists once believed only living things could make organic compounds. In 1828 Friedrich Wöhler heated an inorganic salt and made urea.Chemistry
Why Carbon Forms Millions of CompoundsOver 100 elements, yet most known compounds contain carbon. Four reasons, and why silicon can't do the same.Chemistry
Shapes of Carbon Compounds: sp³, sp², spMethane, ethene and ethyne all contain carbon but have different shapes. Hybridisation, and a quick way to find it.Chemistry · simulation
Drawing Organic Molecules: Four WaysOne butane molecule, four ways to write it: complete, condensed, bond-line and 3D. How to read each, and how to count what a zigzag hides.Chemistry · simulation