A homologous series is a family of organic compounds that share the same general formula, the same functional group, and the same chemical reactions, with each successive member differing by one CH₂ unit. Understanding the concept is the key to predicting the properties of any compound from the pattern of the series, rather than memorising each compound individually.
What is a homologous series?
In organic chemistry, a homologous series is a group of compounds with:
- The same general formula — a single algebraic expression that generates the molecular formula of every member by substituting different values of n (where n is the number of carbon atoms, or sometimes the number of a repeating unit).
- The same functional group — the specific arrangement of atoms responsible for the characteristic reactions of the series.
- A difference of CH₂ between consecutive members — each successive compound has one more carbon and two more hydrogen atoms than the previous one, adding 14 in relative molecular mass.
- Similar chemical properties — all members react in the same ways because they have the same functional group.
- Gradually changing physical properties — boiling points, melting points, and viscosity increase steadily up the series as chain length increases.
What are the main homologous series at GCSE?
The four series you need to know are:
| Series | Functional group | General formula | Example (n = 1) |
|---|---|---|---|
| Alkanes | None (C–C and C–H only) | CₙH₂ₙ₊₂ | Methane, CH₄ |
| Alkenes | C=C double bond | CₙH₂ₙ | Ethene, C₂H₄ |
| Alcohols | –OH (hydroxyl) | CₙH₂ₙ₊₁OH | Methanol, CH₃OH |
| Carboxylic acids | –COOH (carboxyl) | CₙH₂ₙ₊₁COOH | Methanoic acid, HCOOH |
Note that alkenes begin at n = 2 (ethene, C₂H₄) because a C=C double bond requires at least two carbon atoms.
Why do physical properties change gradually up a series?
The gradual change in physical properties is one of the defining features of a homologous series, and it arises directly from the increase in chain length.
Boiling point increases because:
- Longer chains have more electrons and greater surface area.
- This means stronger instantaneous dipole–induced dipole (London dispersion) forces between molecules.
- More energy is needed to overcome these intermolecular forces, so the boiling point rises.
Viscosity increases because longer hydrocarbon chains tangle with one another more readily, increasing resistance to flow.
Flammability decreases as chain length increases — short-chain members (methane, ethane, propane) are gases at room temperature and highly flammable; long-chain members are liquids or waxy solids that burn less readily.
Solubility in water decreases up the alkane series as the hydrocarbon chain grows longer and the molecule becomes more non-polar.
How do you use a general formula?
The general formula lets you write the molecular formula of any member without memorising it.
Example — alkanes (CₙH₂ₙ₊₂):
| n | Name | Molecular formula |
|---|---|---|
| 1 | Methane | CH₄ |
| 2 | Ethane | C₂H₆ |
| 3 | Propane | C₃H₈ |
| 4 | Butane | C₄H₁₀ |
| 5 | Pentane | C₅H₁₂ |
Worked example: What is the molecular formula of the alkane with 7 carbon atoms? Using CₙH₂ₙ₊₂ with n = 7: C₇H(2×7+2) = C₇H₁₆ (heptane).
Worked example: An unknown alkene has the molecular formula C₅H₁₀. Is it a member of the alkene series? Check: CₙH₂ₙ with n = 5 gives C₅H₁₀. Yes — it fits the general formula and contains a C=C double bond.
Why do all members of a series have similar chemical reactions?
The chemical reactions of an organic compound are almost entirely determined by its functional group. All alkenes undergo addition reactions across the C=C double bond (e.g. with bromine water — the orange-brown colour decolourisesto colourless). All alcohols undergo combustion, oxidation to carboxylic acids, and condensation to form esters. All carboxylic acids react with carbonates to produce CO₂ and with alcohols to produce esters.
Because every member of a series shares the same functional group, the reactions are the same regardless of chain length. The products differ in chain length but not in reaction type.
How do alkanes differ from alkenes chemically?
A key distinction often tested at GCSE is the difference in reactivity between alkanes (saturated) and alkenes (unsaturated):
- Alkanes — all C–C single bonds; they are relatively unreactive. They undergo combustion and (in the presence of UV light) substitution reactions with halogens.
- Alkenes — contain a C=C double bond; they are more reactive. They undergo addition reactions, where the double bond opens up and atoms add across it. Testing with bromine water distinguishes them: alkenes decolourise bromine water; alkanes do not.
Frequently asked questions
What is a homologous series in GCSE chemistry?
A homologous series is a family of organic compounds that share the same general formula and functional group, differ by CH₂ between consecutive members, have similar chemical properties, and show a gradual trend in physical properties such as boiling point. Examples include the alkanes (CₙH₂ₙ₊₂), alkenes (CₙH₂ₙ), alcohols, and carboxylic acids. The concept allows chemists to predict the formula, name, and reactions of any member in the series.
How does boiling point change up a homologous series?
Boiling point increases with each additional CH₂ group because the longer chain has more electrons and greater surface area, resulting in stronger London dispersion (van der Waals) forces between molecules. More energy is required to separate the molecules from one another, raising the boiling point. The increase per CH₂ unit is approximately 20–30 °C for smaller members, but the gap narrows slightly as chains become very long.
What is the difference between a saturated and an unsaturated hydrocarbon?
A saturated hydrocarbon contains only single carbon–carbon bonds — all available bonding positions are filled by hydrogen atoms. Alkanes are saturated. An unsaturated hydrocarbon contains one or more C=C double bonds (or C≡C triple bonds), meaning not all positions carry hydrogen. Alkenes are unsaturated. Unsaturated compounds are generally more reactive than saturated ones because the electron-rich double bond is a site for addition reactions.
How do you identify which homologous series a compound belongs to?
First, look at the molecular formula and apply each general formula to see which fits. Then look for the functional group: –OH indicates an alcohol; –COOH indicates a carboxylic acid; a C=C double bond indicates an alkene. If the compound has only C–C and C–H bonds with no double bonds, it is an alkane. Finally, verify that it fits the pattern of the series (e.g. check the molecular formula matches CₙH₂ₙ₊₂ for an alkane with the expected value of n).
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