You are walking up a staircase. Each step is a little higher than the one before, but the shape of the step — the rise, the tread, the material — is exactly the same. If you know how to climb the first step, you know how to climb the hundredth. That is the intuition behind a homologous series.
In organic chemistry, a homologous series is a family of organic compounds that share the same general formula and the same functional group, and whose successive members differ by a constant unit: a −CH2− group. The word "homologous" comes from Greek — homos (same) and logos (proportion) — meaning "having the same ratio or pattern."
A homologous series is defined by three fixed rules:
- All members have the same general formula (e.g., CnH2n+2 for alkanes).
- Each member differs from the next by exactly one −CH2− unit (i.e., a mass difference of 14 u).
- All members contain the same functional group (if any), so their chemical properties are similar.
Let’s see it in action with the simplest series: the alkanes.
- Methane: CH4 (n=1)
- Ethane: C2H6 (n=2)
- Propane: C3H8 (n=3)
- Butane: C4H10 (n=4)
Notice the pattern: each step adds one carbon and two hydrogens — exactly a −CH2− group. The general formula is CnH2n+2. If you know the formula for one member, you can write the formula for the next by adding CH2.
Now, what does this mean for properties? Because the functional group is the same, the chemical behaviour of all members is similar. All alkanes burn in oxygen to give CO2 and water; all react with halogens in the same way (substitution). But physical properties change gradually — this is called gradation. As the chain gets longer, boiling points rise, melting points rise, and the compounds become less volatile and more viscous. Methane is a gas at room temperature; octane (C8H18) is a liquid; waxes with very long chains are solids.
A common mistake is to think that "same general formula" means all members have identical chemical properties. They do not — the type of reaction is the same, but the rate and conditions can change. For example, larger alkanes ignite less easily than smaller ones. The similarity is in the kind of chemistry, not the exact numbers.
Other important homologous series you will meet:
| Series | General Formula | Functional Group | First Member |
|---|
| Alkenes | CnH2n | C=C double bond | Ethene (C2H4) |
| Alkynes | CnH2n−2 | C≡C triple bond | Ethyne (C2H2) |