What is a homologous series?

Features of a homologous series

1. The same general formula. Every alkane fits CₙH₂ₙ₊₂, for example.
2. The same functional group, the atom or group of atoms that gives the compounds their characteristic reactions, such as the C=C double bond in alkenes or the −OH group in alcohols.
3. Similar chemical properties, because the functional group decides how they react. All alkenes decolourise bromine water, for example.
4. A gradual trend in physical properties. As the chain gets longer, boiling point and viscosity rise and the compounds become less flammable.
5. Each member differs from the next by CH₂, adding one carbon and two hydrogens.

The alkanes: a homologous series

Alkanes are hydrocarbons with only single bonds, and AQA's GCSE Chemistry gives their general formula as CₙH₂ₙ₊₂. Most of the hydrocarbons in crude oil are alkanes.

Each formula has one more CH₂ than the one before, so each relative formula mass is 14 more. The boiling point climbs steadily too: the first four are gases at room temperature, and pentane and hexane are liquids. Longer molecules have stronger forces of attraction between them, which take more energy to overcome.

A handy memory aid for the first four names is "Mice Eat Peanut Butter": methane, ethane, propane, butane. From five carbons, the names use Greek number prefixes: pent-, hex-, hept-, oct-.

Using the general formula

Example: what is the formula of the alkane with 10 carbon atoms? n = 10, so the formula is C₁₀H_2×10+2 = C₁₀H₂₂, which is decane, with a relative formula mass of 10 × 12 + 22 × 1 = 142. Try writing an alkane formula yourself, then press Mark to check it in our Organic Chemistry course.

Other homologous series

In the carboxylic acid formula, n starts at 0, so the first member, methanoic acid, is HCOOH. Ethanol, C₂H₅OH, the alcohol in drinks, is the second alcohol. You can build these molecules and compare them in the Molecule Builder.

Why homologous series are useful

There are millions of organic compounds. Sorting them into homologous series means you don't have to learn each one separately: if you know how one alcohol reacts, you can predict how the others will, and the trend tells you roughly how their boiling points compare. It is also why crude oil can be separated by fractional distillation, since alkanes of different chain lengths boil at different temperatures.

Common mistakes

• Saying members have the same physical properties. Chemical properties are similar, but physical properties change gradually along the series.
• Getting the general formula wrong: alkanes are CₙH₂ₙ₊₂ and alkenes CₙH₂ₙ. An alkene has two fewer hydrogens than the alkane with the same number of carbons.
• Forgetting the CH₂ difference. Neighbouring members differ by one carbon and two hydrogens, a relative formula mass of 14.
• Mixing up the series. The name's ending tells you: -ane is an alkane, -ene an alkene, -ol an alcohol and -oic acid a carboxylic acid.
• Saying all homologous series are hydrocarbons. Alcohols and carboxylic acids contain oxygen too.

Read next

Chemistry · CoursesOrganic chemistry course: alkanes, alkenes and isomersAn interactive organic chemistry course in 23 short sections: formulas, alkanes, alkenes, functional groups, naming, isomers, combustion and polymers.