Topic 4 · page 2 of 17
Homologous series and functional groups
Organic compounds come in families. The functional group sets what a family does in a reaction, and the length of the carbon chain sets the gradual trend in physical properties.
Homologous series 4.4
There are millions of organic compounds, but they fall into a small number of families. A family of this kind is a homologous series.
Methane, ethane and propane are the first three members of the alkane series. Each has one more CH₂ unit than the one before.
Two things follow from sharing a functional group and a general formula:
- The members have similar chemical properties, because the same group does the reacting. Every alkane burns in oxygen and reacts with chlorine in UV light in the same way.
- The members show a gradual trend in physical properties, such as boiling temperature, as the chain gets longer.
The trend in boiling temperature
| Alkane | Formula | Boiling temperature / °C |
|---|---|---|
| methane | CH₄ | −162 |
| ethane | C₂H₆ | −89 |
| propane | C₃H₈ | −42 |
| butane | C₄H₁₀ | −1 |
| pentane | C₅H₁₂ | 36 |
| hexane | C₆H₁₄ | 69 |
Alkane molecules are held together by weak London forces between molecules. A longer chain has more electrons, so the London forces are stronger and more energy is needed to separate the molecules. The covalent bonds inside the molecules are not broken when an alkane boils.
Functional groups 4.4
The carbon chain mainly affects the physical properties. The functional group decides how the molecule reacts. Ethanol and ethane have the same two-carbon chain, but only ethanol has an –OH group, so they behave very differently.
| Series | Functional group | General formula | Example |
|---|---|---|---|
| Alkanes | none (only C–C and C–H bonds) | CₙH₂ₙ₊₂ | ethane |
| Alkenes | C=C | CₙH₂ₙ | ethene |
| Halogenoalkanes | C–X (X = F, Cl, Br, I) | CₙH₂ₙ₊₁X | chloroethane |
| Alcohols | –OH | CₙH₂ₙ₊₁OH | ethanol |
Alkenes, halogenoalkanes and alcohols are studied in detail in later topics. For now you only need to recognise the group and know that it controls the chemistry.
Worked exampleUsing a general formula
An alkane has 12 carbon atoms. Give its molecular formula, and the formula of the next member of the series.
- The general formula of an alkane is CₙH₂ₙ₊₂.
- For n = 12: H = 2 × 12 + 2 = 26. The formula is C₁₂H₂₆.
- The next member has one more CH₂: C₁₃H₂₈. Check with the formula: 2 × 13 + 2 = 28.
Check yourself
- Define the term homologous series.
- State two features of a homologous series.
- What is the molecular formula of the alkane with 20 carbon atoms?
- Predict, with a reason, which has the higher boiling temperature: pentane or hexane.
- Define functional group, and identify the functional group in each of ethene and ethanol.
- A student says that propane and propene belong to the same homologous series because both have three carbon atoms. Explain why this is wrong.
Answers cover these until you have tried the questions
- A homologous series is a family of compounds with the same general formula and the same functional group, in which each member differs from the next by CH₂.
- Any two of: same general formula; same functional group; members differ by CH₂; similar chemical properties; a gradual trend in physical properties.
- H = 2 × 20 + 2 = 42, so C₂₀H₄₂.
- Hexane is higher. It has a longer chain and more electrons, so the London forces between its molecules are stronger and more energy is needed to separate them (69 °C against 36 °C).
- A functional group is an atom or group of atoms in a molecule that is responsible for its characteristic chemical reactions. Ethene: C=C. Ethanol: –OH.
- Members of a series must share the same functional group and general formula. Propane (C₃H₈) is an alkane, with general formula CₙH₂ₙ₊₂. Propene (C₃H₆) has a C=C group and general formula CₙH₂ₙ, so it is an alkene.