Introduction to Organic Chemistry
5. Homologous Series
Learning outcomes
- I can define a homologous series.
- I can identify common characteristics of compounds within a homologous series.
- I can recognize repeating patterns in molecular formulas.
- I can explain how physical properties change within a homologous series.
- I can classify simple organic compounds into their correct homologous series.
Introduction
Organic chemistry contains millions of different compounds, yet chemists are able to organize them into a relatively small number of families. These families are called homologous series. Compounds within the same homologous series have similar structures and behave in similar ways during chemical reactions.
Learning about homologous series makes organic chemistry much easier to understand. Instead of studying every compound individually, chemists can learn the common properties of an entire family and predict the behaviour of new compounds based on their structure.
What Is a Homologous Series?
A homologous series is a family of organic compounds that have:
- the same functional group
- the same general formula
- similar chemical properties
- a gradual change in physical properties as the molecules become larger
- neighbouring members that differ by one –CH₂– group
A homologous series follows a regular pattern, making it easier to identify and predict the properties of its members.
Definition:
A homologous series is a family of organic compounds with the same functional group and general formula, in which successive members differ by a –CH₂– unit.
The Alkane Homologous Series
The alkanes are one of the simplest homologous series.
They contain:
- only carbon and hydrogen atoms
- only single covalent bonds
- the general formula:
CₙH₂ₙ₊₂
The first few members are shown below.
| Name | Molecular Formula |
|---|---|
| Methane. | CH₄ |
| Ethane | C₂H₆ |
| Propane | C₃H₈ |
| Butane | C₄H₁₀ |
| Pentane | C₅H₁₂ |
| Hexane | C₆H₁₄ |
Notice that each new member contains one additional carbon atom and two additional hydrogen atoms—together, this is one –CH₂– unit.
The Repeating Pattern
Successive members of a homologous series differ by –CH₂–.
For example:
CH₄ → C₂H₆
Difference:
- CH₂
C₂H₆ → C₃H₈
Difference:
- CH₂
C₃H₈ → C₄H₁₀
Difference:
- CH₂
This repeating pattern is one of the easiest ways to recognize a homologous series.
Worked Example 1
Complete the sequence.
Methane
CH₄
Ethane
C₂H₆
Propane
C₃H₈
Butane
?
Solution
One more carbon atom:
C₄
Add two more hydrogen atoms:
H₁₀
Answer: C₄H₁₀
Common Characteristics
Every member of a homologous series has several features in common.
They have the same functional group.
The functional group determines the chemical behaviour of the compound.
For example:
- All alkanes contain only C–C and C–H single bonds.
- All alcohols contain the –OH functional group.
- All alkenes contain at least one carbon–carbon double bond.
Because of this, members of the same homologous series undergo similar chemical reactions.
They have the same general formula.
Examples include:
| Homologous Series. | General Formula |
|---|---|
| Alkanes | CₙH₂ₙ₊₂ |
| Alkenes | CₙH₂ₙ |
| Alcohols | CₙH₂ₙ₊₁OH |
The general formula allows chemists to predict the molecular formula of any member of the series.
Physical Properties Change Gradually
Although members have similar chemical properties, their physical properties change gradually as the molecules become larger.
As the number of carbon atoms increases:
- boiling point increases
- melting point generally increases
- density usually increases slightly
- viscosity increases
- volatility decreases
This happens because larger molecules experience stronger intermolecular forces, requiring more energy to separate them.
For example:
| Compound. | Approximate Boiling Point (°C) |
|---|---|
| Methane | –162 |
| Ethane | –89 |
| Propane | –42 |
| Butane | –1 |
| Pentane | 36 |
Notice that the boiling point rises steadily as the molecules become larger.
Real-World Example
This gradual change explains why:
- methane is a gas used for cooking and heating,
- propane is stored as liquefied petroleum gas (LPG),
- butane is commonly used in cigarette lighters,
- larger alkanes become liquids used in petrol and diesel,
- even larger hydrocarbons are waxes and bitumen used for candles and road surfaces.
Classifying Organic Compounds
Chemists classify compounds by identifying their functional groups.
| Compound. | Homologous Series |
|---|---|
| CH₄ | Alkane |
| C₂H₄ | Alkene |
| CH₃OH | Alcohol |
| C₂H₅OH | Alcohol |
| C₃H₈ | Alkane |
Recognising the functional group is often enough to determine the homologous series.
Worked Example 2
Identify the homologous series for each compound.
a) C₄H₁₀
General formula:
CₙH₂ₙ₊₂
Answer:
Alkane
b) C₃H₆
General formula:
CₙH₂ₙ
Answer:
Alkene
c) C₂H₅OH
Contains:
–OH
Answer:
Alcohol
Why Homologous Series Matter
Grouping compounds into homologous series helps chemists:
- predict chemical reactions,
- estimate physical properties,
- identify unknown compounds,
- design new medicines,
- manufacture fuels and plastics,
- communicate molecular structures efficiently.
Rather than memorizing millions of compounds individually, chemists learn the rules that apply to each family.
Real-World Connection
Many everyday products are members of homologous series. Natural gas is composed mainly of methane, camping stoves often use propane, disposable lighters contain butane, alcoholic drinks contain ethanol, and many plastics are produced from alkenes such as ethene. Recognizing the homologous series helps scientists understand how these substances are made, stored, transported, and used safely.
Did You Know?
The word homologous comes from Greek words meaning "same relationship." Chemists noticed over 150 years ago that many organic compounds followed predictable patterns in both structure and properties. Today, these patterns are used to organise millions of known organic compounds into manageable families.
Key Terms
- Homologous series — a family of organic compounds with the same functional group and general formula.
- General formula — a formula representing every member of a homologous series.
- Functional group — the atom or group of atoms responsible for a compound's characteristic chemical reactions.
- Physical properties — characteristics such as boiling point, melting point, density, and state of matter.
- Chemical properties — how a substance reacts with other substances.
- –CH₂– unit — the repeating group that distinguishes successive members of a homologous series.
Key Takeaways
- A homologous series is a family of related organic compounds.
- Members of a homologous series have the same functional group and the same general formula.
- Successive members differ by one –CH₂– unit.
- Members have similar chemical properties but show gradual changes in physical properties as molecular size increases.
- Recognizing homologous series allows chemists to predict the structures, properties, and reactions of organic compounds.