Homologous Series and Functional Group
Functional Group
Definition
A functional group is an atom or group of atoms in an organic compound that determines its characteristic
chemical properties and reactions.
Simple Meaning
A functional group is the reactive part of an organic molecule. It controls how the compound behaves in
chemical reactions.
Even if the carbon chain changes, compounds with the same functional group react in similar ways. That is
why it is called a functional group—it gives the molecule its chemical "function."
Examples of Functional Groups
• Alcohols: contain the –OH group
• Methanol: CH₃OH
• Ethanol: C₂H₅OH
• Carboxylic acids: contain the –COOH group
• Alkenes: contain the C=C double bond
Key Idea
The functional group:
• determines the type of chemical reactions a compound undergoes
• gives the compound its characteristic properties
• helps classify organic compounds into families
Easy Analogy
Think of the functional group as the engine of a car. Cars may look different on the outside, but if they
have the same engine, they perform in similar ways. In the same way, organic compounds with the same
functional group behave similarly.
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Homologous Series
Definition
A homologous series is a family of organic compounds that:
• have the same functional group
• have the same general formula
• have similar chemical properties
• show a gradual change in physical properties
• differ from one member to the next by a –CH₂– unit
Simple Meaning
A homologous series is a group of organic compounds that belong to the same family because they have
the same functional group.
Since they have the same functional group, they react in similar ways. The only difference is that each next
member has one extra –CH₂– group.
Example: Alkane Series
• Methane: CH₄
• Ethane: C₂H₆
• Propane: C₃H₈
• Butane: C₄H₁₀
General formula: CₙH₂ₙ₊₂
Each successive member differs by –CH₂–.
Another Example: Alcohol Series
• Methanol: CH₃OH
• Ethanol: C₂H₅OH
• Propanol: C₃H₇OH
General formula: CₙH₂ₙ₊₁OH
All members contain the –OH functional group.
Why Do Members Have Similar Chemical Properties?
Because they all contain the same functional group, which is the reactive part of the molecule.
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Trend in Physical Properties
As the carbon chain gets longer:
• boiling point increases
• melting point generally increases
• viscosity increases
• volatility decreases
Relationship Between the Two
A functional group determines how a compound reacts. A homologous series is a family of compounds
that all contain the same functional group.
So:
• same functional group → similar chemical reactions
• different carbon chain lengths → gradual change in physical properties
Quick Summary
• Functional group = the reactive part of an organic molecule.
• Homologous series = a family of organic compounds with the same functional group and general
formula.
• Successive members differ by –CH₂–.
• They have similar chemical properties but gradually changing physical properties.
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