A homologous series is one of the simplest—but most foundational—ideas in IB Chemistry Topic 10 (Organic Chemistry). It explains why organic compounds behave in predictable ways, why properties change gradually, and how functional groups determine reactions. Once you understand what a homologous series is, the entire structure of organic chemistry becomes easier to navigate.
What Is a Homologous Series?
A homologous series is a family of organic compounds that share the same functional group and general formula, with each successive member differing by a CH₂ unit.
This means every compound in the series:
- Has the same functional group (e.g., –OH, –COOH)
- Has the same chemical properties
- Differs by one carbon and two hydrogens from the next member
- Shows gradual, predictable trends in physical properties
Examples of homologous series:
- Alkanes: CₙH₂ₙ₊₂
- Alkenes: CₙH₂ₙ
- Alcohols: CₙH₂ₙ₊₁OH
- Carboxylic acids: CₙH₂ₙ₊₁COOH
These repeating patterns make organic chemistry systematic rather than random.
Why Homologous Series Differ by CH₂
The –CH₂– unit is the structural building block of carbon chains.
Adding a CH₂:
- Increases the carbon chain by one
- Increases molar mass
- Slightly changes physical properties
- Leaves functional groups and reactivity unchanged
This repetition creates predictable patterns across the series.
Characteristics of a Homologous Series
A homologous series has several defining features.
1. Same Functional Group
All members contain the same reactive part of the molecule.
For example:
- All alcohols have –OH
- All carboxylic acids have –COOH
This means they react in similar ways.
2. Same General Formula
Each series follows a mathematical relationship for C and H.
Examples:
- Alkanes: CₙH₂ₙ₊₂
- Ketones: CₙH₂ₙO
- Esters: CₙH₂ₙO₂
This general formula applies no matter how long the carbon chain becomes.
3. Gradual Trends in Physical Properties
As chain length increases:
- Boiling point increases
- Melting point increases
- Solubility in water decreases
- Density may increase
Why?
Because longer chains have:
- More electrons
- Stronger London dispersion forces
- Larger surface areas
These gradual changes help identify unknown compounds in IB questions.
4. Similar Chemical Properties
Because functional groups remain the same, reactions stay similar.
Example:
All alcohols undergo:
- Combustion
- Oxidation
- Esterification
All alkenes undergo:
- Addition reactions
- Polymerization
The consistency of functional groups means consistent reactivity.
Examples of Common Homologous Series
Alkanes (CₙH₂ₙ₊₂)
Properties:
- Non-polar
- Low reactivity
- Combust easily
- Increase in boiling point with chain length
Alkenes (CₙH₂ₙ)
Properties:
- Contain C=C double bond
- Undergo addition reactions
- More reactive than alkanes
Alcohols (CₙH₂ₙ₊₁OH)
Properties:
- Polar –OH group
- Hydrogen bonding
- Higher boiling points
- Undergo oxidation
Carboxylic Acids (CₙH₂ₙ₊₁COOH)
Properties:
- Very polar
- High boiling points
- Acidic
- Form esters with alcohols
These predictable trends help IB students classify compounds quickly.
Homologous Series and Isomerism
As the number of carbons increases:
- Chain isomers become possible
- Structural diversity increases
- Physical properties change
For example, C₄H₁₀ has two isomers, butane and methylpropane, yet both belong to the alkane series.
Why Homologous Series Matter in IB Chemistry
Homologous series help you:
- Predict boiling points, solubility, polarity
- Understand organic reaction pathways
- Recognize functional groups
- Classify unknown compounds in exams
- Connect molecular structure to behavior
Nearly all organic chemistry questions rely on understanding these patterns.
Common IB Misunderstandings
“Members of a homologous series have identical properties.”
False. They have the same chemical properties but gradually different physical properties.
“Adding CH₂ changes the functional group.”
No—the functional group stays the same.
“Isomers belong to different homologous series.”
They belong to the same homologous series if the functional group is unchanged.
FAQs
Why do boiling points increase in a homologous series?
Because longer chains have stronger London forces, requiring more energy to separate molecules.
Does adding CH₂ always increase reactivity?
No. Reactivity depends on the functional group, not chain length.
Do all organic compounds belong to homologous series?
Most do, especially simple functional groups taught in IB Chemistry.
Conclusion
A homologous series is a family of organic compounds with the same functional group, the same general formula, and members that differ by a CH₂ unit. They show gradual physical trends but share consistent chemical behavior. Understanding homologous series makes organic chemistry predictable, logical, and much easier to master in IB Chemistry.
