Homologous Series in Organic Chemistry
Specification Reference S3.2.3
Quick Notes
- A homologous series is a family of organic compounds with:
- The same functional group
- The same general formula
- Similar chemical properties
- Gradual physical property trends (e.g. boiling point)
- Each successive member differing by a CH2 unit
- You must identify the following homologous series:
- Alkanes – CnH2n+2
- Alkenes – CnH2n
- Alkynes – CnH2n−2
- Halogenoalkanes
- Alcohols
- Aldehydes
- Ketones
- Carboxylic acids
- Ethers
- Amines
- Amides
- Esters
Full Notes
What Is a Homologous Series?
A homologous series is a group of compounds that:
- Share the same functional group
- Have a general formula
- Show a gradual change in physical properties (e.g. melting/boiling point increases with size)
- Differ by a –CH2– unit between successive members
- Exhibit similar chemical reactivity
Recognising and Identifying Homologous Series
| Series Name | Functional Group | General Formula | Example Member |
|---|---|---|---|
| Alkanes | None (single bonds) | CnH2n+2 | CH4 (methane) |
| Alkenes | C=C double bond | CnH2n | CH2=CH2 (ethene) |
| Alkynes | C≡C triple bond | CnH2n−2 | HC≡CH (ethyne) |
| Halogenoalkanes | –X (F, Cl, Br, I) | Varies | CH3Cl (chloromethane) |
| Alcohols | –OH (hydroxyl) | CnH2n+1OH | CH3OH (methanol) |
| Aldehydes | –CHO (carbonyl at end) | CnH2nO | HCHO (methanal) |
| Ketones | >C=O (carbonyl in chain) | CnH2nO | CH3COCH3 (propanone) |
| Carboxylic Acids | –COOH | CnH2nO2 | HCOOH (methanoic acid) |
| Ethers | –O– (alkoxy) | CnH2n+2O (simplified) | CH3OCH3 (dimethyl ether) |
| Amines | –NH2 (amino) | CnH2n+1NH2 | CH3NH2 (methylamine) |
| Amides | –CONH2 (amido) | CnH2nONH2 | HCONH2 (methanamide) |
| Esters | –COOR | CnH2nO2 | CH3COOCH3 (methyl ethanoate) |
Properties Across a Homologous Series
- Boiling and melting points increase with molecular mass due to greater van der Waals forces between molecules.
- Solubility in water often decreases as molecules get larger because the non-polar region increases relative to the polar region.
- Chemical properties stay similar because the functional group stays the same.
Summary
- Homologous series are structural families in organic chemistry.
- Identify each series by its name, general formula and functional group.
- Successive members differ by a CH2 unit.
- They show trends in physical properties but similar chemical reactivity.