what is homologous series explain with an example
Understanding organic chemistry’s fundamental classification system
What is a Homologous Series?
A homologous series is a group of organic compounds with:
- The same general formula
- Similar chemical properties
- Gradually changing physical properties
- The same functional group
- Each member differs by -CH2– (methylene group) from the next
Key Concept:
Homologous series allow chemists to predict properties of compounds based on their position in the series.
Characteristics of Homologous Series
1. Uniform Functional Group
All members contain the same functional group that determines their chemical properties.
2. General Formula
All compounds can be represented by a common general formula (e.g., CnH2n+2 for alkanes).
3. Gradual Physical Changes
Physical properties (mp, bp, density) change gradually with increasing molecular mass.
4. Similar Reactions
All members undergo similar chemical reactions due to the same functional group.
Exam Tip:
Remember that the difference between consecutive members is always CH2 (methylene group).
Example: Alkane Series
General Formula: CnH2n+2
(where n = number of carbon atoms)
| Name | Molecular Formula | Structural Formula |
|---|---|---|
| Methane | CH4 | CH4 |
| Ethane | C2H6 | CH3-CH3 |
| Propane | C3H8 | CH3-CH2-CH3 |
| Butane | C4H10 | CH3-(CH2)2-CH3 |
| Pentane | C5H12 | CH3-(CH2)3-CH3 |
Pattern Observation:
- Each new member adds CH2 to the chain
- Molecular formula increases by CH2
- Boiling point increases with chain length
Physical Properties Trend
Boiling Point
Increases with molecular mass due to stronger van der Waals forces
Solubility
Decreases with increasing chain length (more non-polar)
| Alkane | BP (°C) | MP (°C) | Density (g/mL) |
|---|---|---|---|
| Methane | -161.5 | -182.5 | 0.000656 |
| Ethane | -88.6 | -182.8 | 0.00126 |
| Propane | -42.1 | -187.7 | 0.00183 |
| Butane | -0.5 | -138.3 | 0.00248 |
| Pentane | 36.1 | -129.7 | 0.626 |
Other Important Homologous Series
1. Alkenes
General formula: CnH2n
Example: Ethene (C2H4), Propene (C3H6)
2. Alkynes
General formula: CnH2n-2
Example: Ethyne (C2H2), Propyne (C3H4)
3. Alcohols
General formula: CnH2n+1OH
Example: Methanol (CH3OH), Ethanol (C2H5OH)
4. Carboxylic Acids
General formula: CnH2n+1COOH
Example: Methanoic acid (HCOOH), Ethanoic acid (CH3COOH)
Practice Questions
Q1. What would be the molecular formula of the next alkane after pentane (C5H12)?
Q2. Which of these is NOT a characteristic of homologous series?
a) Same functional group
b) Differ by CH3 group
c) Similar chemical properties
d) Gradual change in physical properties
Q3. Give the general formula for alkenes and name the first three members.
A1. Hexane (C6H14)
A2. b) Differ by CH3 group (correct difference is CH2)
A3. General formula: CnH2n
Members: Ethene (C2H4), Propene (C3H6), Butene (C4H8)
