Homologous Series : Introduction
A homologous series is a group of organic compounds having the same functional group, similar chemical properties, and a regular difference of –CH₂ (methylene group) between consecutive members.
According to NCERT, members of a homologous series can be represented by a common general formula and show a gradual change in physical properties.
Definition of Homologous Series
Homologous Series:
A series of organic compounds in which each successive member differs from the previous member by a –CH₂ unit and contains the same functional group is called a homologous series.
Example
| Compound | Molecular Formula |
|---|---|
| Methane | CH₄ |
| Ethane | C₂H₆ |
| Propane | C₃H₈ |
| Butane | C₄H₁₀ |
Here, each compound differs from the next by CH₂.
Characteristics of a Homologous Series
1. Same Functional Group
All members contain the same functional group.
Example:
Alcohols contain the –OH group.
- Methanol (CH₃OH)
- Ethanol (C₂H₅OH)
- Propanol (C₃H₇OH)
2. Same General Formula
All members follow a common formula.
| Homologous Series | General Formula |
|---|---|
| Alkanes | CₙH₂ₙ₊₂ |
| Alkenes | CₙH₂ₙ |
| Alkynes | CₙH₂ₙ₋₂ |
| Alcohols | CₙH₂ₙ₊₁OH |
3. Difference of CH₂ Group
Adjacent members differ by one carbon atom and two hydrogen atoms (CH₂).
4. Similar Chemical Properties
Because the functional group remains the same, their chemical behavior is similar.
5. Gradual Change in Physical Properties
Properties such as:
- Boiling point
- Melting point
- Density
- Solubility
change gradually with increase in molecular mass.
Why Do Members Have Similar Chemical Properties?
Chemical properties depend mainly on the functional group.
Since all members possess the same functional group, they undergo similar chemical reactions.
Example
Alcohols:
- Methanol
- Ethanol
- Propanol
All contain the –OH group, so they show similar chemical reactions.
Examples of Homologous Series
1. Alkanes
| Name | Formula |
|---|---|
| Methane | CH₄ |
| Ethane | C₂H₆ |
| Propane | C₃H₈ |
| Butane | C₄H₁₀ |
2. Alkenes
| Name | Formula |
|---|---|
| Ethene | C₂H₄ |
| Propene | C₃H₆ |
| Butene | C₄H₈ |
3. Alkynes
| Name | Formula |
|---|---|
| Ethyne | C₂H₂ |
| Propyne | C₃H₄ |
| Butyne | C₄H₆ |
4. Alcohols
| Name | Formula |
|---|---|
| Methanol | CH₃OH |
| Ethanol | C₂H₅OH |
| Propanol | C₃H₇OH |
Importance of Homologous Series
- Helps classify organic compounds.
- Makes study of organic chemistry easier.
- Compounds can be predicted using general formulas.
- Chemical properties become easier to understand.
- Useful in industrial and laboratory applications.
Difference Between Consecutive Members
| First Member | Second Member | Difference |
|---|---|---|
| CH₄ | C₂H₆ | CH₂ |
| C₂H₆ | C₃H₈ | CH₂ |
| C₃H₈ | C₄H₁₀ | CH₂ |
Thus, every next member contains one additional carbon atom and two hydrogen atoms.
Homologous Series of Alkanes
| Number of Carbon Atoms | Compound | Formula |
|---|---|---|
| 1 | Methane | CH₄ |
| 2 | Ethane | C₂H₆ |
| 3 | Propane | C₃H₈ |
| 4 | Butane | C₄H₁₀ |
| 5 | Pentane | C₅H₁₂ |
| 6 | Hexane | C₆H₁₄ |
General Formula:
C_nH_{2n+2}
NCERT Key Points
- Members of a homologous series have the same functional group.
- Consecutive members differ by CH₂.
- They possess similar chemical properties.
- Physical properties change gradually.
- Each series follows a common general formula.
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Practice Questions
Short Answer Questions
1. What is a homologous series?
Answer:
A group of organic compounds having the same functional group and differing by CH₂ between consecutive members.
2. Why do members of a homologous series show similar chemical properties?
Answer:
Because they possess the same functional group.
3. What is the common difference between two consecutive members?
Answer:
CH₂ (one carbon atom and two hydrogen atoms).
4. Give one example of a homologous series.
Answer:
Methane, Ethane, Propane, Butane (Alkanes).
5. What is the general formula of alkanes?
Answer:
CₙH₂ₙ₊₂
Fill in the Blanks
- Consecutive members of a homologous series differ by CH₂.
- Alkanes have the general formula CₙH₂ₙ₊₂.
- Members of a homologous series have the same functional group.
- Ethane belongs to the alkane series.
- Physical properties show a gradual change in a homologous series.
Multiple Choice Questions (MCQs)
1. What is the difference between two consecutive members of a homologous series?
A. CH₄
B. CH₃
C. CH₂
D. C₂H₂
Answer: C. CH₂
2. Which of the following is a homologous series?
A. NaCl, KCl
B. CH₄, C₂H₆, C₃H₈
C. H₂O, CO₂
D. HCl, HBr
Answer: B. CH₄, C₂H₆, C₃H₈
3. The general formula of alkanes is:
A. CₙH₂ₙ
B. CₙH₂ₙ₋₂
C. CₙH₂ₙ₊₂
D. CₙH₂ₙ₊₁OH
Answer: C. CₙH₂ₙ₊₂
4. Which functional group is present in alcohols?
A. –COOH
B. –OH
C. –CHO
D. –NH₂
Answer: B. –OH
5. Propane belongs to which homologous series?
A. Alkanes
B. Alkenes
C. Alkynes
D. Alcohols
Answer: A. Alkanes
6. Members of a homologous series have:
A. Different functional groups
B. Same functional group
C. Same molecular mass
D. Same formula
Answer: B. Same functional group
7. Ethene belongs to:
A. Alkane
B. Alcohol
C. Alkene
D. Alkyne
Answer: C. Alkene
8. Chemical properties mainly depend on:
A. Colour
B. Molecular mass
C. Functional group
D. Density
Answer: C. Functional group
9. Which compound follows the alkane formula?
A. C₂H₄
B. C₂H₂
C. C₂H₆
D. C₂H₅OH
Answer: C. C₂H₆
10. Which statement is correct?
A. Consecutive members differ by CO₂
B. Functional groups are different
C. Consecutive members differ by CH₂
D. Physical properties remain exactly the same
Answer: C. Consecutive members differ by CH₂
Quick Revision
✔ Homologous series = compounds with same functional group and similar chemical properties.
✔ Consecutive members differ by CH₂.
✔ Same general formula.
✔ Physical properties change gradually.
✔ Examples: Alkanes, Alkenes, Alkynes, Alcohols.
✔ Important NCERT topic from Carbon and Its Compounds.
Other Related Notes
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