Homologous Series - Introduction, Examples, Properties & Uses, FAQs

A homologous series is a collection of compounds with the same general formula that differ only in carbon chain length. Compounds in a homogenous series often have a fixed set of functional groups, resulting in chemical and physical properties that are comparable. A homologous series is a collection of molecules with comparable chemical characteristics and functional groups that differ by$\mathrm{CH}_2$ from one another.

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This Story also Contains

1. Homologous Series
2. Homologous series examples
3. Solved Examples Based On Homologous Series
4. Conclusion

In organic molecules with the same general formula, carbon chains of various lengths have been discovered. A series of organic compounds is formed by organic compounds that differ from one another by a repeating unit and have the same general formula. The most fundamental homologous series in organic chemistry is formed of alkanes.

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The functional groups are the same for all members of this series. They have identical physical qualities that increase in mass in a fixed gradation. The characteristics of $\mathrm{CH}_3 \mathrm{OH}, \mathrm{C}_2 \mathrm{H}_5 \mathrm{OH}$, and $\mathrm{C}_3 \mathrm{H}_7 \mathrm{OH}$ are similar and vary gradually as the molecular mass of subsequent members of the series increases. This is because the number of bonds increases as the molecular mass of the molecule increases.

In this article, we will cover the topic (Homologous Series). This topic falls under the broader category of (Some Basic Principles of Organic Chemistry), which is a crucial chapter in (Class 11 Chemistry).

Homologous Series

A group or a series of organic compounds each containing a characteristic functional group forms a homologous series and the members of the series are called homologues. The members of a homologous series can be represented by general molecular formula and the successive members differ from each other in molecular formula by $\mathrm{a}-\mathrm{CH}_2$ unit. There are a number of homologous series of organic compounds. Some of these are alkanes, alkenes, alkynes, haloalkanes, alkanols, alkanals, alkanones, alkanoic acids, amines etc.
It is also possible that a compound contains two or more identical or different functional groups. This gives rise to polyfunctional compounds.

Related Topics Link

• Electronic Configuration of First 30 Elements
• Rutherfords Model of Atoms and Its Limitations
• Modern Periodic Table Modern Periodic Law
• Periodic Trends in Ionisation Enthalpy of Elements
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Homologous series examples

Each subsequent member of homologous series differs from the previous by one CH₂ unit. The difference between CH₄ and C₂H₆ is -CH₂ unit, while the difference between C₂H₆ and C₃H₈ is also -CH₂ unit. As a result, CH₄, C₂H₆, and C₃H₈ are all homologous. The same effect may be seen in alkenes with ethene as the first member and C₃H₆, C₄H₈, and C₅H₁₀ as the subsequent members. By a –CH₂ unit, they differ from one another. C_nH_2n is the formula for alkenes.

The functional groups are the same for all members of this homologous series. They have identical physical qualities that increase in mass in a fixed gradation. The characteristics of CH₃OH, C₂H₅OH, and C₃H₇OH are similar and vary gradually as the molecular mass of subsequent members of the homologous series increases. This is due to the fact that as the molecular mass of the compounds grows, so does the number of bonds. As a result, qualities such as melting and boiling points, solubility, and other properties that are dependent on the mass and total number of bonds in a compound alter gradually as the molecular masses of the compounds grow.

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Recommended topic video on (Homologous series)

Solved Examples Based On Homologous Series

Q.1 Which of the following compounds belong to the same homologous series?

1) Methane $\left(\mathrm{CH}_4\right)$ and Ethane $\left(\mathrm{C}_2 \mathrm{H}_6\right)$

2) Ethene $\left(\mathrm{C}_2 \mathrm{H}_4\right)$ and Propane $\left(\mathrm{C}_3 \mathrm{H}_8\right)$

3) Ethanol $\left(\mathrm{C}_2 \mathrm{H}_5 \mathrm{OH}\right)$ and Propanoic acid $\left(\mathrm{C}_2 \mathrm{H}_5 \mathrm{COOH}\right)$

4) Propene $\left(\mathrm{C}_3 \mathrm{H}_6\right)$ and Butyne $\left(\mathrm{C}_4 \mathrm{H}_6\right)$

Solution:

As we learned -

A group or a series of organic compounds each containing a characteristic functional group forms a homologous series and the members of the series are called homologues. The members of a homologous series can be represented by general molecular formula and the successive members differ from each other in molecular formula by $\mathrm{a}-\mathrm{CH}_2$ unit.

Hence, the answer is the option (1).

Q.2 Which of the following is the general formula for alkanes?

A) $\mathrm{C}_n \mathrm{H}_{2 n+2}$

B) $\mathrm{C}_n \mathrm{H}_{2 n}$

C) $\mathrm{C}_n \mathrm{H}_{2 n-2}$

D) $\mathrm{C}_n \mathrm{H}_{2_n+1}$

Solution:

As we learned -

Alkanes are the Unsaturated Hydrocarbon with formula:$\mathrm{C}_n \mathrm{H}_{2_n+2}$

Hence, the answer is the option (1).

Also read :

• NCERT notes Class 11 Chemistry Chapter 3 Classification of Elements and Periodicity in Properties
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• NCERT Exemplar Class 11 Chemistry Solutions Chapter 3 Classification of Elements and Periodicity in Properties
  

Conclusion

Because the functional group of the molecule does not vary, homologous groups have comparable chemical properties. Keep in mind that the functional group is just the part of the molecule that is reactive or can change in chemical processes. Because each molecule in a homologous series has the same functional group, the homologous have identical reactivities. Homologous series aid in the determination of the structure of each consecutive member of the series, as well as the prediction of the properties of those members.

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