Homologous Series

Edited By Team Careers360 | Updated on Jul 02, 2025 04:34 PM IST

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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Homologous Series
Homologous series examples
Solved Examples Based On Homologous Series
Conclusion

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

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).

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.

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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.

NCERT Chemistry Notes:

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 -

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).

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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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Frequently Asked Questions (FAQs)

1. What does it mean when one series is homologous?

A homologous series is a collection of hydrocarbons that have the same general formula and have similar chemical characteristics. Alkanes, alkenes, and cycloalkanes are the three hydrocarbon series we'll study. Only hydrogen and carbon make up hydrocarbons.

2. What is the generic aldehyde formula?

The carbonyl group in aldehydes has one hydrogen atom linked to it, as well as a second hydrogen atom or a hydrogen group that can be an alkyl group or one that contains a benzene ring. Because alkene has the universal formula C_nH_2n+1, aldehyde has the general formula C_nH_2n+1 CHO or C_nH_2nO.

3. What are the advantages and disadvantages of homologous series?

Because they all have the same functional groups, the chemical characteristics of members of a homologous series are the identical. This series has allowed scientists and engineers to investigate several chemical substances in depth.

4. What is the significance of the term "homologous series"?

In the homologous series of carbon compounds are dictated by the same functional group regardless of the length of the carbon chain. This is why they are referred to as a "homologous succession of carbon compounds."

5. What characterizes homologous series?

The following are the characteristics:-

The members of the homologous series all belong to the same functional group.
The general formula is the same for all members.
Due to the same functional group, members exhibit nearly identical chemical characteristics.
Members share a general method of preparation in common.

6. What is a homologous series in chemistry?

A homologous series is a group of organic compounds with similar chemical properties, sharing the same functional group and general formula. Each member differs from the next by a constant unit, typically a -CH2- group.

7. What is the difference between a functional group and a homologous series?

A functional group is a specific arrangement of atoms within a molecule that gives it characteristic chemical properties, while a homologous series is a group of compounds that share the same functional group and general formula, differing by a constant unit.

8. What is meant by the term "consecutive members" in a homologous series?

Consecutive members in a homologous series are compounds that differ by one -CH2- group (methylene group) in their molecular structure. For example, methane (CH4) and ethane (C2H6) are consecutive members in the alkane series.

9. Why is understanding homologous series important in organic chemistry?

Understanding homologous series is crucial because it helps predict properties and reactions of related compounds, simplifies the study of organic chemistry, and allows for systematic naming and classification of compounds.

10. How does the reactivity of compounds change within a homologous series?

The reactivity of compounds within a homologous series often follows a trend. Generally, as the carbon chain length increases, reactivity decreases due to factors like steric hindrance and electron distribution changes.

11. How does the concept of homologous series apply to organic compounds other than hydrocarbons?

The concept of homologous series applies to various organic compounds beyond hydrocarbons. For example, alcohols (R-OH), carboxylic acids (R-COOH), and amines (R-NH2) all form homologous series, where R represents the variable hydrocarbon chain.

12. How does the concept of homologous series help in predicting chemical properties?

The concept of homologous series helps predict chemical properties because members of the same series share similar chemical behavior due to the presence of the same functional group. This allows chemists to extrapolate properties and reactions from known compounds to unknown ones within the series.

13. What is the general formula for alkanes?

The general formula for alkanes is CnH2n+2, where n represents the number of carbon atoms in the molecule.

14. How do the physical properties of compounds in a homologous series change as the carbon chain length increases?

As the carbon chain length increases in a homologous series, physical properties generally change in a predictable manner: boiling and melting points increase, volatility decreases, and solubility in water decreases while solubility in organic solvents increases.

15. Can you explain the concept of isomerism in relation to homologous series?

Isomerism occurs when compounds have the same molecular formula but different structural arrangements. In homologous series, as the carbon chain length increases, the possibility of isomers also increases, leading to compounds with the same formula but different structures and properties.

16. How does the concept of homologous series aid in spectroscopic analysis?

The concept of homologous series aids in spectroscopic analysis by providing predictable patterns in spectra. As the chain length increases within a series, characteristic shifts or additional peaks appear in NMR, IR, or mass spectra, helping in structure elucidation and compound identification.

17. How does the concept of homologous series aid in understanding intermolecular forces?

The concept of homologous series aids in understanding intermolecular forces by demonstrating how these forces change with molecular size. As the chain length increases, van der Waals forces become stronger, leading to predictable trends in physical properties like boiling points and viscosity.

18. How does the concept of homologous series relate to drug design and pharmacology?

In drug design and pharmacology, the concept of homologous series is used to develop and optimize new medications. By studying how changes in molecular structure affect biological activity within a series, researchers can fine-tune drug properties like potency, selectivity, and bioavailability.

19. What is the significance of dipole moment trends in homologous series?

Dipole moment trends in homologous series are significant as they reflect changes in charge distribution within molecules. These trends help in understanding molecular polarity, intermolecular interactions, and the behavior of compounds in electric fields.

20. What is the importance of understanding homologous series in the field of biochemistry?

Understanding homologous series is important in biochemistry as many biological molecules, such as fatty acids and amino acids, form homologous series. This concept helps in predicting and explaining the behavior of these molecules in biological systems and their roles in metabolic processes.

21. What is the significance of melting point trends in homologous series?

Melting point trends in homologous series are significant as they reflect the strength of intermolecular forces and molecular packing. These trends help in predicting physical states, purification methods, and understanding the solid-state behavior of compounds.

22. What is the significance of surface tension trends in homologous series?

Surface tension trends in homologous series are significant as they reflect changes in intermolecular forces at liquid-air interfaces. These trends help in understanding phenomena like capillary action, wetting behavior, and the design of surfactants and cleaning agents.

23. What is the significance of the -CH2- group in homologous series?

The -CH2- group, known as the methylene group, is significant in homologous series as it represents the constant difference between consecutive members. Its addition or removal allows for the systematic building or breakdown of molecules within the series.

24. What is the relationship between homologous series and the periodic table?

While homologous series primarily relate to organic compounds, there are similarities with the periodic table. Both systems organize elements or compounds based on recurring patterns and properties, allowing for predictions and systematic study.

25. How does the concept of homologous series aid in the naming of organic compounds?

The concept of homologous series aids in naming organic compounds by providing a systematic approach. Compounds within a series follow similar naming conventions, with prefixes or suffixes changing based on the number of carbon atoms or specific functional groups present.

26. Can a compound belong to multiple homologous series?

Yes, a compound can belong to multiple homologous series if it contains more than one functional group. For example, amino acids belong to both the carboxylic acid and amine homologous series.

27. What is the importance of the first member of a homologous series?

The first member of a homologous series is important as it often exhibits unique properties compared to subsequent members. It serves as a reference point for understanding trends within the series and may have distinct applications or reactivity.

28. How does the concept of homologous series relate to the idea of "family resemblance" in chemistry?

The concept of homologous series embodies the idea of "family resemblance" in chemistry. Just as family members share similar traits, compounds in a homologous series share similar chemical properties and structures, allowing for predictable behavior and systematic study.

29. What role does electron distribution play in the properties of compounds within a homologous series?

Electron distribution plays a crucial role in determining properties within a homologous series. As the carbon chain length increases, electron density and distribution change, affecting properties such as polarity, reactivity, and intermolecular forces.

30. How does the concept of homologous series contribute to green chemistry?

The concept of homologous series contributes to green chemistry by allowing for the prediction of properties and reactions of compounds. This can lead to more efficient synthesis methods, reduced waste, and the development of environmentally friendly alternatives by extrapolating from known compounds.

31. What is the significance of boiling point trends in homologous series?

Boiling point trends in homologous series are significant as they reflect the increasing intermolecular forces as the molecular size grows. This trend helps in predicting physical properties, separation techniques, and understanding the behavior of compounds in different phases.

32. How does the concept of homologous series apply to polymers?

The concept of homologous series applies to polymers as they are essentially long chains of repeating units. Understanding the properties of smaller molecules in a series can help predict and explain the behavior of larger polymeric structures composed of similar units.

33. What is the relationship between homologous series and functional group chemistry?

Homologous series and functional group chemistry are closely related. Each homologous series is defined by a specific functional group, and the properties and reactions of compounds within the series are largely determined by this functional group's characteristics.

34. How does the concept of homologous series aid in understanding reaction mechanisms?

The concept of homologous series aids in understanding reaction mechanisms by allowing chemists to study trends in reactivity. As the carbon chain length changes within a series, subtle differences in reaction rates or product distributions can provide insights into the underlying mechanisms.

35. How does the concept of homologous series relate to the idea of structure-property relationships?

The concept of homologous series is a prime example of structure-property relationships in chemistry. It demonstrates how systematic changes in molecular structure (increasing chain length) lead to predictable changes in physical and chemical properties.

36. What role does homologous series play in the petroleum industry?

Homologous series play a crucial role in the petroleum industry. Understanding the properties of hydrocarbon series helps in the separation, processing, and utilization of various fractions of crude oil, as well as in the development of new petroleum-based products.

37. What is the significance of solubility trends in homologous series?

Solubility trends in homologous series are significant as they reflect the balance between the polar and non-polar portions of molecules. These trends help in predicting solubility behavior, designing extraction processes, and understanding the environmental fate of compounds.

38. What is the importance of understanding homologous series in environmental chemistry?

Understanding homologous series is important in environmental chemistry for predicting the behavior of pollutants. It helps in assessing the persistence, bioaccumulation, and transport of organic compounds in the environment based on their structure and position within a series.

39. How does the concept of homologous series contribute to the development of new materials?

The concept of homologous series contributes to the development of new materials by providing a systematic approach to tuning properties. By varying chain length or functional groups within a series, scientists can design materials with specific characteristics for applications in areas like polymers, surfactants, and liquid crystals.

40. What is the relationship between homologous series and the concept of molecular polarity?

The relationship between homologous series and molecular polarity is evident in how polarity changes as the carbon chain length increases. Generally, as the non-polar hydrocarbon portion grows within a series, the overall polarity of the molecule decreases, affecting properties like solubility and boiling point.

41. What is the significance of vapor pressure trends in homologous series?

Vapor pressure trends in homologous series are significant as they reflect the ease with which molecules escape from the liquid phase. Generally, vapor pressure decreases as the chain length increases, affecting properties like volatility and evaporation rates.

42. How does the concept of homologous series relate to the idea of structural isomerism?

The concept of homologous series relates to structural isomerism as the possibility of isomers increases with chain length within a series. This relationship helps in understanding the diversity of organic compounds and the importance of molecular structure in determining properties.

43. What role does homologous series play in the field of analytical chemistry?

Homologous series play an important role in analytical chemistry by providing predictable patterns in chemical and physical properties. This aids in the development of separation techniques, calibration standards, and analytical methods for identifying and quantifying organic compounds.

44. How does the concept of homologous series contribute to our understanding of reaction kinetics?

The concept of homologous series contributes to our understanding of reaction kinetics by allowing the study of how reaction rates change with molecular structure. As the chain length increases within a series, factors like steric hindrance and electronic effects can influence reaction rates in predictable ways.

45. How does the concept of homologous series aid in the study of organic reaction mechanisms?

The concept of homologous series aids in the study of organic reaction mechanisms by allowing chemists to observe how reactivity changes with molecular structure. By comparing reactions across a series, insights can be gained into the factors influencing reaction pathways and transition states.

46. How does the concept of homologous series relate to the principle of chemical equilibrium?

The concept of homologous series relates to chemical equilibrium by demonstrating how structural changes affect equilibrium constants. As the chain length increases within a series, subtle shifts in equilibrium positions can provide insights into the thermodynamics and kinetics of reactions.

47. What role does homologous series play in the field of organic synthesis?

Homologous series play a crucial role in organic synthesis by providing a framework for designing and optimizing synthetic routes. Understanding trends within a series can guide the selection of reagents, reaction conditions, and purification methods for target compounds.

48. How does the concept of homologous series contribute to our understanding of molecular orbital theory?

The concept of homologous series contributes to molecular orbital theory by allowing the study of how electronic structure changes with molecular size. As the chain length increases, patterns in orbital energies and electron distributions can provide insights into bonding and reactivity.

49. What is the significance of viscosity trends in homologous series?

Viscosity trends in homologous series are significant as they reflect the strength of intermolecular forces and molecular size. These trends help in understanding fluid behavior, designing lubricants, and predicting the flow properties of organic liquids.

50. How does the concept of homologous series aid in the development of chromatographic techniques?

The concept of homologous series aids in the development of chromatographic techniques by providing predictable patterns in retention times and separation factors. This allows for the optimization of separation methods and the identification of unknown compounds based on their behavior relative to known series members.

51. What is the importance of understanding homologous series in the field of polymer chemistry?

Understanding homologous series is important in polymer chemistry as it provides insights into how monomer structure affects polymer properties. This knowledge aids in the design of new polymers, the prediction of material characteristics, and the optimization of polymerization processes.

52. How does the concept of homologous series relate to the idea of chemical periodicity?

The concept of homologous series relates to chemical periodicity by demonstrating recurring patterns in organic chemistry, similar to the periodic trends observed in elements. Both concepts allow for the systematic study and prediction of properties based on structural or elemental patterns.

53. What role does homologous series play in the field of computational chemistry?

Homologous series play an important role in computational chemistry by providing a systematic way to study how molecular properties change with structure. This aids in the development and validation of computational models, as well as in the prediction of properties for larger or more complex molecules.

54. How does the concept of homologous series contribute to our understanding of molecular symmetry?

The concept of homologous series contributes to our understanding of molecular symmetry by demonstrating how symmetry elements change as molecules grow within a series. This helps in predicting spectroscopic properties, understanding chirality, and explaining reactivity patterns.

55. How does the concept of homologous series aid in the study of reaction thermodynamics?

The concept of homologous series aids in the study of reaction thermodynamics by allowing the observation of trends in enthalpy, entropy, and Gibbs free energy changes. As the chain length increases within a series, patterns in these thermodynamic parameters can provide insights into the energetics of reactions and the stability of compounds.