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अभ्यासक्रम

Homologous Series

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Topics

  • Definition: Homologous Series
  • Key Points: Homologous Series
Maharashtra State Board: Class 10

Homologous Series in Organic Chemistry

A homologous series is a group of organic compounds having the same functional group and similar chemical properties but differing in the length of the carbon chain. Each successive member differs by a -CH₂- unit and has a molecular mass increase of 14 u.

Characteristics of a Homologous Series:

  • Regular Addition of CH₂ Units: Each successive compound differs by one -CH₂- unit.
  • Gradation in Physical Properties: Physical properties (e.g., boiling points) show a gradual increase or decrease along the series.
  • Similar Chemical Properties: All members have the same functional group, leading to similar chemical reactions.
  • General Molecular Formula: Members of the series can be represented by a single general formula.

General Formula for Homologous Series:

Alkanes: CₙH₂ₙ₊₂, where n = 1 for the first member (Methane: CH₄).

Alkenes: CₙH₂ₙ, where n = 2 for the first member (Ethene: C₂H₄).

Alkynes: CₙH₂ₙ₋₂, substituting n = 2, 3, 4:

  • 1st Member: Ethyne (C₂H₂)
  • 2nd Member: Propyne (C₃H₄)
  • 3rd Member: Butyne (C₄H₆)
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Examples of Homologous Series

(a) Homologous Series of Alkanes

General formula: CₙH₂ₙ₊₂

First member: Methane (CH₄) (n = 1)

Name Molecular Formula Condensed Structural Formula No. of Carbon Atoms No. of -CH₂- Units Boiling Point (°C)
Methane CH₄ CH₄ 1 - -162
Ethane C₂H₆ CH₃-CH₃ 2 1 -88.5
Propane C₃H₈ CH₃-CH₂-CH₃ 3 2 -42
Butane C₄H₁₀ CH₃-CH₂-CH₂-CH₃ 4 3 0
Pentane C₅H₁₂ CH₃-CH₂-CH₂-CH₂-CH₃ 5 4 36

(b) Homologous Series of Alcohols

General formula: CₙH₂ₙ₊₁OH

Name Molecular Formula Condensed Structural Formula No. of Carbon Atoms No. of -CH₂- Units Boiling Point (°C)
Methanol CH₄O CH₃-OH 1 1 63
Ethanol C₂H₆O CH₃-CH₂-OH 2 2 78
Propanol C₃H₈O CH₃-CH₂-CH₂-OH 3 3 97
Butanol C₄H₁₀O CH₃-CH₂-CH₂-CH₂-OH 4 4 118

(c) Homologous Series of Alkenes

General formula: CₙH₂ₙ

Name Molecular Formula Condensed Structural Formula No. of Carbon Atoms No. of -CH₂- Units Boiling Point (°C)
Ethene C₂H₄ CH₂=CH₂ 2 - -102
Propene C₃H₆ CH₃-CH=CH₂ 3 1 -48
1-Butene C₄H₈ CH₃-CH₂-CH=CH₂ 4 2 -6.5
1-Pentene C₅H₁₀ CH₃-CH₂-CH₂-CH=CH₂ 5 3 30
CBSE: Class 10
Maharashtra State Board: Class 10
CISCE: Class 10

Definition: Homologous Series

The same functional group substitutes for hydrogen in a carbon chain is called a homologous series.

OR

A homologous series is a group of organic compounds having  similar structure and similar chemical properties in which the successive compounds differ by a CH2 group.

OR

The series of compounds formed by joining the same functional group in the place of a particular hydrogen atom on the chains having sequentially increasing length is called homologous series.

Maharashtra State Board: Class 10
CISCE: Class 10

Key Points: Homologous Series

  • A homologous series is a group of compounds with the same functional group and similar chemical properties.
  • Successive members differ by one CH₂ (methylene) unit, increasing the molecular mass by 14 u.
  • All members of a homologous series follow the same general molecular formula (e.g., alkanes CnH2n+2).
  • Physical properties such as boiling and melting points show a gradual change with increasing molecular mass.
  • Knowing the properties of a few members helps predict the properties of other members in the series.

Video Tutorials

We have provided more than 1 series of video tutorials for some topics to help you get a better understanding of the topic.

Series 1


Series 2


Shaalaa.com | Carbon and Compounds part 18 (Homologous series)

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Shaalaa.com


Carbon and Compounds part 18 (Homologous series) [00:03:20]
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Carbon and Compounds part 18 (Homologous series)
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Carbon and Compounds part 19 (Functional group-1)
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Carbon and Compounds part 20 (Functional group-2)
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Homologous series
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Related QuestionsVIEW ALL [88]

 Fill in the following blank with suitable word:

Carbon compounds have usually ... melting points and boiling points because they are ...... in nature.

A carbon compound ‘A’ having melting point 156K and boiling point 351K, with molecular formula C2H6O is soluble in water in all proportions.

  1. Identify ‘A’ and draw its electron dot structure.
  2. Give the molecular formulae of any two homologues of ‘A’.

Define Homologous series.

Classify the following carbon compounds into two homologous series and name them.

C3H4, C3H6, C4H6, C4H8, C5H8, C5H10

Give the names of the first four members of the homologous series of
alkynes.

Name the following organic compound:

The first homologue whose general formula is CnH2n.

Which of the following does not belong to the same homologous series?

Complete the following table for homologous series of alcohols.

Name Molecular formula Condensed structural formula Number of carbon atom Number of -CH2- units Boiling point °C
Methanol CH4O CH3-OH 1 1 63
Ethanol C2H6O CH3–CH2-OH 2 2 78
Propanol C3H8O CH3–CH2–CH2-OH ______ ______ 97
Butanol C4H10O CH3–CH2–CH2–CH2–OH ______ ______ 118
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