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Homologous Series of Carbon Compound

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  • Homologous Series in Organic Chemistry
  • Examples of Homologous Series

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)

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

 

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Carbon and Compounds part 18 (Homologous series) [00:03:20]
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