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What is a Homologous Series?

  • It is a series of organic compounds having similar structures and similar chemical properties .
  • 2 successive compounds differ by a CH 2 in a series.
  • For Example, Alkanes, Alkenes and Alkynes are homologous series.

Characteristics of Homologous Series:

  • Difference between 2 successive compounds of any homologous series is 14u and a CH 2  group .
  • It is because the mass of carbon is 12u and hydrogen is 1u . Hence mass of CH 2 will be 12+1*2=12+2=14u.

Example:

  • Mass of Methane (CH 4 ): 12 + 1 × 4 = 12 + 4 = 16u
  • Mass of Ethane (C 2 H 6 ): 12 × 2 + 1 × 6 = 24 +6 = 30u
  • Difference in Mass = 30 - 16 = 14u

Homologous Series of Alkanes:

  • If we compare  Methane (CH 4 ) and Ethane (C 2 H 6 ):
  • Ethane has 1 Carbon atom more than Methane. Also, Ethane has 2 Hydrogen atoms more than Methane. Hence, we can say that Ethane and Methane are successive compounds which differ by CH 2 group .
  • All members of a series can be represented by a general formula : C n H 2n+2 .

Homologous Series of Alkenes:

  • If we compare Ethene (C 2 H 4 ) and Propene (C 3 H 6 ) :

Propene has 1 Carbon atom and 2 Hydrogen atoms more than Ethene.

Hence, we can say that Propene and Ethene  are successive compounds which differ by CH 2 group.

  • All the alkenes can be represented by the general formula : C n H 2n .

Homologous Series of Alkynes:

HOMOLOGOUS SERIES OF ALKYNES - Teachoo.jpg

  • If we compare Ethyne (C 2 H 2 ) and Propyne (C 3 H 4 ):
  • The latter ha s 1 carbon atom and 2 hydrogen atoms more as compared to the former 
    i.e., propyne. Therefore, we can say that
    Propene and Ethene are two successive compounds which differ by CH 2 group.  
  • All the alkynes can be represented by the general formula: C n H 2n-2 .
  • All compounds of a chemical series show similar chemical properties . Example: Methane and ethane are both flammable gasses.
  • As the molecular mass increases in any homologous series, a gradation in physical properties is seen. This is because the melting and boiling points increase with increasing molecular mass .
  • Other physical properties such as solubility in a particular solvent also show a similar gradation. 
  • But the chemical properties, which are determined solely by the functional group, remain similar in a homologous series .

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CA Maninder Singh is a Chartered Accountant for the past 12 years and a teacher from the past 16 years. He teaches Science, Economics, Accounting and English at Teachoo