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.
- 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:
- 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 .