Question

What is meant by homologous series ? Write the first four members of homologous series that begins with :

A. CH₃CHO 

B. H-C ≡ C-H

Also write down their general molecular formula.

Answer 1

Homologous series : 

A series of compounds of the same family in which each member has the same type of carbon skeleton and functional group, and differs from the next member by a constant difference of one methylene group (-CH₂-) in its molecular and structural formula is called as homologous series.

A. CH₃CHO :

No.	Name of compound and molecular formula	Structural formula
a.	Acetaldehyde (CH3CHO)
b.	Propionaldehyde (C2H5CHO)
c.	Butyraldehyde (C3H7CHO)
d.	Valeraldehyde (C4H9CHO)

Comparing these molecular formulae and assigning the number of carbon atoms as ‘n’, the following general formula is deduced : 

C_nH_2nO/C_nH_2n-1CHO (where n = 1, 2, 3, …).

B. H-C ≡ C-H :

No.	Name of compound and molecular formula	Structural formula
a.	Ethyne (C2H2)	HC ≡ CH
b.	Propyne (C3H4)	CH3-C ≡ CH
c.	But-1-yne (C4H6)	CH3-CH2-C ≡ CH
d.	Pent-1-yne (C5H8)	CH3-CH2-CH2-C ≡ CH

Comparing these molecular formulae and assigning the number of carbon atoms as ‘n’, the following general formula is deduced :

C_nH_2n-2 (where n = 2, 3,4,….).