Homologous Series

Take a look at the table above

Methane has 1 carbon atom and 4 hydrogen atoms, and ethane has 2 carbon atoms and 6 hydrogen atoms.

If we remove one carbon and four hydrogen atoms (i.e., CH₄) from ethane, we are left with one carbon and two hydrogen atoms.

In other words,

C₂H₆ (Ethane) −  CH₄ (methane)  =  CH₂  unit.

Similarly,

C₃H₈ (Propane) −  C₂H₆ (ethane)  =  CH₂  unit.

C₄H₁₀ (Butane) −  C₃H₈ (propane)  =  CH₂  unit.

C₅H₁₂ (Pentane) −  C₄H₁₀ (butane)  =  CH₂  unit.

Clearly, each successive member of alkane family is differ by a CH₂ unit. Such type of groups or series of organic compounds where adjacent members are differ by a CH₂ unit, form a homologous series and the members of the series are called homologues.

Homologous series

A homologous series is defined as a group or series of organic compounds all the members of which contain same functional group and any two members of which differ by a -CH₂ unit.

Examples : Alkanes, Alkenes, Alkynes etc.

Below are the examples of organic compounds containing functional groups forming a homologous series.