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Homologous Series

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Compound Name Formula
Methane CH4
Ethane C2H6
Propane C3H8
Butane C4H10
Pentane C5H12

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.

Homologous Series exmple : Methane and ethane comparison

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

Homologous Series exmple : Methane and ethane are differ by CH2 unit

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.

Homologous series

Alcohols

CH3−OH,   CH3CH2−OH,   CH3CH2CH2−OH .....

General formula : CnH2n+1OH

Homologous series

Carboxylic acid

CH3−COOH,   CH3CH2−COOH,   CH3CH2CH2−COOH .....

General formula : CnH2n+1COOH

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