Which of the following forms a homologous series?

Which of the following forms a homologous series?

Ethane, ethylene, acetylene

Ethane, propane, butanone

Methane, ethanol, propanoic acid

Butane, 2-methyl butane, 2,3-dimethyl butane

Answer:

A homologous series is a series of organic compounds having similar structures and similar chemical properties . Two successive compounds differ by CH 2 Group .

1. If we consider ethane, ethylene and acetylene , ethane is an alkane, ethylene is an alkene and acetylene is an alkyne. Since all three of them have different chemical properties , they do not form a homologous series.

2. Among ethane, propane and butanone , ethane and propane are alkanes but butanone is a ketone. Because their chemical properties are different , they do not form a homologous series.

3. Methane, ethanol and propanoic acid do not form a homologous series because methane is an alkane, ethanol and alcohol and propanoic acid is a carboxylic acid and so they have different chemical properties . 

4. Butane, 2-methyl butane and 2,3-dimethyl butane all are alkanes and they also have a difference of CH 2 group between two successive compounds . Therefore, they belong to the same homologous series.

So, the correct option - (4) Butane, 2-methyl butane, 2,3-methyl butane