What is a homologous series? Explain with an example.

Answer

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Hint: It is a series which we usually specify in organic chemistry.
- It is a series in which the subsequent members will differ by a common unit.

Complete step by step answer:
- In the question it is asked that we have to define what homologous series is and explain it with an example.
- We generally have heard the term homologous series while discussing the organic compounds. The homologous series is a series in organic chemistry in which the subsequent chemical compounds differ by a common unit. The compounds belonging to the homologous series have the same functional group.
The compounds in the homologous series will have fixed same set of functional groups i.e. in all the compounds that belong to the same series will consist of the same functional groups only. And because as they have the same functional group in them the compounds will possess some similar physical properties and chemical properties. But the chemical properties differ gradually for subsequent compounds, since the molecular mass will be increasing.
- Now let’s discuss this idea of homologous series with an example which will give more clarity on the topic.
Here we take the case of alkanes. Alkanes are homologous series which have a general formulae,

C_{n} H_{2 n + 2}

The first compound in alkane is methane, then ethane, propane butane etc.
Methane -

C H_{4}

Ethane-

C_{2} H_{6}

C H_{3} - C H_{3}

Propane-

C_{3} H_{8}

C H_{3} - C H_{2} - C H_{3}

Butane-

C_{4} H_{10}

C H_{3} - C H_{2} - C H_{2} - C H_{3}

- If we consider these compounds which belongs to alkane series they differ by a

- C H_{2}

unit (methylene unit) or we can say that they differ by an atomic mass of 14 unit (C-12amu and H-1 amu).
- The alkanes show almost similar set of physical and chemical properties but also show a gradual change in the properties as the molecular mass is increasing and also the size of the molecule is increasing.

Note: The adjacent homologous compound is called as adjacent homologue and the reaction by which we obtain the next homologous compound is called as the homologous reaction.