Question

What is an homologous series? Explain with an example.

Answer 1

There are many series which explain behaviours of elements and molecules. One such series is homologous series in organic chemistry. It is a series of organic compounds with certain similarities in them.

Complete step by step answer:

In the field of organic chemistry, a homologous series is a sequence of compounds which has the same functional group and same chemical properties in which the members of the series can be branched or unbranched. It can be the length of the carbon chain, for example in straight chain alkanes, or could be the number of monomers in a homopolymer.
Compounds in a homologous series would have a fixed set of functional groups, which give them a similar set of chemical and physical properties. These properties gradually change along the series and this change is due to the increase in their sizes and mass.
Chemical process that converts one member of a homologous series to the next member is called a homologous reaction.
Some of the examples of homologous series are,
i) In case of straight chained alkanes, this series begins with methane, ethane, propane, butane and pentane. The successive members differ in mass by addition of an extra methylene bridge ($-C{{H}_{2}}-$ unit) inserted in the chain.
ii) So, the molecular mass of each member differs by 14 atomic mass units. Adjacent members are called adjacent homologues, for example methane and ethane. In the series, the physical properties such as boiling point etc, changes as the molecular mass increases.
iii) Ethane has a higher boiling point than methane because the London force between ethane molecules is higher than it is between methane molecules. This results in stronger forces of intermolecular attraction, thus raising the boiling point.

Note: We have to keep in mind that in the series, as the chain length increases, there is small variation seen in the property. But mostly its members show similar properties.