Question

What is meant by homologous series of organic compounds? Write the chemical formulae of two members of a homologous series and state which part determines the $\left( i \right)$ Physical properties $\left( {ii} \right)$ Chemical properties, of these compounds:

Answer 1

Hint:The word homologous describes itself as something that is common and similar in the series of organic compounds. One can think about the similarities and differences in properties such as physical and chemical properties as well as some structural changes which can be present in the series.
Complete step by step answer:1) A homologous series can be explained as a family of organic compounds that have the same functional group and the adjacent or successive members of the series differ by a $ - C{H_2}$ group.
2) In the homologous series, the two adjacent homologous compounds have a difference in molecular mass of ${\text{14}}$ which is similar to $ - C{H_2}$ the group.
3) The term homologous series is also sometimes used for any collection or group of compounds that are similar in their structures or having the same functional group in their structures. Some compounds such as the general alkane, alkene (olefins), and the carbohydrates, etc show the presence of homologous series.
4) Let’s see the physical and chemical properties of a homologous series of straight-chain alkane, the homologous series of straight-chained alkanes begins with methane which has only one carbon in it. Let’s take two members of the series butane $\left( {{C_4}{H_{10}}} \right)$ and pentane $\left( {{C_5}{H_{12}}} \right)$ which are successive members of the series.
5) In this series, the successive members' butane and pentane differ in mass by an extra methylene group $\left( { - C{H_2} - } \right)$ added in the chain. Therefore, making the change in the molecular mass of each consecutive member by ${\text{14}}$ atomic mass units. The adjacent members in that series, such as butane and pentane, are called adjacent homologs of the series.
$(i)$ Physical properties: In the homologous series of alkane, there are many physical properties that change with the change of increase in mass such as boiling point. Let’s take an example of butane and pentane where pentane has a higher boiling point than butane.
$\left( {ii} \right)$ Chemical properties: The chemical properties of all the members of the homologous series are the same as they have the same functional groups. The members of homologous series butane and pentane don’t differ in chemical properties.


Note: In a homologous series many physical properties such as boiling point changes as mass increases. For example, the ethane molecule has a higher mass than methane and shows a higher boiling point. This happens due to stronger forces of intermolecular attraction in the ethane molecule increasing the boiling point. Every member which is present in the homologous series shows different physical properties but they all show similar chemical properties due to the same functional group.