Question

Formula of methane is $C{{H}_{4}}$ and its succeeding members is expressed in ${{C}_{2}}{{H}_{6}}$. The common difference of succession between them is:
(A) $C{{H}_{2}}$
(B) ${{C}_{2}}{{H}_{2}}$
(C) $C{{H}_{4}}$
(D) None of the above

Answer 1

Both $C{{H}_{4}}$and ${{C}_{2}}{{H}_{6}}$ are the members of the same homologous series. A homologous series are those that have the same functional group. There should be a gradation in the properties of a respective homologous series. They must also have the same structural properties.

Complete step by step solution:
Let us see about homologous series first.
To simplify and to systematize the study of organic chemistry, all the organic compounds have been grouped into families called homologous series.
In other words, compounds that structurally similar compounds and these compounds having the same functional group can be called as a family or a group and these must show a gradation in physical and chemical properties must be similar. Each member of a homologous series is called homologs and the phenomenon is called homology.
For example, the alkane is a homologous series. The first six members of this series are:
$\text{C}{{\text{H}}_{\text{4}}}\text{(methane) }{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{6}}}\text{(ethane) }{{\text{C}}_{\text{3}}}{{\text{H}}_{\text{8}}}\text{(propane) }{{\text{C}}_{\text{4}}}{{\text{H}}_{\text{10}}}\text{(butane) }{{\text{C}}_{\text{5}}}{{\text{H}}_{\text{12}}}\text{(pentane) }{{\text{C}}_{\text{6}}}{{\text{H}}_{\text{14}}}\text{(hexane)}$
There are many examples of homologous series and some of them are alkenes, alkynes, ketones, aldehydes, esters, etc.
Characteristics of a Homologous series:
There is a general formula by which each homologous series can be represented. For example, ${{C}_{n}}{{H}_{2n+1}}COOH$ is the general formula of carboxylic acids.
All the members of a given homologous series will have the same functional group. For example, -OH is the functional group of alcohol.
Most of the physical properties like density, boiling point, the melting point of the members of a homologous series will have regular gradation with a rise in molecular mass.
The difference in the successive members of a homologous series is of $C{{H}_{2}}$ group or by 14 mass units.
As stated above, successive members differ by a $C{{H}_{2}}$, therefore, the difference between $C{{H}_{4}}$and ${{C}_{2}}{{H}_{6}}$will be$C{{H}_{2}}$.

Therefore, the correct answer is an option (A)- $C{{H}_{2}}$.

Note: The individual members of a homologous series can be prepared by the general method of preparation. The chemical properties of the members of a homologous series similar though the first member may vary considerably from the rest of the members.