Question

The ${4^{{\text{th}}}}$ higher homologue of ethane is:
A. butane
B. pentane
C. hexane
D. heptane

Answer 1

Hint: A series of compounds with similar chemical properties, in which members differ from one another by the possession of an additional ${\text{C}}{{\text{H}}_2}$ group is called homologous series. It has a difference in relative mass of $14$.

Complete step by step answer:
Alkanes are the compounds which contain only hydrogen and carbon with single bonds between the carbon atoms. They are named by:
-counting the number of carbon atoms in the longest chain.
-Giving the appropriate prefix.
The suffix –ane is used to show that the molecule is alkane.
For example, one carbon is named as meth, two carbon is named eth, three carbon as prop, four carbon as but, etc. The table showing the number of carbon atoms and corresponding names are given below:

Number of carbon atoms	Prefix given
1	Meth
2	Eth
3	Prop
4	But
5	Pent
6	Hex
7	Hept
8	Oct
9	Non
10	Dec

Ethane can be represented as ${\text{C}}{{\text{H}}_3} - {\text{C}}{{\text{H}}_3}$. Its formula is ${{\text{C}}_2}{{\text{H}}_6}$. General formula of alkanes is ${{\text{C}}_{\text{n}}}{{\text{H}}_{2{\text{n}} + 2}}$. Each member differs by ${\text{C}}{{\text{H}}_2}$ group or a mass of $14$. Such compounds are called homologous series. These members of homologous series behave in a similar way.
Homologous series of alkane is ${\text{C}}{{\text{H}}_4},{{\text{C}}_2}{{\text{H}}_6},{{\text{C}}_3}{{\text{H}}_8},{{\text{C}}_4}{{\text{H}}_{10}},$ etc.
 The 4th higher homologue of ethane can be identified by adding ${\text{C}}{{\text{H}}_2}$ groups four times to ethane.
i.e. ${{\text{C}}_2}{{\text{H}}_6} + 4\left( {{\text{C}}{{\text{H}}_2}} \right)$
Now adding the number of carbon and hydrogen, we get the formula as ${{\text{C}}_6}{{\text{H}}_{14}}$. It is named as hexane since it has six carbon.
So, the correct answer is option C.

Additional information Alkane molecules are made up by the following steps:
-Draw the carbon atoms side by side in a single chain.
-Link them with a single covalent bond.
-Add hydrogen atoms until all carbon atoms have their four bonds since carbon has four valencies.

Note:
We can also add four carbons to ethane directly and satisfy the general formula of alkane. This method can also be used for finding the homologue.