Question

Following are the structures of four isomers of hexane. Among the names given below, which correctly identifies the fifth isomer?
$C{H_3}C{H_2}C{H_2}C{H_2}C{H_2}C{H_3}$
$\left( {C{H_3}} \right)CC{H_2}C{H_3}$
${\left( {C{H_3}} \right)_2}CHC{H_2}C{H_2}C{H_3}$
${\left( {C{H_3}} \right)_2}CHCH{\left( {C{H_2}} \right)_2}$
A) ${\text{2 - }}$Methyl pentane
B) ${\text{2 - }}$Ethyl butane
C) ${\text{2,3 - }}$Dimethyl butane
D) ${\text{3 - }}$Methyl pentane

Answer 1

Hint: The above isomers given are structural isomers of each other. The structural isomers have the same molecular formula but a different arrangement of atoms and molecules. One can think about the possibilities of the isomer and find out the correct choice.

Complete step by step answer:
1) First of all we will discuss the concept of isomerism. The isomers are the molecules that have the same molecular formula but have a different structural formula. There may be a different arrangement of atoms or molecules present within the structure. Now let's analyze the given structures and look for the fifth isomer of the hexane.
2) In the structure of $C{H_3}C{H_2}C{H_2}C{H_2}C{H_2}C{H_3}$ the carbon atoms are arranged in a simple straight-chain and they have all the primary carbon atoms present.
3) In the structure of $\left( {C{H_3}} \right)CC{H_2}C{H_3}$ there are three methyl groups present at the first carbon. There is one quaternary carbon atom present and there are two primary carbon atoms present in the structure.
4) In the structure of ${\left( {C{H_3}} \right)_2}CHC{H_2}C{H_2}C{H_3}$ there are two methyl groups are present at the first carbon. There is one tertiary carbon atom present and there are three primary carbon atoms present in the structure.
5) In the molecular structure of ${\left( {C{H_3}} \right)_2}CHCH{\left( {C{H_2}} \right)_2}$ there are two tertiary carbon atoms present. Now let's relate the structures in the given options and see which can be the fifth isomer of butane.
6) The fifth isomer of hexane will be as ${\text{3 - }}$Methyl pentane as there are no other possible isomers for the structure of hexane. ${\text{3 - }}$Methyl pentane has a methyl group attached to the third carbon in the pentane chain.
Therefore, option D is the correct choice.

Note: The isomers have the same molecular formula but a different structural formula that may have different functional groups. Hence, isomers do not necessarily have the same physical and chemical properties.