Question

Arrange the following alkanes in the increasing order of their boiling point:
Pentane, hexane, $2,3 - dimethyl - butane$
A) Pentane< 2, 3-dimethyl-butane< hexane
B) Hexane< 2, 3-dimethyl-butane< pentane
C) Pentane < hexane < 2, 3-dimethyl-butane
D) None of the above

Answer 1

Alkanes are non-polar compounds and hence their boiling point mostly depends on their molecular weight.
While if two alkanes have similar molecular weight then to compare their boiling point the degree of branching is considered.

Complete step by step answer:
Alkanes are the organic compound consisting only of carbon-carbon single bonds.
They have general formula ${C_n}{H_{2n + 2}}$ and are non-polar in nature due to almost same electronegativity of carbon and hydrogen
Thus they cannot show intermolecular or intramolecular hydrogen bonding
Hence their boiling point solely depends on their molecular weight
So in the given question since pentane has the lowest molecular weight of $72$ ,it has the lowest boiling point among the three compound given
The other two left have the same molecular weight of $86$ since both have the same molecular formula
${C_6}{H_{14}}$ But different structure due to presence of branching in $2,3 - dimethyl - butane$
As we all know that boiling point depends upon surface area such that decrease in surface area of an organic compound leads to decrease in its boiling point
And with branching the surface area of an organic compound decreases leading to decrease in its boiling point
So the boiling point of $2,3 - dimethyl - butane$ is lesser than hexane making the order of boiling point of all three organic compound as follow
Pentane< 2, 3-dimethyl-butane< hexane
Making option ‘C’ as the correct solution for the given question.

Note:
Compounds that have same molecular formula but different structure are known as structural isomers and the case of hexane and $2,3 - dimethyl - butane$ is a special type of structural isomerism known as chain isomerism in which two compounds have same molecular formula but have different structure due to difference in degree of branching while the latter is also called n-hexane due to no branching the former is also an isomer of hexane with two branches.