Question

Based on branching, the isomers of hexane can be divided into three distinct classes as shown in the figure. What is the correct order of their boiling points?

• A. I > II > III
• B. II > III > I
• C. III > II > I
• D. III > I > II

Answer 1

The correct option is C III > II > I

For the hydrocarbons with the same molar mass, as the branching increases, the overall surface area decreases. This leads to a reduction in the intermolecular forces (London dispersion forces), decreasing the boiling point of hydrocarbons.
$\text{Boiling point}\propto \text{Molecular size}\propto \text{van der Waals force}\propto \text{Molar mass}$

The increasing order of branching of the given set of molecules is as follows:

I > II > III

Therefore, the order of boiling point is as follows:

III > II > I

Hence, option (c) is the correct answer.