Which of the following compounds has the highest boiling point ?

Options

A. `2,2`-Dimethylpropane
B. `n`-Hexane
C. `n`-Pentane
D. `2`-Methylbutane

Answer

B

Explanation

With unbranched alkanes, as the molecular mass increases, so does the molecular size, and even more importantly molecular surface areas in contact. With increasing surface area, the van der Waals forces between molecules increase, therfore, more energy ( a higher temperature) is required to seprate molecules from one another and produce boiling. Chain branching makes a molecule more compact reducing its surface area and with it the strength of the van der Waals forces operating between it and the adjacent molecules. This has the effect of lowering the boliling point. Thus, the order of boiling point is `(2) gt (4) gt (3) gt (1)`.