Question

Arrange the following in decreasing order of their boiling points.
(I) n-Butane
(II) 2-Methylbutane
(III) n-Pentane
(IV) 2,2-Dimethylpropane

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

Answer 1

Answer: (D)

III > II > IV > I

The boiling point of alkanes increases with increase in molecular mass and for the same alkane, the boiling point decreases with branching. Thus, the decreasing order of their boiling points is:

$\underset{III}{n-Pentane}>\underset{II}{2-Methylbutane}>\underset{IV}{2,2-Dimethylpropane}>\underset{I}{n-Butane}$

Hence, $D$ is the correct answer.