Question

Arrange the following in decreasing order of their boiling points.

(A) n-butane

(B) 2-methylbutane

(C) n-pentane

(D) 2, 2-dimethylpropane

Options

• A > B > C > D

• B > C > D > A

• D > C > B > A

• C > B > D > A

Answer 1

C > B > D > A

Explanation:

As the number of carbon atom increases, boiling point increases. Boiling point decreases with branching.

2, 2-dimethyl propane

\[\begin{array}{cc}
\phantom{...}\ce{CH3}\\
\phantom{}|\\
\ce{H3C - C - CH3}\\
\phantom{}|\\
\phantom{...}\ce{\underset{b.pt = 282.5 K}{CH3}}
\end{array}\]

n-pentane

\[\ce{\underset{b.pt = 309.1 K}{H3C - CH2 - CH2 - CH2 - CH3}}\]

2-methyl butane

\[\begin{array}{cc}
\ce{H3C - H2C - HC - CH3}\\
\phantom{.........}|\\
\phantom{............}\ce{\underset{b.pt = 301 K}{CH3}}
\end{array}\]

n-butane

\[\ce{\underset{(4 carbon atoms with no branching)}{\underset{b.pt = 273 K}{H3C - H2C - CH2 - CH3}}}\]