Question

Following are the isomers of molecular formula for $C_5{H}_{12}$.
Decreasing order of their boiling points is:

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

Answer 1

The correct option is A I $<$ II $<$ III

(I) $H_{3}C-\stackrel{H}{\stackrel{|}{\underset{H}{\underset{|}{C}}}}-\stackrel{H}{\stackrel{|}{\underset{H}{\underset{|}{C}}}}-\stackrel{H}{\stackrel{|}{\underset{H}{\underset{|}{C}}}}-C{H}_3$

(II) $H_{3}C-\stackrel{C{H}_3}{\stackrel{|}{\underset{H}{\underset{|}{C}}}}-\stackrel{H}{\stackrel{|}{\underset{H}{\underset{|}{C}}}}-C{H}_3$

III) $H_{3}C-\stackrel{C{H}_3}{\stackrel{|}{\underset{C{H}_3}{\underset{|}{C}}}}-C{H}_3$

In these structures, branching increasing from II and isomeric alkanes boiling point decreases with branching. This is due to the reason that with branching surface area decreases and magnitude of vander waals forces of attraction decreases too.