Which of the following has the highest boiling point ?
(i) 2-Methylpentane (ii) 2, 3-Dimethylbutane (iii) 2, 2-Dimethylbutane

Explanation

As the branching increases , surface area decreases. As a result, magnitude of van der Waals forces of attraction decreases and hence the boiling point decreases. Now 2-methylpentane has the largest surface area and hence has the highest boiling point. Further because of the presence of two branches on the same carbon, 2,2-dimethylbutane has lower surface area and hence has lower boiling point than that of 2,3-dimethylbutane. Thus, the overall order of decreasing boiling points is : 2-methylpentane (333 K) gt 2,3- dimethylbutane (331 K) gt 2,2-dimethylbutane (323 K)