Question

Out of $\text{2, 2-dimethylbutane},\text{2-methylpentane}$ and $\text{2, 3-dimethylbutane}$, which one is the correct decreasing order of boiling point?

• A. $\text{2,3-dimethylbutane > 2,2-dimethylbutane > 2-methylpentane}$
• B. $\text{2-methylpentane > 2,2-dimethylbutane > 2,3-dimethylbutane}$
• C. $\text{2,2-dimethylbutane > 2-methylpentane > 2, 2-dimethylbutane}$
• D. $\text{2-methylpentane > 2,3-dimethylbutane > 2,2-dimethylbutane}$

Answer 1

The correct option is D $\text{2-methylpentane > 2,3-dimethylbutane > 2,2-dimethylbutane}$

Boiling point in alkane depends upon the chains of atoms and the molecular mass of the molecules. Straight-chain molecules have higher boiling point than the branched-chain compounds.
Among given compounds, $\text{2-methylpentane}$ has the least number of branches thus it has the highest boiling point.
For the same molar mass, as branching decreases, the overall surface area increases. This leads to a reduction in the intermolecular forces, thereby decreasing boiling point. So between $\text{2,3-dimethylbutane}$ and $\text{2,2-dimethylbutane},\text{2,2-dimethylbutane}$ is more branched and has large surface area thus it lower boiling point than $\text{2,3-dimethylbutane}$.
Further, the boiling point of these compounds follows the order : $\text{2-methylpentane > 2,3-dimethylbutane > 2,2-dimethylbutane}$.