Question

which one has the highest boiling point and why
2-methylpentane
2,3-dimethylbutane
2,2-dimethylbutane

Answer 1

Dear Student,

​The boiling point of alkanes is based on the Vanderwaal force, which depends on:

• The surface are of the molecule
• The number of electrons around the molecule, which increase the alkane's MW

Branching leads to decrease in the total surface area of the molecule, and thus lower Vanderwaal forces between two alkane molecules.

In the given compounds, 2-Methylpentane has only one branched group, whereas 2,2-Dimethylbutane and 2,3-Dimethylbutane have two branched methyl groups.

Thus, 2-Methylpentane has a higher Vanderwaal force due to higher surface area for contact and hence a higher boiling point.

Regards