IMAT 2015 Q46 [Boiling Points]

2,2 – dimethylpropane, C(CH_3)_4, is an isomer of pentane, CH_3(CH_2)_3CH_3.

Pentane has a boiling point of 36 °C whilst the boiling point of 2,2 dimethylpropane is 10 °C.

Which statement below explains the difference in the boiling points for these two substances?

A. 2,2 – dimethylpropane has stronger intermolecular forces.

B. The molecules have different relative molecular masses.

C. Isomers have different chemical properties.

D. Pentane has permanent dipoles.

E. Longer chain, less branched molecules have stronger spontaneous/induced dipoles.

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In organic compounds, the boiling point is affected by the branching and length of the molecules.

The longer and less branched the molecules, the higher the boiling point. The shorter and more branched the molecules, the lower the boiling point.

Let’s look at the structures of each compound mentioned:

Pentane- CH_3(CH_2)_3CH_3


2,2-Dimethylpropane- C(CH_3)_4


As we can see, pentane has fewer branches and a longer chain, this causes a higher amount of induced dipoles, which is the perfect formula for a higher boiling point in comparison to its relevant isomer.

Our answer is, therefore, E.