Question

Explain why n-pentane has a higher boiling point than neopentane.

Answer 1

Hint:Boiling point of a material depends on the weak attractive intermolecular forces existing between molecules. Weaker the attractive intermolecular forces, lesser is the boiling point of a material. For alkanes which are isomers, their structure has a big role to play in deciding the melting and boiling point of the compound.

Complete answer:As mentioned in the hint, it is really about the intermolecular forces and their nature that decides most of the physical properties such as boiling point. Such intermolecular attractive forces that need our attention in this question are ‘London dispersion forces’ which are the weakest known intermolecular forces. The reason that we are talking about the London forces here is because they contribute more in non-polar compounds. (But keep in mind that they are applicable to both polar and non-polar compounds).
When non-polar molecules come in vicinity of one another, they are polarized temporarily (means they become mini magnets for an extremely short duration of time) which results in the attractive forces. More the number of electrons, greater are the forces, since more temporary electric dipoles will form. But since these two compounds are isomers of each other, the difference in the London dispersion forces can only be explained on the basis of the structure or the shape of these molecules, since other properties, such as number of electrons, molar mass etc. will be identical.
In reality, it is really about the surface area of the molecules that is exposed to the other molecules. N-pentane has a more surface area \[\left( {118{\text{ }}{A^{o{\text{ }}2}}} \right)\] as a molecule to offer as compared to neopentane \[\left( {112{\text{ }}{A^{o{\text{ }}2}}} \right)\] . Greater surface area means more attractive forces and since n-pentane has more surface area to offer, it has more attractive forces among molecules and higher boiling point. The boiling point of n-pentane is \[{36.1^o}C\] and the boiling point of neopentane is \[{10^o}C\] .

Hence n-pentane has higher boiling point than neopentane.

Note: The surface area and the shape of molecules also has an impact on the melting point as well. The shape of neopentane is spheroid and hence can be easily packed into a solid structure. On the other hand n-pentane has more possible conformation, hence it can stay in liquid state for greater range of temperature and has comparatively lesser melting point than neopentane.