Question

Butane and 2 -methylpropane, whose space-filling models are shown here, are both nonpolar and have the same molecular formula, \(\mathrm{C}_{4} \mathrm{H}_{10},\) yet butane has the higher boiling point \(\left(-0.5^{\circ} \mathrm{C}\) compared to \(-11.7^{\circ} \mathrm{C}\right) .\) Explain.

Short answer

Although both butane and 2-methylpropane have the same molecular formula \(\mathrm{C}_{4}\mathrm{H}_{10}\) and are nonpolar, butane has a higher boiling point due to its linear structure, which allows for a larger surface area contact between molecules and leads to stronger London dispersion forces. In contrast, 2-methylpropane's branched structure results in weaker London dispersion forces, leading to a lower boiling point.

Step-by-step solution

Understand the molecular structures of butane and 2-methylpropane

First, let's analyze the structures of butane and 2-methylpropane. Both molecules have the molecular formula \(\mathrm{C}_{4}\mathrm{H}_{10}\) and are hydrocarbons. The molecule butane has a linear (or more precisely zigzag) structure, while 2-methylpropane has a branched structure with three carbon atoms in the main chain and one methyl group attached to the second carbon atom.

Analyze the polarity of butane and 2-methylpropane

Both butane and 2-methylpropane are nonpolar molecules. In each case, the carbon-hydrogen bonds within the molecules are not polar enough to give the overall molecule any significant polarity. Additionally, due to the symmetry of both molecules, any small dipoles that may be present will cancel each other out, resulting in nonpolar molecules.

Understand boiling points

Boiling points are determined by the strength of the intermolecular forces between molecules. The stronger the intermolecular forces, the higher the boiling point. The primary types of intermolecular forces are hydrogen bonding, dipole-dipole interactions, and London dispersion forces. Since both butane and 2-methylpropane are nonpolar, their intermolecular forces are primarily due to London dispersion forces. Such forces are temporary, weak attractive forces due to temporary fluctuations in the electron distribution of the molecules.

Relate molecular structures to London dispersion forces

London dispersion forces are influenced by the molecular shape and surface area. In general, larger surface areas tend to create stronger London dispersion forces. Butane's linear structure has a larger surface area in contact with neighboring molecules, which allows for more points of interaction and thus stronger London dispersion forces. In contrast, 2-methylpropane’s branched structure has a smaller surface area of contact due to its more spherical nature, which results in weaker London dispersion forces compared to butane.

Compare boiling points

Due to the stronger London dispersion forces between butane molecules as a result of its larger surface area, butane has a higher boiling point (-0.5°C) than 2-methylpropane (-11.7°C). Therefore, the difference in boiling points between the two molecules can be explained by their differing molecular structures.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces that act between molecules. They play a crucial role in determining the physical properties of substances, such as boiling points.
The main types of intermolecular forces are:

• Hydrogen bonding: An especially strong type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine.
• Dipole-dipole interactions: Attractions between polar molecules with permanent dipoles.
• London dispersion forces: Weak, temporary attractions in nonpolar molecules caused by momentary fluctuations in electron distribution.

In the case of butane and 2-methylpropane, the boiling points primarily depend on London dispersion forces because these molecules do not have hydrogen bonds or significant dipole moments to enable dipole-dipole interactions.
Thus, understanding how these intermolecular forces work helps explain why butane and 2-methylpropane, which are nonpolar, behave differently.

London Dispersion Forces

London dispersion forces, also known as van der Waals forces, are the weakest intermolecular forces. They originate from temporary fluctuations in the electron cloud of molecules.
These temporary changes create instantaneous dipoles, leading to an attraction between nearby molecules. Even though these forces are weak, they are the only type of intermolecular force present in nonpolar molecules, such as butane and 2-methylpropane.

The strength of London dispersion forces is influenced by:

• Molecular Size: Larger molecules have more electrons and surface area, resulting in stronger dispersion forces.
• Molecular Shape: Linear molecules, like butane, have greater surface areas in contact with neighboring molecules, leading to stronger interactions.

For example, the linear structure of butane allows more extensive contact with other molecules compared to the compact, branched shape of 2-methylpropane.
Consequently, butane exhibits stronger London dispersion forces, leading to its higher boiling point.

Molecular Structure

Molecular structure plays a pivotal role in determining how a molecule interacts with its neighbors. It influences both the distribution of electrons and the overall shape of the molecule.

Key aspects of molecular structure include:

• Shape: Influences the surface area available for intermolecular contact. Linear molecules, like butane, allow for more interaction surface, while branched molecules, like 2-methylpropane, occupy less space.
• Symmetry: Affects the ability of polar bonds to generate a net dipole moment across the molecule. Even though both butane and 2-methylpropane are nonpolar, their symmetry is crucial for understanding their boiling points.

The linear structure of butane maximizes its interaction with neighboring molecules due to its larger contact surface area compared to the more compact, branched shape of 2-methylpropane.
This structural difference explains why butane, having stronger London dispersion forces, boils at a higher temperature than 2-methylpropane. Understanding these molecular nuances helps clarify their differing boiling point behaviors.