Question

Butane and 2 -methylpropane, whose space-filling models are shown at the top of the next column, are both nonpolar and have the same molecular formula, ${\mathrm{C}}_4{\mathrm{H}}_{10}$, yet butane has the higher boiling point $(-{0.5}^{\circ }\mathrm{C}$ compared to $-{11.7}^{\circ }\mathrm{C})$. Explain.

Short answer

Although both butane and 2-methylpropane are nonpolar and have the same molecular formula ${\mathrm{C}}_4{\mathrm{H}}_{10}$, the difference in their boiling points can be attributed to their difference in molecular structures. Butane's linear structure has more surface area, leading to stronger London dispersion forces among its molecules, and therefore has a higher boiling point, compared to the branched structure of 2-methylpropane.

Step-by-step solution

Define Boiling Point

Boiling point is the temperature at which a substance changes from a liquid to a gas phase. It is influenced by intermolecular forces, molecular size, molecular weight, and molecular shape.

Examine the Molecular Models

Let's look at the molecular models of butane and 2-methylpropane to understand their structures: Butane: $\phantom{-}C{H}_3-C{H}_2-C{H}_2-C{H}_3$ 2-methylpropane: $\phantom{-}C{H}_3-C(C{H}_3{)}_3$ As we can see, butane is a linear molecule, while 2-methylpropane has a branched structure.

Analyze Intermolecular Forces

Both butane and 2-methylpropane are nonpolar molecules, which means they have no net charge separation. As a result, they do not experience strong dipole-dipole interactions. However, they do experience weak London dispersion forces (LDFs) as their primary intermolecular force. LDFs occur due to the temporary fluctuations in electron distribution, creating temporary dipoles that cause molecules to attract each other.

Relate LDFs to Molecular Structures

The strength of London dispersion forces depends on the surface area of the molecule and the number of electrons. Generally, larger surface area and more electrons lead to stronger LDFs and thus higher boiling points. Butane, being a linear molecule, has a larger surface area than the more compact, branched structure of 2-methylpropane. This increased surface area leads to stronger LDFs between adjacent butane molecules, resulting in a higher boiling point compared to 2-methylpropane.

Explain the Difference in Boiling Points

To summarize, although both butane and 2-methylpropane are nonpolar and have the same molecular formula ${\mathrm{C}}_4{\mathrm{H}}_{10}$, the difference in their boiling points can be attributed to their difference in molecular structures. Butane's linear structure has more surface area, leading to stronger London dispersion forces among its molecules, and therefore has a higher boiling point, compared to the branched structure of 2-methylpropane.

Key concepts

Intermolecular Forces

Intermolecular forces are the attractive forces that occur between molecules. These forces play a vital role in determining the physical properties of a substance, such as boiling points and melting points. There are several types of intermolecular forces, including:

• Dipole-dipole interactions, which occur between polar molecules.
• Hydrogen bonding, a special type of dipole-dipole interaction involving hydrogen atoms.
• London dispersion forces, which occur in all molecules, regardless of polarity.

In the case of butane and 2-methylpropane, both are nonpolar and do not exhibit dipole-dipole interactions or hydrogen bonding. Therefore, the primary intermolecular force at play is the London dispersion force. Despite being weaker than other types of intermolecular forces, London dispersion forces are significant in nonpolar molecules, affecting their boiling points and other properties.

London Dispersion Forces

London dispersion forces (LDFs) are the weakest type of intermolecular force and are present in all molecules, whether they are polar or nonpolar. These forces arise from temporary fluctuations in the electron distribution within a molecule, which creates temporary dipoles. These temporary dipoles induce corresponding dipoles in nearby molecules, leading to attraction.The strength of LDFs depends on several factors, including:

• The molar mass of the molecules, as more electrons allow for larger fluctuations in electron distribution.
• The shape and surface area of the molecules, where larger surface areas promote stronger LDFs.

In our example with butane and 2-methylpropane, although both substances have the same molecular formula (${\mathrm{C}}_4{\mathrm{H}}_{10}$), the difference in their structures results in variations in their boiling points. The linear structure of butane allows for greater surface contact between molecules, resulting in stronger LDFs and a higher boiling point than the branched structure of 2-methylpropane.

Molecular Structure

The molecular structure plays a crucial role in determining the physical properties of a compound, such as its boiling point. The arrangement of atoms within the molecule influences the surface area available for intermolecular interactions, such as London dispersion forces. Different molecular structures can affect properties such as:

• Linear structures, like butane, typically allow more surface area for intermolecular interactions, leading to stronger dispersion forces.
• Branched structures, like 2-methylpropane, tend to have less surface area, resulting in weaker intermolecular forces.

In our comparison, butane's linear arrangement provides more surface area for other molecules to come in contact and experience London dispersion forces. This increased interaction leads to a higher boiling point. Meanwhile, the compact and branched nature of 2-methylpropane results in fewer contacts, thus less interaction, and a lower boiling point.