Problem 21 - Chemistry: The Central Science

Question

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

Step-by-Step Solution

Answer

The higher boiling point of butane (-0.5°C) compared to 2-methylpropane (-11.7°C) can be explained by their molecular structures. Butane has a linear structure with a larger surface area, leading to stronger London dispersion forces, while 2-methylpropane has a more compact, branched structure. The stronger intermolecular forces in butane require more energy to overcome, resulting in a higher boiling point.

Step 1: Analyze the molecular structures of butane and 2-methylpropane

Both butane and 2-methylpropane have the same molecular formula, C4H10, but different structures. Butane is a linear, unbranched molecule, while 2-methylpropane is a branched molecule. Their structures are shown below: Butane: \(–\mathrm{CH_3}\!\mathrm{–}\!\mathrm{CH_2}\!\mathrm{–}\!\mathrm{CH_2}\!\mathrm{–}\!\mathrm{CH_3}\) 2-methylpropane: \(\mathrm{CH_3}\!\mathrm{(}\!\mathrm{CH_3}\!\mathrm{)_2}\!\mathrm{C}\)–\(\mathrm{CH_3}\)

Step 2: Identify the intermolecular forces

Both compounds are nonpolar, as there are no significantly electronegative atoms present which would cause a dipole moment. The primary intermolecular force in these molecules is Van der Waals or London dispersion forces, which are temporary, induced dipoles due to the interactions between electrons. The strength of these forces depends on the surface area and molecular size.

Step 3: Compare molecular sizes and surface area

Butane being a linear molecule has a larger surface area than 2-methylpropane, which is a more compact, branched molecule. The larger surface area of butane allows more interaction between molecules and therefore stronger London dispersion forces.

Step 4: Relate intermolecular forces to boiling points

Boiling points are related to the strength of the intermolecular forces between molecules. Stronger intermolecular forces require more energy to overcome, resulting in higher boiling points. In this case, the larger surface area of butane results in stronger London dispersion forces compared to 2-methylpropane.

Step 5: Conclusion

Although butane and 2-methylpropane have the same molecular formula and both are nonpolar, the difference in their molecular structures affects their boiling points. Butane has a linear structure, resulting in a larger surface area and stronger London dispersion forces compared to the more compact, branched structure of 2-methylpropane. This explains why butane has a higher boiling point (-0.5°C) than 2-methylpropane (-11.7°C).

Key Concepts

Intermolecular Forces

Intermolecular forces are the attractive forces between molecules, which are critical in determining a substance's physical properties, such as boiling and melting points, vapor pressure, and solubility. In the case of hydrocarbons like butane and 2-methylpropane, the type of intermolecular force involved is particularly significant.
In our textbook exercise, butane and 2-methylpropane are both nonpolar hydrocarbons that rely on London dispersion forces as their primary means of intermolecular attraction. Despite having the same molecular formula, their boiling points differ due to the varying strengths of these forces, influenced by their molecular structure. Understanding why these variations occur helps in grasping how molecular interactions affect physical properties.

Molecular Structure

The molecular structure of a compound plays a pivotal role in dictating its physical attributes, including the boiling point. Although butane and 2-methylpropane have identical molecular formulas, their structural differences lead to distinctions in their boiling points. Butane is a linear molecule, while 2-methylpropane has a more compact, branched configuration.

Molecular Shape and Boiling Point

The shape of a molecule directly influences its surface area, which in turn affects the strength of intermolecular forces. In linear molecules like butane, the greater surface area allows for increased interactions between adjacent molecules, heightening the London dispersion forces. The impact of molecular structure, therefore, is a fundamental concept for understanding how the shape of molecules can lead to variations in properties like boiling points among substances with similar chemical compositions.

London Dispersion Forces

London dispersion forces are a type of intermolecular force present in all molecular substances, notably in nonpolar compounds like hydrocarbons. These forces originate from the momentary shifts in electron clouds within molecules, which produce temporary dipoles that induce attraction between neighboring molecules.

Factors Affecting Strength of London Dispersion Forces

• Molecular Size: Larger molecules tend to have stronger London dispersion forces due to their greater electron count, which increases the likelihood of dipole formations.
• Molecular Shape: The surface area of a molecule impacts the extent of contact it can have with its neighbors. Molecules with larger surface areas typically exhibit stronger dispersion forces.

The recognition of London dispersion forces as a determinant of boiling point underlines the importance of considering these transient forces when comparing molecules of similar types, such as isomers.

Surface Area and Boiling Point

The boiling point of a substance is intricately linked to the strength of its intermolecular forces; substances with higher boiling points generally have stronger intermolecular forces. Surface area is a determining factor in the strength of these forces, especially London dispersion forces which are prevalent in nonpolar hydrocarbons.
In the comparison between butane and 2-methylpropane, butane's linear structure yields a greater surface area, enabling more substantial intermolecular interactions, which translates to stronger London dispersion forces. Consequently, more energy (in the form of heat) is required to separate butane molecules during the phase transition from liquid to gas, thus it possesses a higher boiling point.
This relationship between surface area and boiling point is a critical concept in understanding not only the physical properties of hydrocarbons but also the broader implications for various substances across chemistry.