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 \(\left(-0.5^{\circ} \mathrm{C}\right.\) compared to \(\left.-11.7^{\circ} \mathrm{C}\right) .\) Explain.

Short answer

Butane has a higher boiling point than 2-methylpropane due to its linear structure, which results in a larger surface area in contact between molecules and therefore stronger London dispersion forces. In contrast, the compact, branched structure of 2-methylpropane reduces the surface area in contact with neighboring molecules, leading to weaker London dispersion forces. The stronger intermolecular forces in butane result in its higher boiling point (\(-0.5^{\circ} \mathrm{C}\)) compared to 2-methylpropane (\(-11.7^{\circ} \mathrm{C}\)).

Step-by-step solution

Identify the molecular structure of butane and 2-methylpropane

Butane and 2-methylpropane both have the molecular formula \(\mathrm{C}_{4}\mathrm{H}_{10}\), but they have different structural formulas due to being structural isomers. Butane has a linear carbon chain: \( \mathrm{CH}_{3}\mathrm{-CH}_{2}\mathrm{-CH}_{2}\mathrm{-CH}_{3} \) 2-methylpropane has a branched carbon chain: \( \mathrm{CH}_{3} \) \( \, | \) \( \mathrm{CH}_{3}\mathrm{-C}\mathrm{-CH}_{3} \) \( \, | \) \( \mathrm{CH}_{3} \)

Identify the intermolecular forces acting on the molecules

Since both butane and 2-methylpropane are nonpolar molecules, the primary type of intermolecular force acting between their molecules is London dispersion forces. London dispersion forces are weak intermolecular forces resulting from temporary dipoles created due to fluctuations in the electron cloud surrounding the molecules. The larger the surface area of the molecule, the stronger the London dispersion forces will be.

Compare the strength of London dispersion forces in both isomers

Now let's compare butane and 2-methylpropane in terms of their London dispersion forces. Butane is a linear carbon chain, while 2-methylpropane has a more compact, branched structure. Due to being linear, butane molecules have a larger surface area in contact with each other, resulting in stronger London dispersion forces between the molecules. On the other hand, the compact and spherical-like structure of 2-methylpropane reduces the surface area in contact with its neighboring molecules, leading to weaker London dispersion forces.

Relate the strength of London dispersion forces to boiling points

The boiling point of a substance is related to the strength of the intermolecular forces acting on its molecules. The stronger the intermolecular forces, the higher the boiling point. In our case, as we have discussed before, butane has stronger London dispersion forces than 2-methylpropane.

Explain the difference in boiling points

Due to the linear structure of butane and the larger surface area in contact between its molecules, the London dispersion forces acting on butane are stronger than those acting on 2-methylpropane. This results in a higher boiling point for butane, which boils at \(-0.5^{\circ} \mathrm{C}\), compared to 2-methylpropane, which boils at \(-11.7^{\circ} \mathrm{C}\).

Key concepts

London Dispersion Forces

London dispersion forces are a fascinating aspect of chemistry. They are the weakest of the intermolecular forces, yet they play a vital role, especially in nonpolar substances. These forces arise from the momentary fluctuations in electron distribution within molecules. Because electrons are in constant motion, there can be an instantaneous dipole moment when the electron density is uneven. When this occurs, it induces similar dipoles in neighboring molecules.

• This induced moment creates an attraction between molecules, which are London dispersion forces.
• Though individually weak, they collectively have a substantial impact on properties like melting and boiling points.
• The strength of London dispersion forces is directly linked to the surface area and the polarizability of the molecule.

Understanding these forces helps explain why substances with larger surface areas, like linear molecules, experience stronger London dispersion forces.

Boiling Points

Boiling points give insights into the intermolecular forces at work within a substance. When a substance boils, its molecules need to overcome the forces holding them together in a liquid state. So, the stronger these forces, the more energy—or higher temperature—required to separate them.

• For instance, the strength of London dispersion forces has a direct effect on boiling points.
• In our example, butane, which is linear, has stronger forces than its isomer due to more surface contact.
• Therefore, butane needs more heat to reach its boiling point compared to 2-methylpropane.

This difference in boiling points between the two isomers, butane and 2-methylpropane, illustrates the concept well.

Isomerism

Isomerism is a key concept in organic chemistry. It explains how compounds with the same molecular formula can have different structures and properties. Butane and 2-methylpropane are prime examples of structural isomers.

• They share the same formula, \(\mathrm{C}_{4}\mathrm{H}_{10}\), yet differ in their atomic arrangement.
• Butane has a straightforward, linear arrangement, causing it to have greater London dispersion forces.
• In contrast, 2-methylpropane exhibits a compact, branched structure.

This structural variance leads to distinct physical and chemical properties despite having an identical formula. Isomerism thus highlights the diversity and complexity within organic molecules.

Molecular Structure

Molecular structure is crucial in determining a compound's physical properties. The way atoms are bonded and arranged within a molecule affects its strength and the type of forces it can exert or experience.

• Butane's linear structure allows for maximum surface interaction among its molecules.
• 2-methylpropane, on the other hand, has a branched structure that limits this interaction.

Consequently, the structural form of a molecule influences not only London dispersion forces but also broader characteristics like boiling points and reactivity. Understanding molecular structure helps predict and explain these outcomes efficiently.