Question

Consider the following formulas for n-pentane and neopentane: Both compounds have the same overall formula \(\left(\mathrm{C}_{5} \mathrm{H}_{12}, \text { molar }\right.\) mass \(=72.15 \mathrm{g} / \mathrm{mol} ),\) yet \(n\) -pentane boils at \(36.2^{\circ} \mathrm{C}\) whereas neopentane boils at \(9.5^{\circ} \mathrm{C} .\) Rationalize the differences in the boiling points between these two nonpolar compounds.

Short answer

The differences in boiling points between n-pentane (\(36.2^{\circ}\mathrm{C}\)) and neopentane (\(9.5^{\circ}\mathrm{C}\)) can be rationalized by considering their molecular structures and the corresponding strengths of London dispersion forces. Although both compounds have the same molecular weight, the linear structure of n-pentane provides a larger surface area of contact between the molecules, leading to stronger London dispersion forces and a higher boiling point. In contrast, the branched structure of neopentane reduces the surface area of contact, resulting in weaker London dispersion forces and a lower boiling point.

Step-by-step solution

1. Understanding n-pentane and neopentane structures

: n-pentane and neopentane are isomers of the same molecular formula, \(\mathrm{C}_{5}\mathrm{H}_{12}\), with different structures. n-pentane: n-pentane is a straight-chain hydrocarbon with 5 carbon atoms connected in a row, and 12 hydrogen atoms attached to the carbon chain. neopentane: neopentane has a branched structure where a central carbon atom is connected to 4 other carbon atoms, each of which is attached to 3 hydrogen atoms.

2. Determine the intermolecular forces present in each compound

: As both n-pentane and neopentane are nonpolar hydrocarbons, they do not have any permanent dipole-dipole forces or hydrogen bonds. The only forces of attraction between the molecules are the London dispersion forces. Although these forces tend to be weaker compared to other intermolecular forces, they are responsible for the differences in boiling points between n-pentane and neopentane.

3. Evaluate the impact of molecular structures on London dispersion forces

: London dispersion forces are influenced by the molecular weight, molecular structure, and the surface area of interaction. Although both compounds have the same molecular weight, their structures result in different strengths of London dispersion forces: n-pentane: The linear structure of n-pentane provides a larger surface area of contact between the molecules, leading to stronger London dispersion forces. neopentane: The branched structure of neopentane reduces the surface area of contact between the molecules, leading to weaker London dispersion forces.

4. Rationalize the difference in boiling points

: Boiling points are a measure of the energy required to overcome the intermolecular forces between molecules and convert a substance from the liquid phase to the gaseous phase. n-pentane: The stronger London dispersion forces in n-pentane due to its linear structure and larger surface area of contact require more energy to overcome, leading to a higher boiling point of \(36.2^{\circ}\mathrm{C}\). neopentane: The weaker London dispersion forces in neopentane due to its branched structure and reduced surface area of contact require less energy to overcome, leading to a lower boiling point of \(9.5^{\circ}\mathrm{C}\). In conclusion, the differences in boiling points between n-pentane and neopentane can be rationalized by considering their molecular structures and the corresponding strengths of London dispersion forces.

Key concepts

London dispersion forces

London dispersion forces are a type of intermolecular force that occurs between all molecules, no matter their polarity. They are especially significant in nonpolar compounds like n-pentane and neopentane. These forces are caused by momentary fluctuations in electron density in a molecule, which create a temporary dipole that can induce a dipole in a neighboring molecule. This interaction leads to an attraction between the molecules, known as a London dispersion force.

While these forces are universally present, their strength can vary based on several molecular characteristics:

• **Molecular Weight:** Larger molecules with more electrons tend to have stronger dispersion forces due to more significant fluctuations in electron distribution.
• **Surface Area:** The larger the surface area of a molecule, the more contact it can make with neighboring molecules, enhancing dispersion forces.

In the case of n-pentane and neopentane, although they share the same molecular weight, their structural differences significantly impact their dispersion forces. N-pentane has a linear shape, allowing for more extensive surface contact, resulting in stronger dispersion forces compared to the more compact, branched structure of neopentane.

Boiling points

Boiling points indicate the temperature at which a liquid changes into vapor. This process requires energy to overcome the intermolecular forces holding the molecules together in the liquid state. In simpler terms, the stronger these forces, the more energy (i.e., a higher temperature) is needed to boil the substance.

When we consider n-pentane and neopentane, we are particularly interested in how their molecular structures affect their boiling points:

• **N-pentane:** Due to its linear structure, it experiences stronger London dispersion forces. It requires more energy to pull these molecules apart and turn the substance into a vapor. Hence, n-pentane has a higher boiling point of 36.2°C.
• **Neopentane:** Its compact, branched structure means it has weaker London dispersion forces. Therefore, less energy is required to separate these molecules, resulting in a lower boiling point of 9.5°C.

Effectively, the boiling point difference between these two isomers is a reflection of the variances in their intermolecular attractions induced by their structural configurations.

Molecular structure

Molecular structure refers to the specific arrangement of atoms within a molecule. This arrangement determines how the molecule behaves: its physical properties, chemical reactivity, and interactions with other molecules. For isomers like n-pentane and neopentane, their distinct molecular structures can lead to significant differences in their physical properties, despite having the same molecular formula.

• **N-pentane Structure:** It has a straight-chain layout, which maximizes the surface area available for interacting with neighboring molecules, thereby enhancing London dispersion forces.
• **Neopentane Structure:** It features a more compact, branched configuration. This configuration results in a reduced surface area for intermolecular interactions, thus resulting in weaker dispersion forces.

Understanding molecular structure helps in predicting the boiling points of molecules. This is due to how it influences the strength of forces like the London dispersion forces between molecules. By analyzing the molecular structure, we gain insight into why certain isomers like n-pentane and neopentane behave differently, including their different boiling points.