Question

In dichloromethane, \(\mathrm{CH}_{2} \mathrm{Cl}_{2}(\mu=1.60 \mathrm{D})\), the dispersion force contribution to the intermolecular attractive forces is about five times larger than the dipole-dipole contribution. Compared to \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\), would you expect the relative importance of the dipole-dipole contribution to increase or decrease (a) in dibromomethane \((\mu=1.43 \mathrm{D})\), (b) in difluoromethane \((\mu=1.93 \mathrm{D})\) ? (c) Explain.

Short answer

(a) The relative importance of the dipole-dipole contribution will decrease in dibromomethane (CH₂Br₂) compared to dichloromethane (CH₂Cl₂) because its dipole moment is smaller, leading to weaker dipole-dipole forces. (b) The relative importance of the dipole-dipole contribution will increase in difluoromethane (CH₂F₂) compared to CH₂Cl₂ due to its stronger dipole moment, which results in stronger dipole-dipole forces. (c) The relative importance decreases in CH₂Br₂ because of its weaker dipole moment, while it increases in CH₂F₂ due to its stronger dipole moment compared to CH₂Cl₂.

Step-by-step solution

Understand Dispersion and Dipole-Dipole Forces

Dispersion forces are weak forces that result from temporary dipoles caused by the random movement of electrons. They exist in all molecules. On the other hand, dipole-dipole forces are interactions between molecules with permanent dipoles, such as polar molecules. The strength of the dipole-dipole force depends on the magnitude of the molecular dipole moment (µ).

Compare Molecular Dipole Moments

We are given the molecular dipole moments as follows: - Dichloromethane (CH₂Cl₂): µ = 1.60 D - Dibromomethane (CH₂Br₂): µ = 1.43 D - Difluoromethane (CH₂F₂): µ = 1.93 D

Analyze Dibromomethane (CH₂Br₂)

Comparing the dipole moments, we see that CH₂Br₂ has a lower dipole moment than CH₂Cl₂. This means that the dipole-dipole forces in dibromomethane are weaker compared to those in dichloromethane. However, both molecules have dispersion forces. Since the dipole-dipole forces are weaker in dibromomethane, the relative importance of dispersion forces will be higher. Therefore, the relative importance of the dipole-dipole forces will decrease in dibromomethane (CH₂Br₂) compared to dichloromethane (CH₂Cl₂). Answer (a): The relative importance of the dipole-dipole contribution will decrease in dibromomethane.

Analyze Difluoromethane (CH₂F₂)

Comparing the dipole moments, we see that CH₂F₂ has a higher dipole moment than CH₂Cl₂. This means that the dipole-dipole forces in difluoromethane are stronger compared to those in dichloromethane. Consequently, the relative importance of the dipole-dipole forces will increase in difluoromethane (CH₂F₂) compared to dichloromethane (CH₂Cl₂), even though both molecules have dispersion forces. Answer (b): The relative importance of the dipole-dipole contribution will increase in difluoromethane.

Explanation

In CH₂Br₂, the dipole moment is smaller than in CH₂Cl₂, leading to weaker dipole-dipole forces. The weaker dipole-dipole forces in CH₂Br₂ make the dispersion forces relatively more dominant compared to CH₂Cl₂. In contrast, CH₂F₂ has a higher dipole moment than CH₂Cl₂, which means stronger dipole-dipole forces. Therefore, the relative importance of the dipole-dipole forces in CH₂F₂ is greater than in CH₂Cl₂, even though both molecules have dispersion forces. Answer (c): The relative importance of the dipole-dipole contribution will decrease in CH₂Br₂ because of its weaker dipole moment compared to CH₂Cl₂. On the other hand, it will increase in CH₂F₂ due to its stronger dipole moment compared to CH₂Cl₂.

Key concepts

Dispersion Forces

Dispersion forces, also known as London dispersion forces, are the weakest type of intermolecular force. Despite their weakness, they are present in all molecules. These forces arise due to the random movement of electrons that create temporary dipoles. These temporary dipoles, in turn, induce dipoles in neighboring molecules, resulting in a small attraction between them. This effect is especially notable in larger and more complex molecules, as the increased electron cloud size enhances the possibility of temporary dipole formation.
The strength of dispersion forces is influenced by:

• Molecular size: Larger molecules have more electrons and are more polarizable, leading to stronger dispersion forces.
• Shape: Long-chain molecules have stronger dispersion forces than compact-shaped molecules due to more surface contact area.

Although dispersion forces are generally weaker than other forces, their contribution can be significant, particularly in large molecules or atoms with high electron densities.

Dipole-Dipole Forces

Dipole-dipole forces occur between molecules that possess permanent dipoles, i.e., polarized molecules. These forces arise because of the electrostatic attraction between the positive end of one polar molecule and the negative end of another. The strength of dipole-dipole interactions is contingent on the dipole moment (\(\mu\)), which indicates the degree of polarity a molecule has.
Some factors that affect dipole-dipole interactions include:

• Dipole moment magnitude: Greater molecular polarity, as seen with a larger dipole moment, leads to stronger dipole-dipole attractions.
• Orientation: Optimally aligned dipoles attract more strongly compared to randomly oriented molecules.

These forces play a crucial role in determining the properties and behaviors of polar molecules such as dissolving and boiling points. Polar molecules, influenced by these interactions, typically have higher boiling points than nonpolar ones due to the stronger attractions holding the molecules together.

Molecular Dipole Moment

The molecular dipole moment (\(\mu\)) is a vector quantity that represents the polarity of a molecule. It is calculated based on the vector sum of the individual bond dipole moments, which arise from the difference in electronegativity between bonded atoms. The dipole moment is expressed in Debye units (D).
The importance of the molecular dipole moment lies in its ability to influence molecular interactions, like dipole-dipole forces. A higher dipole moment generally indicates a more polar molecule and therefore stronger dipole-dipole interactions.
In practical terms, the dipole moment impacts:

• Solubility: Polar molecules (with high dipole moments) tend to be soluble in polar solvents.
• Boiling and melting points: Molecules with higher dipole moments often exhibit higher boiling/melting points.
• Reactivity and physical properties: The polarity can affect how molecules interact and react with each other and different substances.

It's fascinating to see how this singular measurement can offer insights into a molecule's interactions and its physical and chemical properties.