Question

Chloromethane \(\left(\mathrm{CH}_{3} \mathrm{Cl}\right)\) has a much higher boiling point than methane \(\left(\mathrm{CH}_{4}\right)\). Give two reasons for this.

Short answer

Chloromethane \(\left(\mathrm{CH}_{3} \mathrm{Cl}\right)\) has a higher boiling point than methane \(\left(\mathrm{CH}_{4}\right)\) due to two main reasons: \(1)\) Chloromethane is a polar compound with stronger dipole-dipole interactions in addition to London dispersion forces, whereas methane is a nonpolar compound with only weak London dispersion forces. \(2)\) Chloromethane has a larger molecular weight, which contributes to stronger London dispersion forces. The combination of these intermolecular forces leads to a higher boiling point for chloromethane compared to methane.

Step-by-step solution

1. Identify the Types of Intermolecular Forces Present in Both Molecules

In order to provide reasons for the difference in boiling points, we need to first understand the types of intermolecular forces present in both chloromethane and methane. Methane is a nonpolar compound, so the dominant forces between the molecules are weak London dispersion forces. Chloromethane, on the other hand, is a polar compound due to the electronegativity difference between carbon, hydrogen, and chlorine, resulting in stronger dipole-dipole interactions in addition to London dispersion forces.

2. Compare the Strength of Intermolecular Forces

When comparing the intermolecular forces present in the two compounds, we can see that methane only has the presence of weak London dispersion forces, while chloromethane has both dipole-dipole interactions and London dispersion forces. The presence of the stronger dipole-dipole interactions in chloromethane is the first reason for the higher boiling point. In general, stronger intermolecular forces result in higher boiling points because more energy is needed to overcome the forces holding molecules together in the liquid phase.

3. Assess the Impact of Molecular Weight

Another factor that we should consider when comparing boiling points is the molecular weight of the compounds. Chloromethane has a molecular weight of 50.49 g/mol, while methane has a molecular weight of 16.04 g/mol. In general, compounds with larger molecular weights have larger electron clouds and surface areas, which leads to stronger London dispersion forces. Although London dispersion forces are weaker than dipole-dipole interactions, the increased molecular weight of chloromethane contributes to the overall strength of its intermolecular forces, providing a second reason for its higher boiling point. In summary, chloromethane has a much higher boiling point than methane due to the presence of stronger dipole-dipole interactions and London dispersion forces that result from its polar nature and larger molecular weight.

Key concepts

Dipole-Dipole Interactions

Dipole-dipole interactions are a type of intermolecular force that occur between polar molecules. Imagine these forces as tiny magnets inside molecules that attract each other. In polar molecules, this happens because there is an uneven distribution of electrons, which creates a positive and negative side or "dipole" in the molecule.

This is what we call polarity. Chloromethane (\(\mathrm{CH}_3\mathrm{Cl}\)) is a good example as it's a polar molecule. The difference in electronegativity between chlorine, carbon, and hydrogen leads to a dipole moment. These dipoles cause the chloromethane molecules to attract one another more strongly.

Because of these strong forces, chloromethane molecules resist being pulled apart. This requires more energy in the form of heat, resulting in a higher boiling point compared to nonpolar molecules of similar size, like methane (\(\mathrm{CH}_4\)).

London Dispersion Forces

London dispersion forces are fascinating because they occur in all molecules, whether they are polar or nonpolar. These forces arise due to temporary shifts in electron density that create an instantaneous and weak dipole, even in otherwise nonpolar molecules like methane (\(\mathrm{CH}_4\)).

These fleeting dipoles can induce similar dipoles in neighboring molecules. Despite being the weakest type of intermolecular force, London dispersion forces are significant in large molecules. This is because larger molecules have more electrons, leading to stronger temporary attractions. Chloromethane (\(\mathrm{CH}_3\mathrm{Cl}\)), while primarily depending on dipole-dipole interactions, also benefits from London dispersion forces due to its molecular size.

In the case of chloromethane, the combined effect of both dipole-dipole and London dispersion forces result in a much higher boiling point than that of methane, which relies solely on the weaker dispersion forces.

Molecular Weight

Molecular weight plays an essential role in determining the strength of London dispersion forces. Think of molecular weight as the mass of a single molecule, expressed in grams per mole. Chloromethane (\(\mathrm{CH}_3\mathrm{Cl}\)) has a higher molecular weight (50.49 g/mol) compared to methane (\(\mathrm{CH}_4\)) which has a molecular weight of 16.04 g/mol.

The reason larger molecules generally have stronger London dispersion forces is that they have more electrons. More electrons mean a higher probability of temporary dipole formations, which enhance dispersion forces. More substantial forces translate into a higher boiling point.

So, molecular weight is like adding extra weights to the tug of war between molecules, making it harder to separate them, thus requiring more energy to transition into the gas phase.

Polarity

Polarity is a key player in molecular interactions. It determines how molecules interact with each other and is the reason behind the difference in boiling points for compounds like chloromethane (\(\mathrm{CH}_3\mathrm{Cl}\)) and methane (\(\mathrm{CH}_4\)).

A molecule is polar if there is an uneven distribution of electrons across the molecule, resulting in a partial negative and partial positive end. This polarity creates a greater affinity between molecules. Chloromethane’s polarity is due to the difference in electronegativity between chlorine, carbon, and hydrogen atoms.

Polarity enhances the strength of dipole-dipole interactions. Therefore, chloromethane's polar nature is a major reason why it exhibits a higher boiling point compared to the nonpolar methane, which relies purely on weaker London dispersion forces. Polar molecules are like social butterflies that engage strongly with their neighbors, leading to higher energy requirements for physical state changes like boiling.