Question

Arrange these compounds in order of increasing boiling point. Explain your reasoning. a. \(\mathrm{CH}_{4}\) b. \(\mathrm{CH}_{3} \mathrm{CH}_{3}\) c. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\) d. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\)

Short answer

In order of increasing boiling point: \(\mathrm{CH}_{4} < \mathrm{CH}_{3} \mathrm{CH}_{3} < \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl} < \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\). The order is based on increasing strength of intermolecular forces present in each compound.

Step-by-step solution

Understand Boiling Points

Boiling points are influenced by intermolecular forces. The stronger the intermolecular forces, the higher the boiling point. There are three main types of intermolecular forces (from weakest to strongest): London dispersion forces, dipole-dipole interactions, and hydrogen bonding.

Analyze Molecular Structure

Identify the types of intermolecular forces each compound can exhibit based on its molecular structure. (a) \(\mathrm{CH}_{4}\) only has London dispersion forces as it is non-polar. (b) \(\mathrm{CH}_{3} \mathrm{CH}_{3}\) also only exhibits London dispersion but has a greater surface area than \(\mathrm{CH}_{4}\), thus stronger London forces. (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\) exhibits London dispersion and dipole-dipole interactions due to the presence of a polar C-Cl bond. (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) shows London dispersion, dipole-dipole interactions, and hydrogen bonding due to the -OH group.

Rank the Compounds

Rank the compounds in order of increasing boiling point based on the strength of their intermolecular forces. \(\mathrm{CH}_{4}\) has the weakest intermolecular forces, hence the lowest boiling point. Next is \(\mathrm{CH}_{3} \mathrm{CH}_{3}\) due to having larger London forces than \(\mathrm{CH}_{4}\), followed by \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\) with both London and dipole-dipole interactions. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) has the highest boiling point because it exhibits all three types of intermolecular forces, including hydrogen bonding which is the strongest among the listed forces.

Key concepts

Intermolecular Forces

When learning about boiling points in chemistry, one of the critical concepts to grasp is the role of intermolecular forces. Intermolecular forces are the attractions between molecules and they determine how tightly molecules stick together. Generally speaking, the stronger the intermolecular forces, the higher the energy required to separate the molecules, leading to a higher boiling point.

There are three main types of intermolecular forces you should be familiar with:

• London Dispersion Forces: Present in all molecules, they are the weakest type of force and arise due to temporary dipoles created by the movement of electrons.
• Dipole-Dipole Interactions: These occur between polar molecules with permanent dipoles, where the positive end of one molecule is attracted to the negative end of another.
• Hydrogen Bonding: A special type of dipole interaction that's particularly strong, occurring when a hydrogen atom is bonded to an electronegative atom, such as nitrogen, oxygen, or fluorine, and is attracted to another electronegative atom in a different molecule.

Role in Boiling Points

The influence of intermolecular forces on boiling points can be visualized like a team sport where the strength of the team (molecules) depends on the strength of the players’ handshakes (intermolecular forces). Weak handshakes correspond to a low boiling point, while strong handshakes lead to a higher boiling point. Thus, by analyzing the specific intermolecular forces at work within a molecule, we can predict its boiling point relative to others.

Molecular Structure

Molecular structure plays a significant role in defining the types and strengths of intermolecular forces a compound can exhibit. To put it simply, the way atoms are arranged in a molecule determines how they interact with neighboring molecules.

Let's consider molecular size and shape, for instance. Larger molecules with greater surface areas tend to have stronger London dispersion forces because there's more area for temporary dipoles to occur. This is why butane ((CH}_{3})_2CHCH_{3}) has a higher boiling point than methane (CH}_{4}), even though both are non-polar and only exhibit London dispersion forces.

On the other hand, molecules with polar bonds or asymmetrical shapes have permanent dipoles, leading to dipole-dipole interactions. For example, the polar C-Cl bond in chloromethane (CH}_{3}CH_{2}Cl}) makes it have a higher boiling point than both methane and butane. Understanding the 3D arrangement of molecules, like in VSEPR theory, is equally vital for predicting boiling points.

Hydrogen Bonding

As the superhero of intermolecular forces, hydrogen bonding deserves a spotlight for its significant influence on boiling points. It occurs when a hydrogen atom covalently bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine, is attracted to another electronegative atom in a neighboring molecule.

Take water (H}_{2}O}) for instance; each water molecule can form up to four hydrogen bonds, leading to its unexpectedly high boiling point. In the context of our original exercise, ethanol (CH}_{3}CH_{2}OH}) has the highest boiling point among the given compounds partly because of its ability to form hydrogen bonds through the -OH group.

Impact on Physical Properties

Hydrogen bonds greatly affect the physical properties of substances. They are responsible for the unique properties of water, including its high surface tension, specific heat capacity, and solid-state being less dense than its liquid state – that's why ice floats! In biomolecules like DNA, hydrogen bonding is crucial for the double helix structure's stability.