Question

When an atom or group of atoms is substituted for an \(\mathrm{H}\) atom in benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right),\) the boiling point changes. Explain the order of the following boiling points: \(\mathrm{C}_{6} \mathrm{H}_{6}\left(80{ }^{\circ} \mathrm{C}\right), \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}\) \(\left(132^{\circ} \mathrm{C}\right), \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}\left(156^{\circ} \mathrm{C}\right), \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\left(182^{\circ} \mathrm{C}\right)\)

Short answer

The order of boiling points can be explained by considering the molecular weight of each compound and the intermolecular forces present. Benzene has the lowest boiling point due to only having dispersion forces. Chlorobenzene has a higher boiling point due to dispersion forces and dipole-dipole interactions. Bromobenzene has a higher boiling point than chlorobenzene because of stronger dispersion forces and dipole-dipole interactions. Phenol has the highest boiling point due to having all three types of intermolecular forces: dispersion forces, dipole-dipole interactions, and hydrogen bonding. Thus, the order of boiling points is \(C_6H_6 < C_6H_5Cl < C_6H_5Br < C_6H_5OH\).

Step-by-step solution

Identify the compounds and their boiling points

First, let's write down the compounds and their boiling points: 1. Benzene (C6H6): 80°C 2. Chlorobenzene (C6H5Cl): 132°C 3. Bromobenzene (C6H5Br): 156°C 4. Phenol (C6H5OH): 182°C

Understand how molecular weight affects boiling point

Generally, larger molecules with higher molecular weights have higher boiling points. This is because larger molecules have greater surface areas, leading to stronger van der Waals forces. In this case, the molecular weights of Cl, Br, and OH groups are more significant than that of the hydrogen atom (H) they replaced. Therefore, we expect the boiling points of the substituted benzene to be higher than that of the original benzene.

Analyze intermolecular forces

In addition to molecular weights, let's consider the intermolecular forces in each compound: 1. Benzene - only dispersion forces (weakest intermolecular force) 2. Chlorobenzene - dispersion forces and dipole-dipole interactions (due to the electronegative Cl atom) 3. Bromobenzene - dispersion forces and dipole-dipole interactions (due to the electronegative Br atom) - note that Br is more massive than Cl, so we expect the boiling point to be higher than that of chlorobenzene. 4. Phenol - dispersion forces, dipole-dipole interactions, and hydrogen bonding (due to the polar OH group) - hydrogen bonding is the strongest intermolecular force among these, so its boiling point will be higher than the other compounds.

Summarize the explanation

The order of the boiling points can be explained as follows: 1. The original benzene has the lowest boiling point (80°C) because it only has the weakest intermolecular force - dispersion forces. 2. Chlorobenzene has a higher boiling point (132°C) due to the presence of dispersion forces and dipole-dipole interactions. 3. Bromobenzene has higher boiling points than chlorobenzene (156°C) due to stronger dispersion forces and dipole-dipole interactions resulting from the larger Br atom. 4. Phenol has the highest boiling point (182°C) because it has all three types of intermolecular forces: dispersion forces, dipole-dipole interactions, and hydrogen bonding. Thus, the increasing order of boiling points is C6H6 < C6H5Cl < C6H5Br < C6H5OH.

Key concepts

Intermolecular Forces

Intermolecular forces are the attractive forces that occur between neighboring molecules. They play a crucial role in defining the physical properties of substances, such as boiling points. Boiling points are directly linked to the strength of these forces; stronger intermolecular forces require more energy (heat) to overcome before the substance can transition from liquid to gas.

It is important to understand that there are different types of intermolecular forces, each contributing to different extents. The weakest of these forces are dispersion forces, while the strongest are hydrogen bonds. A molecule can exhibit multiple types of intermolecular forces, such as in the case of phenol, which has both dipole-dipole interactions and hydrogen bonding. These different forces combined explain the variations in boiling points among similar compounds.

Molecular Weight

Molecular weight is another factor that influences the boiling point of a compound. Larger molecules typically have higher molecular weights, which usually translates to higher boiling points. This occurs because larger molecules have a greater surface area over which dispersion forces can act, leading to stronger cumulative intermolecular forces.

In the context of benzene and its derivatives, replacing hydrogen with heavier atoms like chlorine or bromine increases the molecular weight. The increased molecular weight from substituents like Cl and Br results in stronger dispersion forces when compared to benzene alone, thereby raising the boiling point of these substituted compounds.

• For instance, bromobenzene has a higher boiling point than chlorobenzene because bromine is heavier than chlorine.

Hydrogen Bonding

Hydrogen bonding is one of the strongest types of intermolecular forces. This occurs when a hydrogen atom is covalently bonded to a highly electronegative atom, such as oxygen, nitrogen, or fluorine, and interacts with another electronegative atom in a nearby molecule.

In phenol, the presence of an -OH group allows for hydrogen bonding. These bonds require substantial energy to break due to their strength, leading to a higher boiling point for phenol compared to other benzene derivatives that cannot form such bonds. The presence of hydrogen bonds is a key reason why phenol has the highest boiling point among the compounds discussed.

Dipole-Dipole Interactions

Dipole-dipole interactions occur when molecules with permanent dipoles (positive and negative ends) attract each other. These interactions are generally stronger than dispersion forces but weaker than hydrogen bonds.

In chlorobenzene and bromobenzene, the chlorine and bromine atoms create a partial permanent dipole because they are more electronegative than hydrogen, thereby attracting adjacent molecules. This increases the boiling point when compared to benzene, which only has weaker dispersion forces.

• Chlorobenzene and bromobenzene both have dipole-dipole interactions, but the greater mass of bromine compared to chlorine gives bromobenzene a higher boiling point.

Dispersion Forces

Dispersion forces, also known as London dispersion forces, are the weakest form of intermolecular forces. They arise due to the temporary fluctuations in electron density in an atom or molecule, resulting in temporary dipoles.

All molecules experience dispersion forces regardless of their polarity. However, in non-polar molecules such as benzene, these are the only type of intermolecular force present. As a result, benzene has a much lower boiling point than its heavier and more polar derivatives like chlorobenzene, bromobenzene, and phenol.

Despite their weak nature, dispersion forces become increasingly significant in larger molecules, contributing to higher boiling points as seen in larger benzene derivatives like bromobenzene.