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: \(\mathrm{C}_{6} \mathrm{H}_{6}\left(80{ }^{\circ} \mathrm{C}\right), \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}\) \(\left(132^{\circ} \mathrm{C}\right), \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}\left(156^{\circ} \mathrm{C}\right), \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\left(182^{\circ} \mathrm{C}\right)\)

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

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.

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.

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.