Question

When an atom or group of atoms is substituted for an atom in benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)\), the boiling pointchanges. Explair 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 (Benzene < Chlorobenzene < Bromobenzene < Phenol) can be explained by a combination of factors such as molecular weight, intermolecular forces (London dispersion forces, dipole-dipole interactions, and hydrogen bonding), and the molecular polarity of the compounds. Phenol has the highest boiling point due to the presence of hydrogen bonding, while benzene, chlorobenzene, and bromobenzene have increasing boiling points due to the influence of molecular weight and weak dipole-dipole interactions.

Step-by-step solution

Identify the molecules involved

The compounds mentioned in the exercise are: 1. Benzene: \(\mathrm{C}_{6} \mathrm{H}_{6}\) with boiling point: \(80^{\circ} \mathrm{C}\) 2. Chlorobenzene: \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}\) with boiling point: \(132{ }^{\circ} \mathrm{C}\) 3. Bromobenzene: \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}\) with boiling point: \(156^{\circ} \mathrm{C}\) 4. Phenol: \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\) with boiling point: \(182{ }^{\circ} \mathrm{C}\)

Compare molecular weights

First, let's check if the molecular weights affect the boiling points. The more significant the molecular weight, the stronger the London dispersion forces, leading to a higher boiling point. - Benzene: 78.11 g/mol - Chlorobenzene: 112.56 g/mol - Bromobenzene: 157.01 g/mol - Phenol: 94.11 g/mol We can see that molecular weights follow the order: Benzene < Phenol < Chlorobenzene < Bromobenzene while boiling points follow the order: Benzene < Chlorobenzene < Bromobenzene < Phenol. It suggests that molecular weight is not the only factor influencing the boiling points of these compounds.

Compare the intermolecular forces

Benzene, chlorobenzene and bromobenzene are non-polar molecules, while phenol is a polar molecule. For non-polar molecules, the primary intermolecular forces are London dispersion forces (weakest), whereas polar molecules exhibit dipole-dipole interactions in addition to London dispersion forces. The stronger the intermolecular forces, the higher the boiling point. - Benzene: London dispersion forces - Chlorobenzene: London dispersion forces and weak dipole-dipole interactions due to electronegativity difference in Cl and H - Bromobenzene: London dispersion forces and weak dipole-dipole interactions due to electronegativity difference in Br and H - Phenol: London dispersion forces, dipole-dipole interactions, and hydrogen bonding due to the presence of OH group

Explaining the boiling point order

London dispersion forces increase with increasing molecular weight, explaining why the non-polar chlorobenzene and bromobenzene have higher boiling points than benzene. Since Cl is more electronegative than Br, chlorobenzene is slightly more polar than bromobenzene and therefore has stronger dipole-dipole interactions; however, this effect is not strong enough to overcome the contribution from increased London dispersion forces due to higher molecular weight in bromobenzene. Phenol has the strongest intermolecular forces because the presence of an OH group allows it to form hydrogen bonds, which are significantly stronger than dipole-dipole interactions and London dispersion forces. This is why phenol has the highest boiling point among the given compounds. In conclusion, the order of boiling points: Benzene < Chlorobenzene < Bromobenzene < Phenol, can be explained by a combination of factors such as molecular weight, intermolecular forces (London dispersion forces, dipole-dipole interactions, and hydrogen bonding), and the molecular polarity of the compounds.

Key concepts

Intermolecular forces

Intermolecular forces are the attractions occurring between molecules, greatly influencing their physical properties like boiling and melting points. Each type of intermolecular force varies in strength and forms the basis to understand why some molecules evaporate or boil at lower temperatures than others.

Three main types of intermolecular forces dominate:

• London dispersion forces: These are the weakest and occur in all molecules, whether polar or non-polar. The larger the molecule, the stronger the London dispersion forces, which cause an increase in boiling points.
• Dipole-dipole interactions: These occur between polar molecules due to their permanent dipoles. They are stronger than London dispersion forces and typically lead to higher boiling points.
• Hydrogen bonds: These are a much stronger type of dipole-dipole interaction occurring specifically in molecules where hydrogen is bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine. Hydrogen bonding significantly elevates boiling points compared to molecules lacking these bonds.

To understand the boiling point variations in compounds like chlorobenzene and phenol, it’s crucial to assess how these forces interact within each molecule.

Molecular weight

Molecular weight plays a significant role in determining a compound's boiling point. In general, an increase in molecular weight corresponds to a higher boiling point. This is primarily because larger, heavier molecules have more electrons, leading to stronger London dispersion forces.

By comparing the weights:

• Benzene has a molecular weight of 78.11 g/mol.
• Chlorobenzene has a weight of 112.56 g/mol.
• Bromobenzene weighs 157.01 g/mol.
• Phenol is at 94.11 g/mol.

You’ll notice that while there's a general correlation between weight and boiling point, it isn't always linear due to the effect of other factors. For instance, bromobenzene’s higher molecular weight than chlorobenzene explains its higher boiling point. However, phenol's boiling point is higher than chlorobenzene, despite its lower molecular weight, indicating how intermolecular forces contribute more dominantly to boiling point variation in this case.

Molecular polarity

Molecular polarity is dictated by the differences in electronegativity between atoms within a molecule, resulting in the molecule having a net dipole moment. This polarity affects how molecules interact with each other and aligns with the strength of intermolecular forces.

• Non-polar molecules like benzene rely on London dispersion forces for interactions.
• Chlorobenzene and bromobenzene are slightly polar due to the electronegative Cl and Br atoms, contributing to weak dipole-dipole interactions.
• Phenol, with a noticeably polar O-H bond, engages in hydrogen bonding, which is significantly stronger than dipole-dipole interactions.

The polarity hence ties back to the intermolecular forces, where more polar molecules like phenol exhibit higher boiling points due to the ability to form stronger interactions such as hydrogen bonds. Understanding these connections between polarity and boiling points provides deeper insight into predicting how substitutions in a benzene ring influence its thermal behavior.