Question

Account for the following: (a) \(\mathrm{H}_{2} \mathrm{O}\) has abnormally high boiling point. (b) O-nitrophenol is steam volatile.

Short answer

Water has a high boiling point due to the presence of strong intermolecular hydrogen bonds. O-nitrophenol is steam volatile because the nitro group interferes with the formation of intermolecular hydrogen bonds, making it easier for the molecules to enter the gas phase.

Step-by-step solution

Explain Water’s Boiling Point

\(\mathrm{H}_{2} \mathrm{O}\) or water has an unusually high boiling point due to the presence of hydrogen bonding. This is a type of dipole-dipole interaction that occurs when a hydrogen atom bonded to a highly electronegative atom (like oxygen) is attracted to another electronegative atom in another molecule.

Describe the Structure of Water

In the case of water, each water molecule can form up to four hydrogen bonds; two from the hydrogen atoms to electronegative oxygen atoms in other water molecules, and two from the lone pairs of the oxygen atom to the hydrogen atoms in other water molecules. This unique structure of water molecules and the strong hydrogen bonds between them result in a higher boiling point.

Discuss O-nitrophenol’s Volatility

O-nitrophenol is a type of phenol that has a nitro group (-NO2) attached to its molecule. The presence of the nitro group reduces the ability for phenolic OH groups to form intermolecular hydrogen bonds with each other. Therefore, molecules of o-nitrophenol can escape from the liquid to the gas phase more easily, making this compound volatile.

Explain the Steam Volatility

Steam volatility refers to the ability of a substance to be carried with steam, even though the substance might have a higher boiling point than the boiling point of water. In case of O-nitrophenol, although intramolecular hydrogen bonding is present between the phenolic OH and the nitro -NO2 group, the intermolecular H-bonds are relatively less, which makes O-nitrophenol steam volatile.

Key concepts

Boiling Point of Water

Water is a fascinating compound when it comes to its boiling point. It has a boiling point of 100°C, which is significantly higher than many other similar molecules. The reason behind this lies in its unique capability to form hydrogen bonds.

Hydrogen bonding is a special type of dipole-dipole interaction. Here, the hydrogen atom is covalently bonded to oxygen, a highly electronegative element. This hydrogen atom is then attracted to the oxygen atom of a neighboring water molecule. Thus, each water molecule can make up to four hydrogen bonds. This happens because:

• Two hydrogens form bonds with the oxygen of adjacent molecules.
• The lone pairs on oxygen form bonds with the hydrogens of neighboring molecules.

These bonds need a considerable amount of energy to break, which is why water has a high boiling point. The interconnected network of water molecules means they stick together strongly, resistant to being pulled apart into a gaseous state.

Volatility of O-nitrophenol

Understanding why o-nitrophenol is steam volatile requires a look into its structural features. O-nitrophenol is a type of phenol that includes a nitro group (-NO2) attached to its aromatic ring. This specific arrangement impacts how the molecule interacts with others in the liquid phase.

In the case of o-nitrophenol, the nitro group reduces the molecule's tendency to form intermolecular hydrogen bonds. While the hydroxyl group (OH) is typically good at forming such bonds, the presence of the nitro group inhibits their overall formation between separate o-nitrophenol molecules.

As a result, these molecules are more likely to escape the liquid as they are not tightly bound together by these typical forces. This characteristic makes o-nitrophenol volatile in steam, allowing it to be carried away readily in the vapor phase even if it has a relatively high boiling point.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between neighboring molecules. These forces are fundamental in determining the physical properties of substances, such as boiling points, melting points, and volatility.

There are several types of intermolecular forces, including:

• Hydrogen bonds: Particularly strong among molecules where a hydrogen atom is bonded to an electronegative atom like oxygen or nitrogen.
• Van der Waals forces: Weak interactions which include London dispersion forces and dipole-dipole interactions.
• Dipole-dipole interactions: Occur between molecules that have permanent dipoles.

The strength and type of intermolecular forces present in a substance directly influence its physical characteristics. When it comes to water, hydrogen bonding is the key intermolecular force that leads to its unusually high boiling point.

For o-nitrophenol, the nature of these forces changes due to the presence of the nitro group, altering its hydrogen bonding capabilities and making it more volatile. This illustrates how varying intermolecular forces can lead to significant differences in behavior and properties of chemical compounds.