Question

The melting point of 2 -nitrophenol is lower than that of 4 -nitrophenol and this is because of (1) Intramolecularhydrogen bondingin 2-nitrophenol (2) Intramolecular hydrogen bonding in 4 -nitrophenol (3) Intermolecular hydrogen bonding in 2 -nitrophenol (4) Hydrogen bonding has no relation to the melting point,

Short answer

The melting point of 2-nitrophenol is lower than 4-nitrophenol due to intramolecular hydrogen bonding in 2-nitrophenol.

Step-by-step solution

- Identify the Molecular Structure

2-nitrophenol has the nitro group (-NO2) attached to the second carbon of the benzene ring, while 4-nitrophenol has the nitro group attached to the fourth carbon.

- Determine Types of Hydrogen Bonding

2-nitrophenol forms intramolecular hydrogen bonds, which occur within the same molecule. 4-nitrophenol forms intermolecular hydrogen bonds, which occur between different molecules.

- Compare the Strength of Hydrogen Bonds

Intramolecular hydrogen bonds in 2-nitrophenol are generally weaker and less extensive compared to the intermolecular hydrogen bonds in 4-nitrophenol. Consequently, molecules with intermolecular hydrogen bonds often have higher melting points due to the stronger interactions that need to be broken.

- Conclude the Reason for Lower Melting Point

Since 2-nitrophenol engages in intramolecular hydrogen bonding which is weaker, it has a lower melting point compared to 4-nitrophenol, which engages in stronger intermolecular hydrogen bonding.

Key concepts

Intramolecular Hydrogen Bonding

Intramolecular hydrogen bonding occurs within the same molecule. It happens when a hydrogen atom is covalently bonded to a very electronegative atom (like oxygen or nitrogen) and interacts with another electronegative atom within the same molecule.
This type of bonding often leads to a more compact molecular structure. It's like the molecule 'folds in' on itself.
For example, in 2-nitrophenol, the hydroxyl group (-OH) forms a hydrogen bond with the nitro group (-NO2) located on the same molecule's second carbon.
Because this interaction happens within a single molecule, it doesn't significantly influence the overall structural rigidity.
Therefore, it results in a weaker bond that is easier to break, contributing to a lower melting point.

Intermolecular Hydrogen Bonding

Intermolecular hydrogen bonding occurs between different molecules. Here, a hydrogen atom on one molecule will interact with an electronegative atom on a different molecule, leading to stronger and more extensive bonding.
This type of bonding is crucial in determining the physical properties of substances, such as boiling and melting points.
In the case of 4-nitrophenol, the hydrogen atom from the hydroxyl group can form a bond with the oxygen or nitrogen atoms in the nitro group of another molecule.
This creates a network of strong bonds between molecules, increasing stability and significantly raising the melting point of the substance.
So, it's the network of interactions between multiple molecules that make intermolecular hydrogen bonds much stronger and contribute to higher melting points.

Molecular Structure

The molecular structure of a compound significantly impacts its physical properties. 2-nitrophenol and 4-nitrophenol are isomers, meaning they have the same molecular formula but different structures.
In 2-nitrophenol, the nitro group is attached to the second carbon, while in 4-nitrophenol, it is attached to the fourth carbon.
This difference changes how atoms within the molecule interact with each other and with other molecules.
A more compact structure due to intramolecular hydrogen bonding in 2-nitrophenol leads to weaker overall interactions, influencing the ease of breaking these interactions.
On the other hand, 4-nitrophenol’s structure promotes stronger intermolecular hydrogen bonds. This makes it more difficult to disrupt these interactions, leading to a higher melting point.

Nitrophenol Isomers

Nitrophenol isomers, such as 2-nitrophenol and 4-nitrophenol, have significant differences in their physical properties despite having the same chemical formula. This is due to their structural differences.
The position of the nitro group relative to the hydroxyl group within the benzene ring governs the type of hydrogen bonding.
In 2-nitrophenol, the close proximity of the hydroxyl group and nitro group allows for intramolecular hydrogen bonding.
This internal bonding lowers the melting point as the molecules don't interact as strongly with each other.
4-nitrophenol, however, promotes intermolecular hydrogen bonding because the groups are positioned farther apart, encouraging interactions between molecules.
As a result, substances with intermolecular hydrogen bonds like 4-nitrophenol are more stable and have higher melting points compared to their isomers exhibiting intramolecular hydrogen bonds.