o-Nitrophenoal is less soluble in water than p-nitrophenol because :

o-Nitrophenol shows intramolecular H-bonding

The correct answer is option 2. o-Nitrophenol shows intramolecular H-bonding.

The solubility of a compound in water is influenced by its ability to interact with water molecules, primarily through hydrogen bonding. Let us delve into why o-nitrophenol is less soluble in water compared to p-nitrophenol:

Hydrogen Bonding and Solubility

Intramolecular vs. Intermolecular Hydrogen Bonding

Intramolecular Hydrogen Bonding in o-Nitrophenol:

In o-nitrophenol, the hydroxyl group (-OH) and the nitro group (-NO₂) are positioned ortho (adjacent) to each other on the benzene ring.

This close proximity allows the hydrogen of the -OH group to form a hydrogen bond with the oxygen of the -NO₂ group within the same molecule. This is called intramolecular hydrogen bonding.

As a result of this intramolecular bonding, the -OH group in o-nitrophenol is less available to form hydrogen bonds with water molecules. The molecule is somewhat "self-stabilized" due to these intramolecular interactions, reducing its ability to interact with and dissolve in water.

Intermolecular Hydrogen Bonding in p-Nitrophenol:

In p-nitrophenol, the -OH and -NO₂ groups are para (opposite) to each other on the benzene ring. This arrangement does not favor intramolecular hydrogen bonding.

Consequently, p-nitrophenol molecules are more free to engage in intermolecular hydrogen bonding with water molecules.

The -OH group can form hydrogen bonds with water, enhancing the molecule’s solubility in water.

Solubility Implications

o-Nitrophenol:

Due to intramolecular hydrogen bonding, the molecule is less able to interact with water molecules. This self-stabilization results in decreased solubility.

The strong intramolecular bonds create a less polar structure overall, reducing the molecule’s ability to be surrounded and solvated by water.

p-Nitrophenol:

Without significant intramolecular hydrogen bonding, p-nitrophenol can more readily form hydrogen bonds with water. This makes it more soluble in water.

The molecule’s ability to engage in intermolecular interactions with water outweighs any intramolecular bonding effects.

Addressing the Other Options

(1) Steam Volatility:

Steam volatility relates to how readily a substance evaporates. While volatility might affect physical state changes, it does not directly explain solubility in water.

(3) Intermolecular Hydrogen Bonding in o-Nitrophenol:

Although o-nitrophenol can form some intermolecular hydrogen bonds, the presence of intramolecular hydrogen bonding significantly limits these interactions, thereby affecting solubility more profoundly.

(4) Melting Point:

Melting point is related to the solid state of a substance and does not provide a direct explanation for solubility in water.

In summary, the lower solubility of o-nitrophenol in water compared to p-nitrophenol is primarily due to intramolecular hydrogen bonding, which reduces the molecule’s ability to form hydrogen bonds with water and thus decreases its solubility.