Ortho-Nitrophenol is less soluble in water than p- and m-Nitrophenols because :
Due to intramolecular H-bonding its H-bonding with H2O decreases.
Solubility became 69 less.
The question asks why ortho-nitrophenol is less soluble in water compared to meta- and para-nitrophenols. The key reason involves the type of hydrogen bonding present in these compounds.
Hydrogen bonding is a strong intermolecular force that occurs when a hydrogen atom is bonded to a highly electronegative atom (like O, N, F) and is attracted to another electronegative atom. In phenols, the -OH group can form hydrogen bonds.
In ortho-nitrophenol, the nitro group (-NO2) is adjacent to the hydroxyl group (-OH). The oxygen of the nitro group and the hydrogen of the hydroxyl group are close enough to form an intramolecular hydrogen bond within the same molecule. This is represented as:
This internal bonding reduces the ability of the -OH group to form hydrogen bonds with water molecules.
In meta- and para-nitrophenols, the nitro group is not adjacent to the -OH group. The distance is too great for intramolecular bonding. Instead, these molecules form intermolecular hydrogen bonds with other phenol molecules and, importantly, with water molecules. This enhances their solubility in water.
Since ortho-nitrophenol's -OH group is occupied in an intramolecular bond, it has fewer sites available to form hydrogen bonds with water. Meta- and para-nitrophenols can freely form intermolecular H-bonds with water, making them more soluble.
Ortho-nitrophenol is less soluble in water than meta- and para-nitrophenols because o-nitrophenol shows intramolecular H-bonding, which prevents its -OH group from forming effective hydrogen bonds with water molecules.
Hydrogen Bond Energy: Typically in the range of 4-40 kJ/mol, which is significant enough to affect physical properties like solubility and boiling point.
Solubility Principle: "Like dissolves like." Polar compounds dissolve in polar solvents like water via hydrogen bonding and dipole-dipole interactions.
Effect on Boiling Point: Compounds that form intermolecular hydrogen bonds have higher boiling points. Interestingly, o-nitrophenol has a lower boiling point than its isomers because it cannot form strong intermolecular polymers due to intramolecular H-bonding, making it more volatile. This is related to the third option, but it is a consequence of the same intramolecular bonding, not the direct cause of lower solubility.
Effect on Melting Point: Intramolecular bonding can also lead to lower melting points due to less efficient packing in the solid state, as mentioned in the fourth option. However, this is again an effect, not the primary cause for the difference in solubility.