Question

ASSERTION : P-nitrophenol is a stronger acid than o-nitrophenol.
REASON : Intramolecular hydrogen bonding makes o-isomer weaker than p-isomer.
A. both assertion and reason are correct and reason is the correct explanation for assertion.
B. both assertion and reason are correct and reason is not the correct explanation for assertion.
C. assertion is correct but reason is incorrect.
D. both assertion and reason are incorrect.

Answer 1

Hint:When hydrogen gets bonded between two electronegative atoms, the bonding is known as a hydrogen bond. When a molecule has two groups that are able to form hydrogen bonds, the bonding is known as intramolecular hydrogen bonding.

Complete answer
The electrostatic attraction between hydrogen and the most electronegative elements of the second row is known as hydrogen bonding. The second-row elements involved in hydrogen bonding are nitrogen, oxygen and fluorine.
The hydrogen bonding is of two types. Intramolecular and intermolecular. Intramolecular hydrogen bonding is found between the groups within the molecule. Intermolecular hydrogen bonding is found between two groups of two molecules.
The structure of p-nitrophenol and o-nitrophenol are as follows:

In o-nitrophenol, -OH and are present in close proximity, so hydrogen attached with oxygen in hydroxyl group forms an hydrogen bonding with nitrogen of . The hydrogen bonding in o-nitrophenol is shown as follows:

So, o-nitrophenol has intramolecular hydrogen bonding.
In p-nitrophenol, -OH and are very far from each other, so hydrogen attached with oxygen in the hydroxyl group cannot form intramolecular H-bonding.
Due to intramolecular hydrogen bonding, hydrogen is not easily daunted by o-nitrophenol whereas in p-nitrophenol the hydrogen free so, p-nitrophenol can donate proton easily hence p-nitrophenol is more acidic than o-nitrophenol. So, the assertion, p-nitrophenol is a stronger acid than o-nitrophenol, is true.
P-nitrophenol is a stronger acid than o-nitrophenol, due to intramolecular hydrogen bonding so, reason, Intramolecular hydrogen bonding makes o-isomer weaker than p-isomer is also true and explains the assertion.

Therefore, option (A) both assertion and reason are correct and reason is the correct explanation for assertion.

Note:in p-nitrophenol, intermolecular hydrogen bonding is present. In p-nitrophenol, -OH and ${\text{N}}{{\text{O}}_{\text{2}}}$ are very far from each other, so, hydrogen attached with oxygen in hydroxyl group form and hydrogen bonding with nitrogen of ${\text{N}}{{\text{O}}_{\text{2}}}$of another p-nitrophenol molecule. The hydrogen bonding in p-nitrophenol is shown as follows:

Intramolecular hydrogen bonding is more stable than intermolecular hydrogen bonding. On adding a non-polar solvent the intermolecular hydrogen bonding breaks. The addition of non-polar solvent does not affect intramolecular hydrogen bonding. For hydrogen bonding, hydrogen should be attached with an electronegative atom.