Question

Which of the following has more pK\(_a\) value?

• A. 2-Nitrophenol
• B. 2-Nitro-phenol
• C. 3-Nitrophenol
• D. 4-Nitrophenol

Hint:

In aromatic compounds, the position of electron-withdrawing groups (such as NO\(_2\)) plays a significant role in determining the acidity (pK\(_a\) value) of the compound.

Answer 1

The Correct Option is B

The pK\(_a\) value refers to the acidity of a substance; the lower the pK\(_a\), the stronger the acid. In general, the electron-withdrawing groups such as nitro groups reduce the electron density around the hydroxyl group in phenols, making them more acidic. However, the position of the nitro group matters. The electron-withdrawing nitro group at the ortho position has a lesser effect on the hydroxy group’s acidity than the meta and para positions. Therefore, 2-nitrophenol (ortho position) has the highest pK\(_a\) value among the options. Thus, the correct answer is option (2) 2-Nitrophenol.

Answer 2

Step 1: Understanding pK_a Value
The pK_a value is a measure of the acidity of a compound. It is the negative logarithm of the acid dissociation constant (K_a). A higher pK_a value means a weaker acid, while a lower pK_a indicates a stronger acid. Thus, comparing pK_a values allows us to compare the relative acidity of different compounds.

Step 2: Analyzing Nitro-phenol Isomers
There are different positional isomers of nitrophenol, such as ortho (2-nitro), meta (3-nitro), and para (4-nitro). These positions affect the electronic interactions and hydrogen bonding within the molecule, thereby influencing acidity.

Step 3: Intramolecular vs Intermolecular Hydrogen Bonding
In 2-nitrophenol (ortho isomer), the nitro group and hydroxyl group are adjacent. This allows for intramolecular hydrogen bonding, which stabilizes the molecule but does not stabilize the phenoxide ion formed after deprotonation. As a result, 2-nitrophenol is less acidic compared to its para counterpart and has a relatively higher pK_a value.

In contrast, 4-nitrophenol can form intermolecular hydrogen bonds and the nitro group at the para position exerts a strong electron-withdrawing effect through resonance and inductive effects, stabilizing the phenoxide ion. Hence, 4-nitrophenol is more acidic and has a lower pK_a.

Step 4: Conclusion
Among the nitrophenol isomers, 2-nitrophenol has a higher pK_a value due to intramolecular hydrogen bonding, which makes the release of H⁺ (proton) less favorable. Therefore, 2-nitrophenol is less acidic than other isomers like 4-nitrophenol.