Question

For the given compounds, the correct order of increasing \(pK_a\) value :

• A. \((E)<(D)<(C)<(B)<(A)\)
• B. \((D)<(E)<(C)<(B)<(A)\)
• C. \((E)<(D)<(B)<(A)<(C)\)
• D. \((B)<(D)<(A)<(C)<(E)\)
• E. None of these

Answer 1

Answer: (E)

To determine the order of increasing \(pK_a\) values for the given compounds, we need to evaluate their acidic strength. The \(pK_a\) value is inversely related to acidity; the lower the \(pK_a\), the stronger the acid.

Let's analyze the compounds one by one considering their substituents:

1. Compound (A): Phenol. The hydroxyl group is attached to the benzene ring without any electron-withdrawing or electron-donating groups.
2. Compound (B): 2-nitrophenol. The nitro group is a strong electron-withdrawing group, making the compound more acidic than phenol.
3. Compound (C): 2-hydroxyanisole. The methoxy group is an electron-donating group, reducing acidity compared to phenol.
4. Compound (D): 4-nitrophenol. The nitro group increases the acidity significantly compared to phenol due to electron-withdrawing effects.
5. Compound (E): 2-methoxyphenol. The methoxy group donates electrons, reducing acidity compared to phenol.

Based on the substituents, the relative acidic strength is:

• Most acidic: 4-nitrophenol (D) - Strong electron-withdrawing group at para position.
• 2-nitrophenol (B) - Strong electron-withdrawing group at ortho position.
• Phenol (A) - Baseline acidity.
• 2-methoxyphenol (E) - Electron-donating group reducing acidity.
• Least acidic: 2-hydroxyanisole (C) - Electron-donating group reducing acidity further.

Thus, the correct order of increasing \(pK_a\) values is:

\((C) < (E) < (A) < (B) < (D)\)

The provided options do not match this order, confirming the correct answer is "None of these."

Answer 2

Explanation:

1. Definition of \( pK_a \): Lower \( pK_a \) corresponds to a stronger acid.
2. Factors affecting \( pK_a \):
   • Electron-withdrawing groups (EWG) increase acidity by stabilizing the conjugate base.
   • Electron-donating groups (EDG) decrease acidity by destabilizing the conjugate base.
3. Order Analysis:
   • (D) Dinitrophenol: Strongest acid due to two nitro (EWG) groups stabilizing the phenoxide ion.
   • (B) Nitrophenol: Less acidic than (D), but still acidic due to one nitro group.
   • (A) Phenol: Less acidic than (B) due to the absence of strong EWGs.
   • (C) Methoxyphenol: The methoxy group (EDG) destabilizes the phenoxide ion, making it less acidic than phenol.
   • (E) Methoxy-dinitrophenol: The methoxy group reduces the effect of the nitro groups, making it less acidic than dinitrophenol but more acidic than phenol.
4. Final Order: \[ (D) < (B) < (A) < (C) < (E). \]

Final Answer: None of these.