For the given compounds, the correct order of increasing \(pK_a\) value :
\((E)<(D)<(C)<(B)<(A)\)
\((D)<(E)<(C)<(B)<(A)\)
\((E)<(D)<(B)<(A)<(C)\)
\((B)<(D)<(A)<(C)<(E)\)
- None of these
The Correct Option is
Approach Solution - 1
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:
- Compound (A): Phenol. The hydroxyl group is attached to the benzene ring without any electron-withdrawing or electron-donating groups.
- Compound (B): 2-nitrophenol. The nitro group is a strong electron-withdrawing group, making the compound more acidic than phenol.
- Compound (C): 2-hydroxyanisole. The methoxy group is an electron-donating group, reducing acidity compared to phenol.
- Compound (D): 4-nitrophenol. The nitro group increases the acidity significantly compared to phenol due to electron-withdrawing effects.
- 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."
Approach Solution -2
Explanation:
- Definition of \( pK_a \): Lower \( pK_a \) corresponds to a stronger acid.
- 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.
- 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.
- Final Order: \[ (D) < (B) < (A) < (C) < (E). \]
Final Answer: None of these.
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