What is the correct order of acidity for the following compounds?
Show Hint
Picric acid (\(\text{P}\)) is a unique phenol containing three nitro groups, making it extremely acidic (stronger than benzoic acid and many mineral acids).
Remembering that Picric acid is stronger than Benzoic acid (\(\text{P} > \text{N}\)) immediately allows you to narrow down to the correct option (A).
Step 1: Understanding the Question:
The question asks us to arrange four organic compounds—Phenol (M), Benzoic acid (N), 2,4,6-trinitrophenol (P), and 4-nitrophenol (Q)—in decreasing order of their acid strength. Step 2: Key Formula or Approach:
The acidity of a compound depends on the stability of its conjugate base (anion) formed after losing a proton (\(\text{H}^+\)):
1. A more stable conjugate base corresponds to a stronger acid.
2. Carboxylic acids are generally more acidic than phenols because the negative charge in the carboxylate ion is delocalized over two highly electronegative oxygen atoms.
3. However, the presence of strong electron-withdrawing groups (like \(-\text{NO}_2\)) on a phenol ring via resonance (\(-M\) effect) and inductive (\(-I\) effect) stabilizes the phenoxide ion enormously, which can make substituted phenols even stronger than carboxylic acids. Step 3: Detailed Explanation:
Let's analyze the stability of the conjugate bases for each compound:
- M (Phenol): The phenoxide ion is stabilized by resonance, but the negative charge is delocalized onto less electronegative carbon atoms of the benzene ring. It is a weak acid with a \(pK_a \approx 10\).
- N (Benzoic acid): The carboxylate ion (\(\text{C}_6\text{H}_5\text{COO}^-\)) is highly stable due to equivalent resonance structures where the negative charge is shared between two electronegative oxygen atoms. It is much more acidic than phenol, with a \(pK_a \approx 4.2\).
- P (2,4,6-trinitrophenol, or Picric acid): This compound has three highly electron-withdrawing nitro groups (\(-\text{NO}_2\)) at the ortho and para positions relative to the hydroxyl group.
- These groups exert very powerful \(-M\) and \(-I\) effects, stabilizing the negative charge on the phenoxide oxygen to an extreme degree.
- Consequently, picric acid is exceptionally acidic, with a \(pK_a \approx 0.38\). This is much more acidic than benzoic acid.
- Q (4-nitrophenol): The presence of one electron-withdrawing \(-\text{NO}_2\) group at the para position stabilizes the phenoxide ion via resonance, making it significantly more acidic than phenol (\(pK_a \approx 7.15\)). However, with only one nitro group, it remains less acidic than benzoic acid.
Comparing the \(pK_a\) values:
Picric acid (P, \(pK_a \approx 0.38\)) $>$ Benzoic acid (N, \(pK_a \approx 4.2\)) $>$ 4-nitrophenol (Q, \(pK_a \approx 7.15\)) $>$ Phenol (M, \(pK_a \approx 10\)).
Therefore, the decreasing order of acidity is: \(\text{P} > \text{N} > \text{Q} > \text{M}\). Step 4: Final Answer:
The correct order of acidity is \(\text{P} > \text{N} > \text{Q} > \text{M}\), which corresponds to option (A).
