In Chapter 23, we learned about the electrophilic aromatic nitration reaction. When phenol is subjected to these conditions with a large excess of nitric acid, a molecule called picric acid is produced. Predict the product of this reaction and explain why the pK_a value of this compound is 0.38.

Verified step by step guidance

1. Step 1: Recognize that phenol (C₆H₅OH) undergoes electrophilic aromatic nitration when treated with concentrated nitric acid (HNO₃) and sulfuric acid (H₂SO₄). The hydroxyl (-OH) group on phenol is an activating group, making the aromatic ring highly reactive towards electrophilic substitution reactions.
2. Step 2: Understand the mechanism of nitration. The reaction involves the generation of the nitronium ion (NO₂⁺) as the electrophile, which is formed by the interaction of HNO₃ and H₂SO₄. The nitronium ion attacks the aromatic ring, leading to substitution at positions that are ortho and para to the hydroxyl group due to its electron-donating nature.
3. Step 3: Note that under the reaction conditions (large excess of nitric acid and elevated temperature), multiple nitration occurs. Phenol reacts to form 2,4,6-trinitrophenol, commonly known as picric acid. The hydroxyl group directs the nitro groups to the ortho and para positions relative to itself.
4. Step 4: Explain the pKa value of picric acid (0.38). The presence of three electron-withdrawing nitro groups (-NO₂) significantly increases the acidity of the hydroxyl group. These nitro groups stabilize the negative charge on the phenoxide ion formed after deprotonation, making picric acid a very strong acid compared to phenol.
5. Step 5: Summarize the reaction: Phenol reacts with concentrated HNO₃ and H₂SO₄ at 100°C to produce picric acid (2,4,6-trinitrophenol). The low pKa value is due to the strong electron-withdrawing effects of the nitro groups, which enhance the acidity of the compound.

Key Concepts

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a fundamental reaction in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. In the case of nitration, the electrophile is the nitronium ion (NO₂⁺), which attacks the electron-rich aromatic system. Understanding EAS is crucial for predicting the products of reactions involving aromatic compounds, such as the formation of picric acid from phenol.

Acidity and pKₐ

The pKₐ value is a measure of the acidity of a compound, indicating how easily it donates a proton (H⁺) in solution. A lower pKₐ value signifies a stronger acid. Picric acid, with a pKₐ of 0.38, is highly acidic due to the presence of multiple electron-withdrawing nitro groups, which stabilize the negative charge on the conjugate base formed after deprotonation, making it more likely to release a proton.

Resonance Stabilization

Resonance stabilization refers to the delocalization of electrons across multiple structures, which can enhance the stability of a molecule. In picric acid, the negative charge on the conjugate base can be delocalized over the nitro groups, leading to increased stability. This stabilization contributes to the compound's strong acidity, as the more stable the conjugate base, the more favorable the deprotonation process.