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Phenol acidity, oxidation to quinones, redox biology, antioxidants, and Kolbe-Schmitt synthesis
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Phenols (ArOH) bridge alcohols and aromatic chemistry. Phenoxide stabilization by resonance into the ring makes phenols ~10⁶× more acidic than alcohols (pKa ≈ 10).
Substituent effects on phenol acidity — EWGs at the ortho/para positions stabilize the conjugate base (picric acid pKa ≈ 0.4); EDGs raise the pKa.
Reactions
Quinones in biology — coenzyme Q, vitamin K, plastoquinone. Reversible 2e⁻/2H⁺ redox couple underlies the electron-transport chain.
Rank the following compounds in order of increasing pKa: phenol, p-methylphenol (p-cresol), p-nitrophenol, p-methoxyphenol, picric acid (2,4,6-trinitrophenol). Justify each ordering with a one-sentence resonance/inductive argument.
Increasing pKa (most acidic → least acidic): picric acid (~0.4) < p-nitrophenol (~7.2) < phenol (~10.0) < p-methylphenol (~10.3) < p-methoxyphenol (~10.2/10.3, ≈ p-cresol)
Avoid these 3 frequent errors
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mol of reacts with mol of . How many grams of water are produced? Which is the limiting reagent? ()
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Propose a synthesis of acetylsalicylic acid (aspirin) starting from phenol. Show the Kolbe-Schmitt step and the final acylation, and explain why bicarbonate (HCO₃⁻) is not used in place of CO₂ in step 1.
Step 1 — Kolbe-Schmitt carboxylation:
Step 2 — Acetylation of the phenolic OH:
Why CO₂ rather than HCO₃⁻? Bicarbonate is a weak electrophile and a poor source of CO₂ for delivery to a phenoxide ring. The Kolbe-Schmitt requires the neutral, electrophilic CO₂ molecule, coordinated by the sodium counterion at high pressure to enforce ortho selectivity. Bicarbonate would either (a) protonate the phenoxide back to phenol or (b) fail to electrophilically attack the ring.