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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 ~10โถร more acidic than alcohols (pKa โ 10).
Substituent effects on phenol acidity โ EWGs at the ortho/para positions stabilize the conjugate base (picric acid pKa โ 0.4); EDGs raise the pKa.
Reactions
Quinones in biology โ coenzyme Q, vitamin K, plastoquinone. Reversible 2eโป/2Hโบ 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 โ least acidic): picric acid (~0.4) < p-nitrophenol (~7.2) < phenol (~10.0) < p-methylphenol (~10.3) < p-methoxyphenol (~10.2/10.3, โ 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 (HCOโโป) is not used in place of COโ in step 1.
Step 1 โ Kolbe-Schmitt carboxylation:
Step 2 โ Acetylation of the phenolic OH:
Why COโ rather than HCOโโป? Bicarbonate is a weak electrophile and a poor source of COโ for delivery to a phenoxide ring. The Kolbe-Schmitt requires the neutral, electrophilic COโ 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.