11.6 Phenols & Biological Redox Centers

Phenols: Structure and Acidity

A phenol has a hydroxyl group bonded directly to an aromatic ring carbon, distinguishing it structurally from an alcohol, where the hydroxyl sits on an sp3 carbon. The parent compound is simply named phenol; substituted derivatives are named as substituted phenols or, when the ring bears other priority groups, as hydroxy-substituted parent compounds.

Phenol's most exam-relevant property is its acidity: phenol has a pKa near 10, dramatically more acidic than a typical alcohol (pKa 16-18) despite both having an O-H bonded to a six-membered ring. Two effects combine to explain the gap. First, induction: the ring carbon bonded to oxygen is sp2-hybridized and holds its electrons closer to the nucleus than an sp3 carbon does, weakly pulling electron density away from the O-H and stabilizing the conjugate base. Second, and more importantly, resonance: deprotonating phenol gives a phenoxide ion whose negative charge is not confined to oxygen — it delocalizes through the ring pi system onto the ortho and para carbons, spreading the charge over four positions instead of one. Cyclohexanol's alkoxide has no comparable delocalization pathway, so it remains a far weaker acid.

Substituents on the ring tune phenol's acidity in the same direction predicted by their electronic character. Electron-withdrawing groups positioned ortho or para to the hydroxyl — nitro, halogens, carbonyls — further stabilize the phenoxide by both induction and additional resonance delocalization onto the substituent itself, and their effect is additive: 2,4,6-trinitrophenol (picric acid) has three nitro groups positioned to stabilize the negative charge and is acidic enough (pKa near 0.4) to rival a strong mineral acid. Electron-donating groups, such as alkyl or amino substituents, do the opposite, destabilizing the phenoxide and raising the pKa. This is the identical substituent logic used for carboxylic acid acidity trends (Section 11.4) — the position and electronic character of a substituent, not its identity alone, determines whether it helps or hurts conjugate-base stability.

Phenol's intermediate acidity also makes it a classic tool in acid-base extraction (Chapter 9): a phenol is acidic enough to be deprotonated by aqueous sodium hydroxide, forming a water-soluble phenoxide salt that partitions into the aqueous layer, but it is not acidic enough to react with the weaker base sodium bicarbonate, which deprotonates only carboxylic acids (pKa about 4-5). This gap lets a mixture containing a carboxylic acid, a phenol, and a neutral compound be separated cleanly by sequential extraction — bicarbonate pulls only the carboxylic acid into the aqueous layer first, and hydroxide is then needed to pull out the phenol, leaving the neutral compound behind in the organic layer.

Because phenol's O-H bond is comparatively weak and the resulting phenoxy radical is resonance-stabilized in the same way the phenoxide anion is, phenols readily donate a hydrogen atom to quench a free radical — the basis of their use as biological and industrial antioxidants. Vitamin E (alpha-tocopherol) is a phenol that protects cell membrane lipids from radical-chain oxidative damage by exactly this mechanism.

Oxidation and Reduction of Phenols: Hydroquinones and Quinones

A hydroquinone is a 1,4-dihydroxybenzene: two phenolic OH groups on opposite ring carbons. Oxidizing hydroquinone removes two hydrogen atoms — two electrons and two protons together — and converts the aromatic ring into 1,4-benzoquinone, a non-aromatic six-membered ring bearing two carbonyl groups conjugated with two remaining C=C double bonds. This hydroquinone/quinone interconversion is fully reversible: quinone plus two electrons and two protons regenerates hydroquinone. This same redox couple has practical applications outside biology: photographic film developers historically relied on hydroquinone as a mild reducing agent that selectively converts exposed silver halide crystals to metallic silver, and hydroquinone appears in some dermatological formulations, where it inhibits the tyrosinase-catalyzed oxidation step of melanin synthesis.

What makes this couple biologically important is that the two-electron transfer does not have to happen in one concerted step. It can proceed through an intermediate one-electron semiquinone radical — quinone picks up one electron and one proton to become the semiquinone, then a second electron and proton complete the reduction to hydroquinone, or the sequence runs in reverse for oxidation. Because that radical intermediate is reasonably stable, delocalized by resonance across the ring, the quinone/hydroquinone system can accept or donate electrons either two at a time or one at a time. This flexibility is exactly why biology relies on quinone-type molecules as two-electron redox centers that interface between two-electron carriers, like the NADH/NAD+ hydride couple, and one-electron carriers, like the iron centers in cytochromes.

Ubiquinone (Coenzyme Q) in the Electron Transport Chain

Ubiquinone, also called coenzyme Q, is a substituted benzoquinone bearing a long hydrophobic isoprenoid tail. That tail has no direct role in the redox chemistry itself, but it keeps the molecule dissolved and laterally mobile within the lipid bilayer of the inner mitochondrial membrane rather than fixed to a single protein complex.

Ubiquinone sits at a genuine convergence point in the electron transport chain: it accepts electron pairs from Complex I (NADH dehydrogenase, oxidizing NADH) and separately from Complex II (succinate dehydrogenase, whose bound FAD cofactor is reduced during the citric acid cycle step that oxidizes succinate), reducing itself to ubiquinol (QH2, the hydroquinone form) through the semiquinone intermediate described above. Because ubiquinone is small and lipid-soluble, it then diffuses through the membrane to Complex III, where it is reoxidized — and where the same semiquinone chemistry lets its stored two-electron pair be handed off as two separate one-electron transfers to the iron-containing cytochromes and the Rieske iron-sulfur protein downstream. This electron transfer at Complex III is directly coupled to proton translocation across the inner mitochondrial membrane, contributing to the proton-motive force that ATP synthase later uses to generate ATP — the oxidative phosphorylation machinery covered in more depth later in this guide. For now, the exam-relevant takeaway is structural: ubiquinone's quinone/hydroquinone redox chemistry, running through a semiquinone radical, is precisely what lets it bridge two-electron and one-electron electron-transport chemistry.

Phenols and biological redox centers — key points:

• Phenols are more acidic than aliphatic alcohols because the phenoxide ion is resonance-stabilized
• Electron-withdrawing ring substituents increase phenol acidity; electron-donating groups decrease it
• Quinones accept 2e⁻/2H⁺ to form hydroquinones; the half-reduced form is a semiquinone radical
• Ubiquinone (coenzyme Q) shuttles electrons between Complexes I/II and III in the ETC
• Catecholamines and other phenolic metabolites can participate in metal chelation and redox cycling