Phenols
A phenol is a hydroxyl group (-OH) attached to an aromatic ring. That single structural change - moving the OH from an sp³ carbon (alcohol) to an sp² aromatic carbon (phenol) - completely transforms the reactivity. Phenols are roughly a million times more acidic than alcohols, and they participate in two-electron biological redox reactions that regular alcohols never see.
Why Phenols Are Much More Acidic Than Alcohols
Phenol has pKa 10. Ethanol has pKa 16. That six-unit gap means phenol is about a million times more acidic (factor of 10⁶ in Ka).
The reason: phenoxide (the conjugate base) has resonance stabilization that alkoxide lacks. The negative charge on phenoxide oxygen can delocalize into the aromatic ring, placing partial negative charge on the ortho and para positions. Four resonance structures total, each contributing to stabilization.
Ethoxide has no such option - the alkyl group has no pi system. The negative charge is localized on one oxygen.
More stable conjugate base = stronger acid. Phenol is much more acidic as a result.
Substituent Effects on Phenol Acidity
Electron-withdrawing groups on the ring stabilize the phenoxide anion (extra delocalization) and lower the pKa:
| Phenol | pKa |
|---|---|
| Phenol | 10.0 |
| 4-chlorophenol | 9.4 |
| 4-nitrophenol | 7.2 |
| 2,4-dinitrophenol | 4.1 |
| 2,4,6-trinitrophenol (picric acid) | 0.4 |
Nitro groups are especially strong electron-withdrawers because they can accept resonance electron density from the phenoxide - the negative charge delocalizes into the nitro oxygens, spreading the charge across SIX atoms total (O, four ring carbons, nitro O). Three nitro groups (picric acid) gives a phenol more acidic than carboxylic acids.
Electron-donating groups (methyl, methoxy) slightly decrease acidity by pushing electron density into the ring, destabilizing the anion.
Nomenclature of Phenols
Phenols are named with the OH group getting locant 1, and ring substituents getting the lowest locants. Common names are often used:
- Phenol = C₆H₅OH.
- o-Cresol, m-cresol, p-cresol = methyl-substituted phenols (2-, 3-, 4-methylphenol).
- Hydroquinone = 1,4-dihydroxybenzene (para-dihydroxybenzene). Aka 1,4-benzenediol.
- Catechol = 1,2-dihydroxybenzene.
- Resorcinol = 1,3-dihydroxybenzene.
These common names appear regularly on the MCAT - know them.
The Biological Redox Role: Hydroquinones and Ubiquinones
Phenols participate in 2-electron redox cycles that are essential in biological electron transport. The key interconversion:
Hydroquinone (reduced) ⇌ Quinone (oxidized) + 2H⁺ + 2e⁻
Hydroquinone has two hydroxyls on a benzene ring (1,4-). Oxidation removes two H atoms (one proton + one electron each), converting both OHs into C=O carbonyls and changing the benzene ring into a cyclohexadiene (1,4-benzoquinone).
This 2-electron, 2-proton redox cycle is the mechanism by which phenolic compounds serve as electron carriers in biology.
Ubiquinone (Coenzyme Q) in the ETC
Ubiquinone (CoQ, Coenzyme Q) is a lipid-soluble phenol derivative with a long hydrophobic isoprenoid tail (CoQ10 in humans). Its core is 1,4-benzoquinone; the tail anchors it in the inner mitochondrial membrane of the electron transport chain (ETC).
CoQ cycles between its quinone (oxidized) and hydroquinone (reduced, “ubiquinol”) forms as it shuttles electrons from Complex I and Complex II to Complex III:
- CoQ + 2e⁻ + 2H⁺ → CoQH₂ (reduced form).
- CoQH₂ → CoQ + 2e⁻ + 2H⁺ (delivered to Complex III).
This is one of the key two-electron redox steps in cellular respiration. The AAMC explicitly lists “Phenols: Oxidation and reduction (e.g., hydroquinones, ubiquinones): biological 2e- redox centers” in the organic chemistry content outline, so recognize this context on passages.
Vitamin E and K: Phenol-Based Antioxidants
Vitamin E (α-tocopherol) is a phenol with a phytol tail. Its phenol -OH donates a hydrogen atom to lipid peroxyl radicals, quenching radical chain reactions in cell membranes. The resulting tocopheroxyl radical is stabilized by resonance into the chroman ring, preventing it from continuing the radical chain.
Vitamin K has a naphthoquinone structure (two fused aromatic rings with a 1,4-quinone on one of them). It cycles between quinone and hydroquinone forms in the blood clotting cascade - specifically in the gamma-carboxylation of glutamate residues in clotting factors.
Both vitamins leverage the same phenol-to-quinone two-electron redox chemistry you just learned.
Phenol as a Disinfectant
Phenol (carbolic acid) was the first effective surgical antiseptic, introduced by Joseph Lister in the 1860s. Its acidity and ability to denature proteins make it bactericidal. Modern phenol derivatives (triclosan, thymol, eugenol) are used in toothpastes, mouthwashes, and household disinfectants.
Reactions of Phenols
Unlike alcohols, phenols:
- Do NOT undergo SN1 or SN2 - you cannot substitute the OH with a halide easily because the aromatic ring stabilizes the phenol OH in place. (The C-O bond of a phenol is actually more like a C=O-ish partial double bond due to resonance, making it much stronger.)
- DO undergo electrophilic aromatic substitution on the ring (not covered in detail on the MCAT - AAMC removed EAS from the content outline).
- Can be oxidized to quinones - the key redox transformation.
- Can be deprotonated by NaOH - because their pKa is 10, less than water’s 15.7.
Distinguishing Phenols from Alcohols
A simple lab test: add NaHCO₃ (sodium bicarbonate) and then NaOH.
- Phenol: NaHCO₃ does NOT dissolve it (phenol is not acidic enough - pKa 10, carbonic acid pKa 6.35). NaOH DOES dissolve it (forms soluble sodium phenoxide).
- Alcohol: neither dissolves it (pKa 16 is too high).
- Carboxylic acid: both dissolve it (pKa 4.8 is below carbonic acid pKa).
This acid-base extraction sequence separates phenols from alcohols from carboxylic acids - a classic lab technique discussed in Ch 12.