Reactions of Phenols

Phenols oxidize to quinones — conjugated diketones — with strong oxidants like Na₂Cr₂O₇/H₂SO₄. 1,4-dihydroxybenzene (hydroquinone) → 1,4-benzoquinone (both OH → C=O).
Quinones are named by the numerical positions of their carbonyls + 'quinone' (e.g. 1,4-benzoquinone).
Extended conjugation makes quinones resonance-stabilized electrophiles, valuable in redox chemistry — the basis of their biological role.
Further oxidation of quinones gives hydroxyquinones (quinone core + extra OH groups), which shift polarity/redox potential/reactivity.
Ubiquinone (coenzyme Q) cycles oxidized (2 C=O) ↔ reduced ubiquinol (2 OH), shuttling electrons between Complexes I, II, III in the electron transport chain to drive ATP production; Vitamin K₁/K₂ are quinones needed for blood clotting.

Quinone: Conjugated diketone formed by oxidizing a phenol (both ring OH groups → C=O); resonance-stabilized electrophile important in redox chemistry.
Hydroxyquinone: A quinone with additional OH group(s) on the ring, from further oxidation; more polar, altered redox potential vs. the parent quinone.
Ubiquinone / Ubiquinol: Coenzyme Q; oxidized form (ubiquinone, 2 carbonyls) and reduced form (ubiquinol, 2 hydroxyls) shuttle electrons between Complexes I, II, and III in oxidative phosphorylation.

This oxidation to a quinone is specific to phenols (aromatic OH) — an ordinary aliphatic alcohol doesn't have an analogous reaction to a quinone.
Don't confuse the oxidized (ubiquinone, carbonyls) and reduced (ubiquinol, hydroxyls) forms of coenzyme Q — reduction adds electrons/hydrogens and converts C=O back to C-OH, same direction as any carbonyl reduction.

What product forms when hydroquinone (1,4-dihydroxybenzene) is oxidized, and what functional group change occurs?

Why are quinones considered resonance-stabilized electrophiles?

What is ubiquinone's role in the electron transport chain, and what is its reduced form called?

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