quinone

Meaning: A quinone is a cyclohexadienedione, a six-membered ring with two carbonyl groups and two C=C bonds. The usual example is 1,4-benzoquinone (para); 1,2 (ortho) quinones also exist. The ring is conjugated but not aromatic, so it readily accepts electrons to regain an aromatic ring. Reduction gives a hydroquinone (benzene-1,4-diol), and oxidizing the hydroquinone reverses the change. Adding only one electron gives a semiquinone radical.

Equation: Q + 2H⁺ + 2e⁻ ⇌ QH₂. Q is the quinone (oxidized form), QH₂ is the hydroquinone (reduced form; ubiquinol for coenzyme Q), H⁺ is a proton and e⁻ is an electron. Each is counted in mol, the SI unit of amount, with 2 mol of H⁺ and 2 mol of e⁻ per 1 mol of Q.

On the MCAT: In biochemistry passages, ubiquinone (CoQ) is a mobile, lipid-soluble carrier that takes electrons from Complex I and Complex II and passes them to Complex III. Questions ask which form is oxidized or reduced, or what happens downstream when a step is blocked. In organic questions, you may need to recognize a quinone from its structure, or label hydroquinone → quinone as an oxidation (loss of H, gain of C=O).

Common confusion: The quinone is the oxidized form, not the reduced form. The hydroquinone is the aromatic diol, and the quinone ring is not aromatic. Do not call a quinone a phenol: it has C=O groups, not O–H groups.