18.25:
Oxidation of Phenols to Quinones
Phenols, like 1° and 2° alcohols, undergo oxidation, even though they do not contain α hydrogens. The electron-donating –OH group promotes easy oxidation of phenols to quinones.
1,2- and 1,4-benzenediols oxidize to o– and p-quinones, respectively. However, the quinones can be readily reduced to benzenediols using mild reducing agents.
Since phenols do not have ɑ hydrogens, their oxidation pathway is slightly different and involves the loss of two electrons and two protons.
The reaction starts with a hydroquinone molecule losing an electron to generate the radical cation. Subsequent deprotonation by water produces a semiquinone radical that is stabilized by resonance.
The loss of another electron generates the protonated quinone, which is finally quenched by water to produce p-quinone.
The redox property of quinones makes them suitable for catalyzing physiological functions, like cellular respiration.
18.25:
Oxidation of Phenols to Quinones
In the presence of oxidizing agents, phenols are oxidized to quinones. Quinones can be easily reduced back to phenols using mild reducing agents. The electron-donating hydroxyl group enhances the reactivity of the aromatic ring, enabling oxidation of the ring even in the absence of an α hydrogen.
o-hydroxy phenols are oxidized to o-quinones and p-hydroxy phenols to p-quinones. Such redox reactions involve the transfer of two electrons and two protons. The reversible redox property is crucial in several physiological systems to catalyze biological reactions. For example, cellular respiration utilizes ubiquinones or coenzyme Q as electron mediators for reducing molecular oxygen to water. The most important functional group in vitamin K2, which is involved in blood clotting, is a quinone. Quinones are also present in menadione, which is a synthetic supplement for vitamin K2.