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.