Hydroquinone, also known as benzene-1,4-diol or quinol, is an aromatic organic compound that is a type of phenol, a derivative of benzene, having the chemical formula C6H4(OH)2.

By acting as a free radical scavenger, hydroquinone serves to prolong the shelflife of light-sensitive resins such as preceramic polymers.

[23][24] In 2006, the United States Food and Drug Administration revoked its previous approval of hydroquinone and proposed a ban on all over-the-counter preparations.

[25] The FDA officially banned hydroquinone in 2020 as part of a larger reform of the over-the-counter drug review process.

[25][29][27] NTP evaluation showed some evidence of long-term carcinogenic and genotoxic effects[30] While hydroquinone remains widely prescribed for treatment of hyperpigmentation, questions raised about its safety profile by regulatory agencies in the EU, Japan, and USA encourage the search for other agents with comparable efficacy.

One of these, 4-butylresorcinol, has been proved to be more effective at treating melanin-related skin disorders by a wide margin, as well as safe enough to be made available over the counter.

[35] In the anthraquinone process substituted hydroquinones, typically anthrahydroquinone are used to produce hydrogen peroxide which forms spontaneously on reaction with oxygen.

Hydroquinones are one of the two primary reagents in the defensive glands of bombardier beetles, along with hydrogen peroxide (and perhaps other compounds, depending on the species), which collect in a reservoir.

When the contents of the reservoir are forced into the reaction chamber, the catalases and peroxidases rapidly break down the hydrogen peroxide and catalyze the oxidation of the hydroquinones into p-quinones.

These reactions release free oxygen and generate enough heat to bring the mixture to the boiling point and vaporize about a fifth of it, producing a hot spray from the beetle's abdomen.