User:ThunderFighs/Quinone

Reduction
Quinones are strong oxidizing agents that undergo reversible reduction reactions. The redox behavior of quinones is sensitive to the pH of a buffered aqueous media. In acidic conditions, quinones undergo a reversible single-step, two-electron and two-proton reduction. This results in the formation of the quinone's corresponding aromatic compound. An example of an acidic reduction reaction is the formation of a hydroquinone from the reduction of 1,4-quinone:

In alkaline conditions, quinones undergo a reversible single-step, two-electron reduction. In neutral conditions, quinones may undergo either a one-proton, two-electron reduction or a two-electron reduction.

In aprotic media, quinones undergo two-step reduction without protons. In the first step, a short-lived semiquinone intermediate is formed. In the second step, the semiquinone is reduced into a quinone dianion.

Addition
Quinones undergo addition reaction to form 1,4-addition products. The 1,4-addition breaks the conjugation of the quinone. An example of 1,4-addition reaction is the addition of hydrogen chloride to form chlorohydroquinone:

Quinones can undergo Diels-Alder reactions. The quinone acts as the dienophile and reacts with a diene at a carbon-carbon double bond.

Total Synthesis
Quinones are used as dienophiles in total synthesis using Diels-Alder reactions and have been historically important in early research in totally synthesizing molecules such as cholesterol, cortisone, morphine, and reserpine.

Quinhydrone Electrode
Quinones may form a quinhydrone species via hydrogen bonding between ρ-quinone and ρ-hydroquinone. An equimolar mix of ρ-quinones and ρ-hydroquinone and their contact with an inert metallic electrode, such as antimony, forms a quinhydrone electrode which can be used to measure the pH of solutions. Quinhydrone electrodes provide fast response times and high accuracy. However, it can only measure pH in the range of 1 to 9 and the solution must not contain a strong oxidizing or reducing agent.

Photography
A commercial application of quinones is in black-and-white photography. In this process, an emulsion containing silver bromide or silver iodide is placed on black-and-white film and activated with light. Hydroquinone is used to reduce the activated silver ions to metallic silver and is oxidized to quinone. Finally, all silver halide not activated by light and reduced by hydroquinone is removed, leaving a negative by deposited silver where the film had been struck by light.