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. It has two hydroxyl groups bonded to a benzene ring in a para position. It is a white granular solid. Substituted derivatives of this parent compound are also referred to as hydroquinones. The name "hydroquinone" was coined by Friedrich Wöhler in 1843. In 2023, it was the 274th most commonly prescribed medication in the United States, with more than 800,000 prescriptions.
Production Hydroquinone is produced industrially in two main ways.
The most widely used route is similar to the cumene process in reaction mechanism and involves the dialkylation of benzene with propene to give 1,4-diisopropylbenzene. This compound reacts with air to afford the bis(hydroperoxide), which is structurally similar to cumene hydroperoxide and rearranges in acid to give acetone and hydroquinone. A second route involves hydroxylation of phenol over a catalyst. The conversion uses hydrogen peroxide and affords a mixture of hydroquinone and its ortho isomer catechol (benzene-1,2-diol): C6H5OH + H2O2 → C6H4(OH)2 + H2O Other, less common methods include:
The 1960s saw extensive research into the synthesis of hydroquinone from acetylene and carbon monoxide via catalytic iron pentacarbonyl. Rhodium or ruthenium can substitute for iron as the catalyst with favorable chemical yields, but are not typically used due to the cost of recovery from the reaction mixture. Hydroquinone and its derivatives can also be prepared by oxidation of various electron-rich benzene derivatives, such as phenols, aniline, and DIPB. Examples include Elbs persulfate oxidation and Dakin oxidation. Hydroquinone was first obtained in 1820 by the French chemists Pelletier and Caventou via the dry distillation of quinic acid. Hydrolysis of chlorophenol. The latter two methods are generally less atom-economical than oxidation with hydrogen peroxide, as are certain industrial implementations of the peroxide oxidation. Their commercial practice in China produced serious pollution in 2022.
Reactions The reactivity of hydroquinone's hydroxyl groups resembles that of other phenols, being weakly acidic. The resulting conjugate base easily undergoes O-alkylation to give mono- and diethers. Similarly, hydroquinone is highly susceptible to ring substitution via Friedel–Crafts alkylation. This reaction is often used for the production of several popular antioxidants, namely 2-tert-butyl-4-methoxyphenol (BHA). The useful dye quinizarin is produced by diacylation of hydroquinone with phthalic anhydride.
Redox Hydroquinone can be reversibly oxidised under mild conditions to give benzoquinone. Naturally occurring hydroquinone derivatives, such as coenzyme Q, exhibit similar reactivity, wherein one hydroxyl group is exchanged for an amino group. Given the conditional reversibility and relative ubiquity of reagents, oxidation reactions of hydroquinones and hydroquinone derivatives are of significant commercial use, often used at an industrial scale. When colorless hydroquinone and benzoquinone—bright yellow in solid form—are cocrystallized at a 1:1 ratio, a dark-green crystalline charge-transfer complex (melting point 171 °C), known as quinhydrone (C6H6O2·C6H4O2), is formed. This complex dissolves in hot water, dissociating both quinone molecules in solution.
Amination An important reaction involves the conversion of hydroquinone to its mono- and di-amine derivatives. One such derivative, methylaminophenol, used in photography, is produced according to the stoichiometry:
C6H4(OH)2 + CH3NH2 → HOC6H4NHCH3 + H2O Diamines—used in the rubber industry as antiozone agents—are aminated from aniline, and are formed via a similar pathway:
C6H4(OH)2 + 2 C6H5NH2 → C6H4(N(H)C6H5)2 + 2 H2O
Uses Hydroquinone has a variety of uses principally associated with its action as a reducing agent that is soluble in water. It is a major component in most black and white photographic developers for film and paper, where, with the compound metol, it reduces silver halides to elemental silver. There are various other uses associated with its reducing power. As a polymerisation inhibitor, exploiting its antioxidant properties, hydroquinone prevents polymerization of acrylic acid, methyl methacrylate, cyanoacrylate, and other monomers that are susceptible to radical-initiated polymerization. By acting as a free radical scavenger, hydroquinone serves to prolong the shelf life of light-sensitive resins such as preceramic polymers. Hydroquinone can lose a hydrogen cation from both hydroxyl groups to form a diphenolate ion. The disodium diphenolate salt of hydroquinone is used as an alternating comonomer unit in the production of the polymer PEEK.
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