What Is Hydroquinone?
Hydroquinone, a divalent phenol, is also known as p-dihydroxybenzene or quinol. It can be synthesized by reducing benzoquinone with sulfurous acid.
Most hydroquinone is produced through the diisopropylbenzene process, utilizing benzene and propylene. It may cause allergic contact dermatitis as a side effect.
Uses of Hydroquinone
Hydroquinone serves as an intermediate raw material in the manufacture of photographic developers, dyes, and pharmaceuticals, and as an antioxidant for rubber and electronic materials. It is also employed as an analytical reagent for determining phosphorus, arsenic, and silicic acid.
As an antioxidant, hydroquinone shows excellent inhibitory effects on the polymerization of monomers like acrylonitrile, acrylates, and styrene when used in very small amounts.
In cosmetics, hydroquinone is used as a hair colorant, skin bleach, antioxidant, and fragrance component. However, its use in concentrations above 2% is restricted due to suspected carcinogenic properties.
Properties of Hydroquinone
At normal pressure, hydroquinone has a melting point of 172°C, a boiling point of 287°C, and a density of 1.3 g/cm3. It is soluble in water, ethanol, and ether, but not in cold benzene.
Being highly reductive, hydroquinone can reduce alkaline silver salt solutions and Fehling’s solution, and it discolors upon gradual oxidation in air.
Hydroquinone is a colorless or white crystal with the chemical formula C6H6O2, a molar mass of 110.11 g/mol, and the specific formula C6H4(OH)2.
Other Information on Hydroquinone
1. Synthesis of Hydroquinone
Hydroquinone can be produced by oxidizing phenol, using hydrogen peroxide with beta-zeolite (H-BEA) as a catalyst and diethyl ketone as an auxiliary catalyst. This method also yields catechol, but selectivity can be improved by ion-exchanging H-BEA with alkaline earth metals.
Another production method is the Elbs persulfate oxidation, utilizing potassium persulfate to oxidize phenol to hydroquinone.
2. Oxidation of Hydroquinone
Easily oxidized to p-benzoquinone, hydroquinone’s reductive properties have given it its name through the reduction process of p-benzoquinone, also known as 1,4-benzoquinone, a single six-membered carbon ring quinone with the molecular formula C6H4O2.
3. Regioisomers of Hydroquinone
Hydroquinone has two regioisomers, catechol and resorcinol. Catechol, with hydroxy groups in the ortho position on the benzene ring, is also known as pyrocatechol. Resorcinol has hydroxy groups in the meta position and is used in tire cord adhesives, UV absorbers for resins, and flame retardants.