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Hydroquinone

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Hydroquinone
Hydroquinone
General
Systematic name Hydroquinone
Other names Quinol
Benzene-1,4-diol
Molecular formula C6H4(OH)2
SMILES Oc1ccc(O)cc1
Molar mass 110.1 g/mol
Appearance white solid
CAS number [123-31-9]
Properties
Density and phase 1.3 g/cm3, solid
Solubility in water 5.9 g/100 ml (15 °C)
Melting point 172 °C
Boiling point 287 °C
Acidity (pKa) ?
Dipole moment zero
Hazards
MSDS External MSDS
EU classification Harmful (Xn)
Carc. Cat. 3
Muta. Cat. 3
Dangerous for
the environment (N)
NFPA 704
NFPA 704
safety square
NFPA 704 four-colored diamondHealth 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformFlammability 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilInstability (yellow): no hazard codeSpecial hazards (white): no code
2
1
R-phrases Template:R22, Template:R40, Template:R41, Template:R43, Template:R50, Template:R68
S-phrases Template:S2, Template:S26, Template:S36/37/39, Template:S61
Flash point 165 °C
RTECS number MX3500000
Supplementary data page
Structure and
properties 		n, εr, etc.
Thermodynamic
data 		Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
Related compounds
Related benzenediols Pyrocatechol
Resorcinol
Related compounds 1,4-benzoquinone
Except where noted otherwise, data are given for
materials in their standard state (at 25 °C, 100 kPa)
Infobox disclaimer and references

Hydroquinone, also benzene-1,4-diol, is an aromatic organic compound which is a type of phenol, having the chemical formula C6H4(OH)2. Its chemical structure, shown in the table at right, has two hydroxyl groups bonded to a benzene ring in a para position. It is a white granular solid at room temperature and pressure.

Nomenclature

Hydroquinone is the name recommended by the International Union of Pure and Applied Chemistry (IUPAC) in its 1993 Recommendations for the Nomenclature of Organic Chemistry.[1]

Properties

Hydroquinone can undergo mild oxidation to convert to the compound parabenzoquinone, C6H4O2, often called p-quinone or simply quinone. Reduction of quinone reverses this reaction back to hydroquinone. Some biochemical compounds in nature have this sort of hydroquinone or quinone section in their structures, such as Coenzyme Q, and can undergo similar redox interconversions.

The hydroxyl groups of hydroquinone are quite weakly acidic. Hydroquinone can lose an H+ from one of the hydroxyls to form a monophenolate ion or lose an H+ from both to form a diphenolate ion.

Uses

Hydroquinone has a variety of uses principally associated with its action as a reducing agent which is soluble in water. It is a major component in most photographic developers where, with the compound Metol, it reduces silver halides to elemental silver.

In human medicine, hydroquinone is used as a topical application in skin whitening to reduce the color of skin as it does not have the same predisposition to cause dermatitis as Metol does. This use is banned in some countries (e.g. France) because of fears of a cancer risk. Hydroquinone is also used for anal bleaching, a cosmetic practice used especially by pornstars.

The disodium diphenolate salt of hydroquinone is used as an alternating comonomer unit in the production of the polymer PEEK.

As a polymerization inhibitor, hydroquinone prevents polymerization of acrylic acid, methyl methacrylate, etc.

It is also used as a raw material of herbicides, rubber antioxidants and dye stuffs.

Natural occurrences

Hydroquinones are one of the two primary reagents in the defensive glands of bombardier beetles, along with hydrogen peroxide (and perhaps other chemicals, depending on the species), which collect in a reservoir. The reservoir opens through a muscle-controlled valve onto a thick-walled reaction chamber. This chamber is lined with cells that secrete catalases and peroxidases. 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.

See also

Reference

  1. ^ Panico, R.; & Powell, W. H. (Eds.) (1994). A Guide to IUPAC Nomenclature of Organic Compounds 1993. Oxford: Blackwell Science. ISBN 0-632-03488-2.{{cite book}}: CS1 maint: multiple names: authors list (link)

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