1,4-Benzoquinone

1,4-Benzoquinone
IUPAC name Cyclohexa-2,5-diene-1,4-dione
Other names p-benzoquinone; p-quinone
Identifiers
CAS number	106-51-4 Y
ChemSpider	4489 Y
UNII	3T006GV98U Y
KEGG	C00472 Y
ChEBI	CHEBI:16509 Y
ChEMBL	CHEMBL8320 Y
RTECS number	DK2625000
Jmol-3D images	Image 1 Image 2
SMILES O=C\1\C=C/C(=O)/C=C/1 C1=CC(=O)C=CC1=O
InChI InChI=1S/C6H4O2/c7-5-1-2-6(8)4-3-5/h1-4H Y Key: AZQWKYJCGOJGHM-UHFFFAOYSA-N Y InChI=1/C6H4O2/c7-5-1-2-6(8)4-3-5/h1-4H Key: AZQWKYJCGOJGHM-UHFFFAOYAR
Properties
Molecular formula	C6H4O2
Molar mass	108.09 g mol−1
Appearance	Yellow solid
Density	1.318 g/cm3 at 20 °C
Melting point	115 °C, 388 K, 239 °F
Boiling point	Sublimes
Solubility in water	Slightly soluble
Solubility	Slightly soluble in petroleum ether; soluble in acetone; very soluble in ethanol, benzene, diethyl ether
Hazards
R-phrases	R23/25 R36/37/38 R50
S-phrases	S26 S28 S45 S61
Main hazards	Toxic
Related compounds
Related compounds	1,2-Benzoquinone
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

1,4-Benzoquinone, commonly known as para-quinone, is a chemical compound with the formula C₆H₄O₂. In a pure state, it forms bright-yellow crystals with a characteristic irritating odor, resembling that of chlorine, bleach, and hot plastic. Impure samples are often dark-colored due to the presence of quinhydrone (1:1 complex of quinone with hydroquinone). This six-membered ring compound is the oxidized derivative of 1,4-hydroquinone. The molecule is multifunctional: it exhibits properties of a ketone, forming an oxime; an oxidant, forming the dihydroxy derivative; and an alkene, undergoing addition reactions, especially those typical for α,β-unsaturated ketones. 1,4-Benzoquinone is sensitive toward both strong mineral acids and alkali, which cause condensation and decomposition of the compound.

Preparation

1,4-Benzoquinone can be prepared from hydroquinone via a number of oxidation methods. One such method makes use of hydrogen peroxide as the oxidizer and iodine or an iodine salt as a catalyst for the oxidation occurring in a polar solvent, e.g., isopropyl alcohol.

A few grams of para-benzoquinone, prepared by the action of iodine and hydrogen peroxide on hydroquinone, being sublimated in a 3l beaker.

Benzoquinone compounds are a metabolite of paracetamol and, as such, it can also be prepared by the oxidation of paracetamol with nitric acid.

A 200mm watch glass covered in freshly sublimated para-benzoquinone.

When heated to its melting point, the product sublimates at atmospheric pressure, and, when prepared from hydroquinone, the substrate boils at a significantly higher temperature than 1,4-benzoquinones melting point, allowing for an effective separation of the two. However, iodine also sublimates within this region and so must be fully removed by filtration prior to sublimation if it has been used as a catalyst during production.

Needles of freshly sublimated para-benzoquinone.

Because the compound degrades to quinhydrone over time, ideally it is prepared immediately before use, but it can also be stored in a freezer and sublimated prior to use at a later date.

It was first commercially prepared in 1919.

Applications in organic synthesis

It is used as a hydrogen acceptor and oxidant in organic synthesis. 1,4-Benzoquinone serves as a dehydrogenation reagent. It is also used as a dienophile in Diels Alder reactions.

Benzoquinone reacts with acetic anhydride and sulfuric acid to the triacetate of hydroxyquinol. This reaction is called the Thiele reaction after Johannes Thiele, who first described the reaction in 1898. An application is found in total synthesis:

Benzoquinone is also used to suppress double-bond migration during olefin metathesis reactions.

An acidic potassium iodide solution reduces a solution of benzoquinone to hydroquinone, which is oxidized back with a solution of silver nitrate.

Due to its ability to function as an oxidizer, 1,4-benzoquinone can be found in methods using the Wacker-Tsuji oxidation, wherein a palladium salt functions to catalytically oxidize an alkene to its corresponding ketone. This reaction is typically carried out using pressurized oxygen as the oxidizer, and can be done instead with methyl nitrite in solution, but benzoquinone can sometimes be preferential, as it does not require the handling of gases, making it easy to measure out by mass.

Metabolism

1,4-Benzoquinone is a toxic metabolite found in human blood and can be used to track exposure to benzene or mixtures containing benzene and benzene compounds, such as petrol. The compound can interfere with cellular respiration, and kidney damage has been found in animals receiving severe exposure. It is excreted in its original form and also as variations of its own metabolite, hydroquinone.

Safety

1,4-Benzoquinone is able to stain skin dark brown, cause erythema (redness, rashes on skin) and lead on to localized tissue necrosis. It is particularly irritating to the eyes and respiratory system. Its ability to sublimate at commonly encountered temperatures allows for a greater airborne exposure risk than might be expected for a room-temperature solid. IARC has found insufficient evidence to comment on the compound's carcinogenicity, but has noted that it can easily pass into the bloodstream and that it showed activity in depressing bone marrow production in mice and can inhibit protease enzymes involved in cellular apoptosis.

Related 1,4-benzoquinones

A variety of derivatives and analogues are known. Illustrative examples:

• 1,4-Naphthoquinone, derived by oxidation of naphthalene with chromium trioxide.
• 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), a stronger oxidant and dehydrogenation agent than 1,4-benzoquinone.
• Ubiquinone-1, a naturally occurring 1,4-benzoquinone.
• Chloro-p-benzoquinone, (CAS no. [695-99-8])
• Chloranil, 1,4-C₆Cl₄O₂, a stronger oxidant and dehydrogenation agent than 1,4-benzoquinone.

See also

• Tetrahydroxybenzoquinone
• Benzoquinonetetracarboxylic acid
• 1,2-Benzoquinone
• Quinones
• Duroquinone