1,4-Benzoquinone

1,4-Benzoquinone

Skeletal formula Skeletal formula Space-filling model Space-filling model
File:Para-Benzoquinone needles.jpg
Names
Preferred IUPAC name Cyclohexa-2,5-diene-1,4-dione
Other names 1,4-Benzoquinone Benzoquinone p-Benzoquinone p-Quinone
Identifiers
CAS Number	106-51-4 checkY
3D model (JSmol)	Interactive image Interactive image
Beilstein Reference	773967
ChEBI	CHEBI:16509 checkY
ChEMBL	ChEMBL8320 checkY
ChemSpider	4489 checkY
ECHA InfoCard
EC Number	203-405-2
E number
Gmelin Reference	2741
IUPHAR/BPS	6307
KEGG	C00472 checkY
PubChem CID	4650
RTECS number	DK2625000
UNII	3T006GV98U checkY
UN number	2587
CompTox Dashboard (EPA)	{{#property:P3117}}
InChI InChI=1S/C6H4O2/c7-5-1-2-6(8)4-3-5/h1-4H checkY Key: AZQWKYJCGOJGHM-UHFFFAOYSA-N checkY InChI=1/C6H4O2/c7-5-1-2-6(8)4-3-5/h1-4H Key: AZQWKYJCGOJGHM-UHFFFAOYAR
SMILES O=C\1\C=C/C(=O)/C=C/1 C1=CC(=O)C=CC1=O
Properties
Chemical formula	C6H4O2
Molar mass	108.096 g·mol−1
Appearance	Yellow solid
Odor	Acrid, chlorine-like
Density	1.318 g/cm3 at 20 °C
Melting point	115 °C (239 °F; 388 K)
Boiling point	Sublimes
Solubility in water	11 g/L (18 °C)
Solubility	Slightly soluble in petroleum ether; soluble in acetone; 10% in ethanol, benzene, diethyl ether
Vapor pressure	0.1 mmHg (25 °C)
Magnetic susceptibility (χ)	−38.4·10−6 cm3/mol
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Toxic
GHS labelling:
Pictograms	GHS06: ToxicGHS07: Exclamation markGHS09: Environmental hazard
Signal word	Danger
Hazard statements	H301, H315, H319, H331, H335, H400
Precautionary statements	P261, P264, P270, P271, P273, P280, P301+P310, P302+P352, P304+P340, P305+P351+P338, P311, P312, P321, P330, P332+P313, P337+P313, P362, P391, P403+P233, P405, P501
Flash point	38 to 93 °C; 100 to 200 °F; 311 to 366 K
Lethal dose or concentration (LD, LC):
LD50 (median dose)	296 mg/kg (mammal, subcutaneous) 93.8 mg/kg (mouse, subcutaneous) 8.5 mg/kg (mouse, IP) 5.6 mg/kg (rat) 130 mg/kg (rat, oral) 25 mg/kg (rat, IV)
NIOSH (US health exposure limits):
PEL (Permissible)	TWA 0.4 mg/m3 (0.1 ppm)
REL (Recommended)	TWA 0.4 mg/m3 (0.1 ppm)
IDLH (Immediate danger)	100 mg/m3
Related compounds
Related compounds	1,2-Benzoquinone
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). ☒N verify (what is checkY☒N ?) 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 or formaldehyde. This six-membered ring compound is the oxidized derivative of 1,4-hydroquinone. The molecule is multifunctional: it exhibits properties of a ketone, being able to form oximes; 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 is prepared industrially by oxidation of hydroquinone, which can be obtained by several routes. One route involves oxidation of diisopropylbenzene and the Hock rearrangement. The net reaction can be represented as follows:

$\mathrm{C}{\phantom{A}}_6\mathrm{H}{\phantom{A}}_4(\mathrm{C}\mathrm{H}\mathrm{M}\mathrm{e}{\phantom{A}}_2){\phantom{A}}_2+3\mathrm{O}{\phantom{A}}_2⟶\mathrm{C}{\phantom{A}}_6\mathrm{H}{\phantom{A}}_4\mathrm{O}{\phantom{A}}_2+2\mathrm{O}\mathrm{C}\mathrm{M}\mathrm{e}{\phantom{A}}_2+\mathrm{H}{\phantom{A}}_2\mathrm{O}$

The reaction proceeds via the bis(hydroperoxide) and the hydroquinone. Acetone is a coproduct.

Another major process involves the direct hydroxylation of phenol by acidic hydrogen peroxide:

$\mathrm{C}{\phantom{A}}_6\mathrm{H}{\phantom{A}}_5\mathrm{O}\mathrm{H}+\mathrm{H}{\phantom{A}}_2\mathrm{O}{\phantom{A}}_2⟶\mathrm{C}{\phantom{A}}_6\mathrm{H}{\phantom{A}}_4(\mathrm{O}\mathrm{H}){\phantom{A}}_2+\mathrm{H}{\phantom{A}}_2\mathrm{O}$

Both hydroquinone and catechol are produced. Subsequent oxidation of the hydroquinone gives the quinone.

Quinone was originally prepared industrially by oxidation of aniline, for example by manganese dioxide. This method is mainly practiced in China where environmental regulations are more relaxed.

Oxidation of hydroquinone is facile. 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.

When heated to near its melting point, 1,4-benzoquinone sublimes, even at atmospheric pressure, allowing for an effective purification. Impure samples are often dark-colored due to the presence of quinhydrone, a dark green 1:1 charge-transfer complex of quinone with hydroquinone.

Structure and redox

File:DistancesQuinone.png

C–C and C–O bond distances in benzoquinone (Q), its 1e reduced derivative (Q⁻), and hydroquinone (H₂Q).

Benzoquinone is a planar molecule with localized, alternating C=C, C=O, and C–C bonds. Reduction gives the semiquinone anion C₆H₄O₂⁻}, which adopts a more delocalized structure. Further reduction coupled to protonation gives the hydroquinone, wherein the C6 ring is fully delocalized.

Reactions and applications

Benzoquinonium is a skeletal muscle relaxant, ganglion blocking agent that is made from benzoquinone.

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 give the triacetate of hydroxyquinol. This reaction is called the Thiele reaction or Thiele–Winter reaction after Johannes Thiele, who first described it in 1898, and after Ernst Winter, who further described its reaction mechanism in 1900. An application is found in this step of the total synthesis of Metachromin A:

An application of the Thiele reaction, involving a benzoquinone derivative.

An application of the Thiele reaction, involving a benzoquinone derivative.

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 can be reoxidized back to the quinone 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 catalyzes the conversion of an alkene to a ketone. This reaction is typically carried out using pressurized oxygen as the oxidizer, but benzoquinone can sometimes preferred. It is also used as a reagent in some variants on Wacker oxidations.

1,4-Benzoquinone is used in the synthesis of Bromadol and related analogs.

File:FAMCOM.png

Structure of Cp*Rh(para-quinone).

Related 1,4-benzoquinones

Further information: Quinone

2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is a stronger oxidant and dehydrogenation agent than 1,4-benzoquinone. Chloranil 1,4-C₆Cl₄O₂ is another potent oxidant and dehydrogenation agent. Monochloro-p-benzoquinone is yet another but milder oxidant.

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

File:Brachinus spPCCA20060328-2821B.jpg

The bombardier beetle sprays 1,4-benzoquinone to deter predators

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 sublime 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.

See also

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