Aromatic hydrocarbon
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2008-07-08T08:52:35Z
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/* Benzene ring model */
{{Redirect|Arene}}
An '''aromatic hydrocarbon''' (abbreviated as AH) or '''arene''' <ref>Definition IUPAC [[Gold Book]] [http://www.iupac.org/goldbook/A00435.pdf Link]</ref> is a [[hydrocarbon]], of which the [[molecular structure]] incorporates one or more planar sets of six [[carbon]] atoms that are connected by [[delocalised electron]]s numbering the same as if they consisted of alternating single and double [[covalent bond]]s. The term 'aromatic' was assigned before the physical mechanism determining [[aromaticity]] was discovered, and was derived from the fact that many of the compounds have a sweet scent. This sweet scent actually came from impurities in the compounds (which are not actually aromatic in the sense initially described). The configuration of six carbon atoms in aromatic compounds is known as a [[benzene|benzene ring]], after the simplest possible such hydrocarbon, [[benzene]]. Aromatic hydrocarbons can be ''monocyclic'' or ''polycyclic''.
Some non-benzene-based compounds called '''heteroarenes''', which follow [[Hückel's rule]], are also aromatic compounds. In these compounds, at least one carbon atom is replaced by one of the [[heteroatom]]s [[oxygen]], [[nitrogen]], or [[sulfur]]. Examples of non-benzene compounds with aromatic properties are [[furan]], a heterocyclic compound with a five-membered ring that includes an oxygen atom, and [[pyridine]], a heterocyclic compound with a six-membered ring containing one nitrogen atom.<ref> [http://www.encyclopedia.com/doc/1E1-aromatic.html HighBeam Encyclopedia: aromatic compound] </ref>
== Benzene ring model ==
[[image:Benzene circle.svg|100px|right|thumb| [[Benzene]] ]]
{{main|aromaticity}}
[[Benzene]], C<sub>6</sub>H<sub>6</sub>, is the simplest AH and was recognized as the first aromatic hydrocarbon, with the nature of its bonding first being recognized by [[Friedrich August Kekulé von Stradonitz]] in the 19th century.
Each carbon atom in the hexagonal cycle has four electrons to share. One goes to the hydrogen atom, and one each to the two neighboring carbons. This leaves one to share with one of its two neighboring carbon atoms, which is why the benzene molecule is drawn with alternating single and double bonds around the hexagon.
Many chemists draw a circle around the inside of the ring to show six electrons floating around in delocalized molecular orbitals the size of the ring itself. This also accurately represents the equivalent nature of the six bonds all of [[bond order]] ~1.5. This equivalency is well explained by [[Resonance (chemistry)|resonance form]]s. The electrons float above and below the ring, and the electromagnetic fields they generate keep the ring flat.
General properties:
# Display [[aromaticity]].
# The Carbon-Hydrogen ratio is very large.
# They burn with a sooty yellow flame because of the high carbon-hydrogen ratio.
# They undergo [[electrophilic substitution reaction]]s and [[nucleophilic aromatic substitution]]s.
==Arene synthesis==
Many laboratory methods exist for the [[organic synthesis]] of arenes from non-arene precursors:
* [[Alkyne trimerization]], [2+2+2] cyclization of three alkynes
* [[Dötz reaction]]
* [[Diels-Alder reaction]]s of [[alkyne]]s with [[pyrone]] or [[cyclopentadienone]] with expulsion of carbon dioxide or carbon monoxide.
* Aromatization of [[cyclohexane]]s and other aliphatic rings: reagents are, catalysts used in [[hydrogenation]] such as platinum, palladium and nickel (reverse hydrogenation), [[quinone]]s and the elements [[sulfur]] and [[selenium]] <ref>Jerry March Advanced Organic Chemistry 3Ed., ISBN 0-471-85472-7</ref>.
* [[Bergman cyclization]], enyne plus hydrogen donor
==Arene reactions==
The main arene reactions are
* [[Electrophilic aromatic substitution]]
* [[Nucleophilic aromatic substitution]]
* Many [[coupling reaction]]s to biraryls
* [[Hydrogenation]] to saturated rings
Lesser-known reactions:
* Unusual thermal Diels-Alder reactivity of arenes can be found in the [[Wagner-Jauregg reaction]]
* Other photochemical cycloaddition reactions with alkenes through [[excimer]]s.
== Benzene and derivatives of benzene ==
[[image:toluene.png|100px|right|thumb| [[Toluene]] ]]
Benzene derivatives have from one to six [[substituent]]s attached to the central benzene core. Examples of benzene compounds with just one substituent are [[phenol]], which carries a [[hydroxyl]] group and [[toluene]] with a [[methyl]] group. When there is more than one substituent present on the ring, their spatial relationship becomes important for which the [[arene substitution patterns]] ''ortho'', ''meta'', and ''para'' are devised. For example, three [[isomer]]s exist for [[cresol]] because the methyl group and the hydroxyl group can be placed next to each other (ortho), one position removed from each other (meta), or two positions removed from each other (para). [[Xylenol]] has two methyl groups in addition to the hydroxyl group, and, for this structure, 6 isomers exist.
Examples of benzene derivatives with alkyl substituents (alkylbenzenes):
*[[Ethylbenzene]] C<sub>6</sub>H<sub>5</sub>-CH<sub>2</sub>-CH<sub>3</sub>
*[[Toluene]] C<sub>6</sub>H<sub>5</sub>-CH<sub>3</sub>
*[[Xylene]], [[m-Xylene]], [[p-Xylene]] C<sub>6</sub>H<sub>4</sub>(-CH<sub>3</sub>)<sub>2</sub>
*[[Mesitylene]], [[Pseudocumene]], [[Hemimellitene]] C<sub>6</sub>H<sub>3</sub>(-CH<sub>3</sub>)<sub>3</sub>
*[[Prehnitene]], [[Isodurene]], [[Durene]] C<sub>6</sub>H<sub>2</sub>(-CH<sub>3</sub>)<sub>4</sub>
*[[Pentamethylbenzene]] C<sub>6</sub>H(-CH<sub>3</sub>)<sub>5</sub>
*[[Mellitene]] C<sub>6</sub>(-CH<sub>3</sub>)<sub>6</sub>
Examples of other aromatic compounds:
*[[Aniline]] C<sub>6</sub>H<sub>5</sub>-NH<sub>2</sub>
*[[Aspirine|Acetylsalicylic acid]] C<sub>6</sub>H<sub>4</sub>(-O-C(=O)-CH<sub>3</sub>)(-COOH)
*[[Benzoic acid]] C<sub>6</sub>H<sub>5</sub>-COOH
*[[Biphenyl]] (C<sub>6</sub>H<sub>5</sub>)<sub>2</sub>
*[[Chlorobenzene]] C<sub>6</sub>H<sub>5</sub>-Cl
*[[Nitrobenzene]] C<sub>6</sub>H<sub>5</sub>-NO<sub>2</sub>
*[[Paracetamol]] C<sub>6</sub>H<sub>4</sub>(-NH-C(=O)-CH<sub>3</sub>)(-OH)
*[[Phenacetin]] C<sub>6</sub>H<sub>4</sub>(-NH-C(=O)-CH<sub>3</sub>)(-O-CH<sub>2</sub>-CH<sub>3</sub>)
*[[Phenol]] C<sub>6</sub>H<sub>5</sub>-OH
*[[Picric acid]] C<sub>6</sub>H<sub>2</sub>(-OH)(-NO<sub>2</sub>)<sub>3</sub>
*[[Salicylic acid]] C<sub>6</sub>H<sub>4</sub>(-OH)(-COOH)
*[[Trinitrotoluene]] C<sub>6</sub>H<sub>2</sub>(-CH<sub>3</sub>)(-NO<sub>2</sub>)<sub>3</sub>
The arene ring has an ability to stabilize charges. This is seen in, for example, phenol (C<sub>6</sub>H<sub>5</sub>-OH), which is [[acidic]] at the hydroxyl (OH), since a charge on this oxygen (alkoxide -O<sup>–</sup>) is partially delocalized into the benzene ring.
==Polyaromatic hydrocarbons==
{{main|Polycyclic aromatic hydrocarbon}}
[[image:napthalene.png|100px|right|thumb| [[Naphthalene]] ]]
Some important arenes are the '''polyaromatic hydrocarbons''' (PAH); they are also called '''polycyclic aromatic hydrocarbons''' and '''polynuclear aromatic hydrocarbons'''. They are composed of more than one aromatic ring. The simplest PAHs are [[benzocyclopropene]] ([[carbon|C]]<sub>7</sub>[[hydrogen|H]]<sub>6</sub>), [[benzocyclopropane]] ([[carbon|C]]<sub>7</sub>[[hydrogen|H]]<sub>8</sub>), [[benzocyclobutadiene]] ([[carbon|C]]<sub>8</sub>[[hydrogen|H]]<sub>6</sub>), and [[benzocyclobutene]] ([[carbon|C]]<sub>8</sub>[[hydrogen|H]]<sub>8</sub>).
Common examples are [[naphthalene]] with two fused rings, [[anthracene]] with three, [[tetracene]] with four, and [[pentacene]] with five linearly fused rings. [[Phenanthrene]] and [[triphenylene]] are examples of non-linear connections. More exotic examples are [[helicene]]s and [[corannulene]].
These compounds are one of the most widespread [[Persistent organic pollutant|organic pollutants]], remaining on beaches and marine environmentals for a long time after an [[oil spill]]. Recent investigations have concluced that their toxicity is up to 100 times worse than first assumed. <ref>{{cite web |url=http://www.fakr.noaa.gov/oil/adn/adn1.htm |title=Sound battles back, but threats linger |accessdate=2008-02-02 |format= |work=[[NOAA Fisheries]] }}</ref>
== See also ==
* [[Asphaltene]]
* [[Simple aromatic rings]]
== External links ==
* [http://members.aol.com/logan20/aromatic.html R. H. Logan's homepage: Aromaticity And Aromatic Hydrocarbons]
* [http://www.pca.state.mn.us/programs/pubs/qa-pahs.pdf Carcinogenic FAC list] in [[Portable Document Format]].
* [http://www.atsdr.cdc.gov/toxprofiles/tp69.html Toxicological profiles of PAH].
* [http://books.nap.edu/books/POD088/html/385.html#pagetop LIST of PAH].
* [http://www.aapg.org/explorer/2002/11nov/abiogenic.cfm Abiogenic Gas Debate 11:2002 (EXPLORER)]
==References==
<references/>
[[Category:Aromatic hydrocarbons]]
[[Category:Hydrocarbons]]
[[ar:هيدروكربون أروماتي]]
[[bs:Aromatski ugljikovodici]]
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[[fr:Hydrocarbure aromatique]]
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