Editing Aromatic compound

{{short description|Compound containing rings with delocalized pi electrons}}
{{Redirect|Arene}}
[[File:Benzene Structural diagram.svg|thumb|class=skin-invert-image|2D model of a benzene molecule. The carbon "ring" is what makes benzene "aromatic".]]
'''Aromatic compounds''' or '''arenes''' are [[organic compound]]s "with a chemistry typified by [[benzene]]" and "cyclically conjugated."<ref>{{cite web |title=Aromatic | website = IUPAC GoldBook |url=https://goldbook.iupac.org/terms/view/A00435. |access-date=2023-11-06}}</ref>
The word "aromatic" originates from the past grouping of molecules based on odor, before their general chemical properties were understood. The current definition of aromatic compounds does not have any relation to their odor. Aromatic compounds are now defined as cyclic compounds satisfying [[Hückel's rule]].
Aromatic compounds have the following general properties:

* Typically unreactive
* Often non polar and hydrophobic
* High carbon-hydrogen ratio
* Burn with a strong sooty yellow flame, due to high C:H ratio
* Undergo [[electrophilic substitution reaction]]s and [[nucleophilic aromatic substitution]]s<ref name=":0">{{March6th}}</ref>

Arenes are typically split into two categories - benzoids, that contain a benzene derivative and follow the benzene ring model, and non-benzoids that contain other aromatic cyclic derivatives. Aromatic compounds are commonly used in organic synthesis and are involved in many reaction types, following both additions and removals, as well as saturation and dearomatization.

== Heteroarenes ==
'''Heteroarenes''' are aromatic compounds, where at least one [[Methine group|methine]] or [[Vinylene group|vinylene]] (-C= or -CH=CH-) group is replaced by a [[heteroatom]]: [[oxygen]], [[nitrogen]], or [[sulfur]].<ref>''IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). Online version (2019-) created by S. J. Chalk. <nowiki>ISBN 0-9678550-9-8</nowiki>. <nowiki>https://doi.org/10.1351/goldbook</nowiki>.''</ref> Examples of non-benzene compounds with aromatic properties are [[furan]], a heterocyclic compound with a five-membered ring that includes a single oxygen atom, and [[pyridine]], a heterocyclic compound with a six-membered ring containing one nitrogen atom. Hydrocarbons without an aromatic ring are called [[Aliphatic compound|aliphatic]]. Approximately half of compounds known in 2000 are described as aromatic to some extent.<ref>{{Cite journal |last1=Balaban |first1=Alexandru T. |last2=Oniciu |first2=Daniela C. |last3=Katritzky |first3=Alan R. |date=2004-05-01 |title=Aromaticity as a Cornerstone of Heterocyclic Chemistry |url=https://pubs.acs.org/doi/10.1021/cr0306790 |journal=Chemical Reviews |language=en |volume=104 |issue=5 |pages=2777–2812 |doi=10.1021/cr0306790 |pmid=15137807 |issn=0009-2665}}</ref>
[[File:Orbital_overlap_of_furan.png|class=skin-invert-image|thumb|168x168px|Electron flow through p orbitals for the heterocycle [[furan]]<ref name=":43">{{Cite book |last=Klein |first=David R. |title=Organic Chemistry |publisher=John Wiley & Sons |year=2017 |isbn=9781119444251 |edition=3rd}}</ref>]]
[[File:Pyridine_line_bond_structure.png|class=skin-invert-image|left|thumb|142x142px|Line bond structure of the heterocycle [[pyridine]]<ref name=":43" />]]
[[File:Furan_line_bond_structure.png|class=skin-invert-image|center|thumb|142x142px|Line bond structure of the heterocycle [[furan]]<ref name=":43" />]]

== Applications ==
Aromatic compounds are pervasive in nature and industry. Key industrial aromatic hydrocarbons are benzene, [[toluene]], [[xylene]] called BTX.  Many biomolecules have phenyl groups including the so-called [[aromatic amino acid]]s.

== Benzene ring model ==
{{main|Aromaticity}}
[[File:Benzene_with_hydrogens.png|class=skin-invert-image|thumb|182x182px|Line bond structure of benzene<ref name=":43" />]]
[[File:Benzene_orbitals.png|class=skin-invert-image|thumb|168x168px|Electron flow through p orbitals showing the aromatic nature of [[benzene]]<ref name=":43" />]]
[[Benzene]], C<sub>6</sub>H<sub>6</sub>, is the least complex aromatic hydrocarbon, and it was the first one defined as such.<ref name=":2">{{Cite web |title=Benzene {{!}} Definition, Discovery, Structure, Properties, & Uses {{!}} Britannica |url=https://www.britannica.com/science/benzene |access-date=2023-11-06 |website=www.britannica.com |language=en}}</ref> Its bonding nature was first recognized independently by [[Johann Josef Loschmidt|Joseph Loschmidt]] and [[August Kekulé]] in the 19th century.<ref name=":2" /> Each carbon atom in the hexagonal cycle has four electrons to share. One electron forms a sigma bond with the hydrogen atom, and one is used in covalently bonding to each of the two neighboring carbons. This leaves six electrons, shared equally around the ring in delocalized pi molecular orbitals the size of the ring itself.<ref name=":43" /> This represents the equivalent nature of the six carbon-carbon bonds all of [[bond order]] 1.5. This equivalency can also explained by [[Resonance (chemistry)|resonance forms]].<ref name=":43" /> The electrons are visualized as floating above and below the ring, with the electromagnetic fields they generate acting to keep the ring flat.<ref name=":43" />

The circle symbol for aromaticity was introduced by [[Robert Robinson (organic chemist)|Sir Robert Robinson]] and his student James Armit in 1925 and popularized starting in 1959 by the Morrison & Boyd textbook on organic chemistry.<ref>{{Cite journal |last1=Armit |first1=James Wilson |last2=Robinson |first2=Robert |date=1925 |title=CCXI.—Polynuclear heterocyclic aromatic types. Part II. Some anhydronium bases |url=http://xlink.rsc.org/?DOI=CT9252701604 |journal=J. Chem. Soc., Trans. |language=en |volume=127 |pages=1604–1618 |doi=10.1039/CT9252701604 |issn=0368-1645}}</ref> The proper use of the symbol is debated: some publications use it to ''any'' cyclic π system, while others use it only for those π systems that obey [[Hückel's rule]]. Some argue that, in order to stay in line with  Robinson's originally intended proposal, the use of the circle symbol should be limited to monocyclic 6 π-electron systems.<ref name=":3">{{Cite journal |last=Jensen |first=William B. |date=April 2009 |title=The Origin of the Circle Symbol for Aromaticity |url=https://pubs.acs.org/doi/abs/10.1021/ed086p423 |journal=Journal of Chemical Education |language=en |volume=86 |issue=4 |pages=423 |doi=10.1021/ed086p423 |bibcode=2009JChEd..86..423J |issn=0021-9584}}</ref> In this way the circle symbol for a six-center six-electron bond can be compared to the Y symbol for a [[three-center two-electron bond]].<ref name=":3" />

== Benzene and derivatives of benzene ==
[[File:Ortho_meta_para.png|thumb|170x170px|class=skin-invert-image|Substitution nomenclature of benzene<ref name=":43" />]]
Benzene derivatives have from one to six [[substituent]]s attached to the central benzene core.<ref name=":0" /> 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.<ref name=":02">{{Cite web |date=2015-05-03 |title=16.5: An Explanation of Substituent Effects |url=https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_(Morsch_et_al.)/16%3A_Chemistry_of_Benzene_-_Electrophilic_Aromatic_Substitution/16.05%3A_An_Explanation_of_Substituent_Effects |access-date=2023-12-03 |website=Chemistry LibreTexts |language=en}}</ref> When reacting to form more complex benzene derivatives, the substituents on a benzene ring can be described as either [[Activating group|activated]] or [[Deactivating group|deactivated]], which are electron donating and electron withdrawing respectively.<ref name=":02" /> Activators are known as ortho-para directors, and deactivators are known as meta directors.<ref name=":02" /> Upon reacting, substituents will be added at the ortho, para or meta positions, depending on the directivity of the current substituents to make more complex benzene derivatives, often with several isomers. Electron flow leading to re-aromatization is key in ensuring the stability of such products.<ref name=":02" />

For example, three [[isomer]]s exist for [[cresol]] because the methyl group and the hydroxyl group (both ortho para directors) can be placed next to each other (''ortho''), one position removed from each other (''meta''), or two positions removed from each other (''para'').<ref name=":14">{{cite book |doi=10.1016/B978-0-12-386454-3.00296-7 |chapter=Cresols |title=Encyclopedia of Toxicology |date=2014 |last1=Badanthadka |first1=M. |last2=Mehendale |first2=H.M. |pages=1061–1065 |isbn=978-0-12-386455-0 }}</ref> Given that both the methyl and hydroxyl group are ortho-para directors, the ortho and para isomers are typically favoured.<ref name=":14" /> [[Xylenol]] has two methyl groups in addition to the hydroxyl group, and, for this structure, 6 isomers exist.{{citation needed|date=December 2023}}

Arene rings can stabilize charges, as seen in, for example, phenol (C<sub>6</sub>H<sub>5</sub>–OH), which is [[acidic]] at the hydroxyl (OH), as charge on the oxygen (alkoxide –O<sup>−</sup>) is partially delocalized into the benzene ring.

<gallery perrow="5" caption="Representative arene compounds">
File:Benzene circle.svg|[[Benzene]]
File:Toluene.svg|[[Toluene]]
File:Ethylbenzene-2D-structure.svg|[[Ethylbenzene]]
File:Para-Xylol - para-xylene.svg|[[Xylene|''p''-Xylene]]
File:Meta-Xylol - meta-xylene.svg|[[Xylene|''m''-Xylene]]
File:Ortho-Xylol - ortho-xylene.svg|[[Xylene|''o''-Xylene]]
File:1,3,5-Trimethylbenzene.svg|[[Mesitylene]]
File:Durene.png|[[Durene]]
File:Biphenyl.svg|[[Biphenyl]]
File:Phenol.svg|[[Phenol]]
File:Aniline.svg|[[Aniline]]
File:Benzaldehyde.svg|[[Benzaldehyde]]
File:Benzoic acid.svg|[[Benzoic acid]]
File:Benzamide.svg|[[Benzamide]]
File:Acetophenone structure.svg|[[Acetophenone]]
</gallery>

== Non-benzylic arenes ==
Although benzylic arenes are common, non-benzylic compounds are also exceedingly important. Any compound containing a cyclic portion that conforms to [[Hückel's rule]] and is not a benzene derivative can be considered a non-benzylic aromatic compound.<ref name=":43" />

=== Monocyclic arenes ===
Of [[annulene]]s larger than benzene, [12]annulene and [14]annulene are weakly aromatic compounds and [18]annulene, [[Cyclooctadecanonaene]], is aromatic, though strain within the structure causes a slight deviation from the precisely planar structure necessary for aromatic categorization.<ref>{{Cite web |date=2013-10-02 |title=What does "aromatic" really mean? |url=https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Arenes/Properties_of_Arenes/Aromaticity/What_does_aromatic_really_mean |access-date=2023-11-06 |website=Chemistry LibreTexts |language=en}}</ref> Another example of a non-benzylic monocyclic arene is the [[Cyclopropenium ion|cyclopropenyl]] (cyclopropenium cation), which satisfies [[Hückel's rule]] with an n equal to 0.<ref name=":4">{{Cite web |date=2013-10-02 |title=What does "aromatic" really mean? |url=https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Arenes/Properties_of_Arenes/Aromaticity/What_does_aromatic_really_mean |access-date=2023-11-29 |website=Chemistry LibreTexts |language=en}}</ref> Note, only the cationic form of this cyclic propenyl is aromatic, given that neutrality in this compound would violate either the octet rule or [[Hückel's rule]].<ref name=":4" />

Other non-benzylic monocyclic arenes include the aforementioned heteroarenes that can replace carbon atoms with other heteroatoms such as N, O or S.<ref name=":43" /> Common examples of these are the five-membered [[pyrrole]] and six-membered [[pyridine]], both of which have a substituted nitrogen<ref>{{Cite web |date=2020-07-30 |title=4.2: Covalent Bonds |url=https://chem.libretexts.org/Courses/Mount_Aloysius_College/CHEM_100%3A_General_Chemistry_(O'Connor)/04%3A_Covalent_Bonding_and_Simple_Molecular_Compounds/4.02%3A_Covalent_Bonds |access-date=2023-11-06 |website=Chemistry LibreTexts |language=en}}</ref>

=== Polycyclic aromatic hydrocarbons ===
[[Image:Hexabenzocoronene-3D-balls.png|thumb|[[Hexabenzocoronene]] is a large polycyclic aromatic hydrocarbon.]]
{{main|Polycyclic aromatic hydrocarbon}}

[[Polynuclear aromatic hydrocarbon|Polycyclic aromatic hydrocarbons]], also known as polynuclear aromatic compounds (PAHs) are aromatic hydrocarbons that consist of fused [[aromatic]] [[Simple aromatic ring|rings]] and do not contain [[heteroatom]]s or carry [[substituent]]s.<ref>{{Cite journal |last=Fetzer |first=John C. |date=2007-04-16 |title=THE CHEMISTRY AND ANALYSIS OF LARGE PAHs |url=http://www.tandfonline.com/doi/abs/10.1080/10406630701268255 |journal=Polycyclic Aromatic Compounds |language=en |volume=27 |issue=2 |pages=143–162 |doi=10.1080/10406630701268255 |s2cid=97930473 |issn=1040-6638}}</ref> [[Naphthalene]] is the simplest example of a PAH. PAHs occur in [[oil]], [[coal]], and [[tar]] deposits, and are produced as byproducts of fuel burning (whether fossil fuel or biomass).<ref name=":5">"Polycyclic Aromatic Hydrocarbons – Occurrence in foods, dietary exposure and health effects" (PDF). European Commission, Scientific Committee on Food. December 4, 2002. Archived (PDF) from the original on 2022-10-09.</ref> As pollutants, they are of concern because some compounds have been identified as [[carcinogen]]ic, [[mutagen]]ic, and [[teratogen]]ic.<ref name=":6">{{Cite journal |last1=Larsson |first1=Bonny K. |last2=Sahlberg |first2=Greger P. |last3=Eriksson |first3=Anders T. |last4=Busk |first4=Leif A. |date=July 1983 |title=Polycyclic aromatic hydrocarbons in grilled food |url=https://pubs.acs.org/doi/abs/10.1021/jf00118a049 |journal=Journal of Agricultural and Food Chemistry |language=en |volume=31 |issue=4 |pages=867–873 |doi=10.1021/jf00118a049 |pmid=6352775 |bibcode=1983JAFC...31..867L |issn=0021-8561}}</ref><ref>Scientific Opinion of the Panel on Contaminants in the Food Chain on a request from the European

Commission on Marine Biotoxins in Shellfish – Saxitoxin Group. The EFSA Journal (2009) 1019, 1-76.</ref><ref>{{Cite journal |last=Keith |first=Lawrence H. |date=2015-03-15 |title=The Source of U.S. EPA's Sixteen PAH Priority Pollutants |url=https://www.tandfonline.com/doi/full/10.1080/10406638.2014.892886 |journal=Polycyclic Aromatic Compounds |language=en |volume=35 |issue=2–4 |pages=147–160 |doi=10.1080/10406638.2014.892886 |issn=1040-6638}}</ref><ref>{{Cite journal |last1=Thomas |first1=Philippe J. |last2=Newell |first2=Emily E. |last3=Eccles |first3=Kristin |last4=Holloway |first4=Alison C. |last5=Idowu |first5=Ifeoluwa |last6=Xia |first6=Zhe |last7=Hassan |first7=Elizabeth |last8=Tomy |first8=Gregg |last9=Quenneville |first9=Cheryl |date=2021-02-01 |title=Co-exposures to trace elements and polycyclic aromatic compounds (PACs) impacts North American river otter (Lontra canadensis) baculum |journal=Chemosphere |volume=265 |pages=128920 |doi=10.1016/j.chemosphere.2020.128920 |issn=0045-6535|doi-access=free |pmid=33213878 |bibcode=2021Chmsp.26528920T }}</ref> PAHs are also found in cooked foods.<ref name=":5" /> Studies have shown that high levels of PAHs are found, for example, in meat cooked at high temperatures such as grilling or barbecuing, and in smoked fish.<ref name=":5" /><ref name=":6" /> They are also a good [[PAH world hypothesis|candidate molecule to act as a basis for the earliest forms of life]].<ref>{{Cite journal |last1=Ehrenfreund |first1=Pascale |last2=Rasmussen |first2=Steen |last3=Cleaves |first3=James |last4=Chen |first4=Liaohai |date=June 2006 |title=Experimentally Tracing the Key Steps in the Origin of Life: The Aromatic World |url=http://www.liebertpub.com/doi/10.1089/ast.2006.6.490 |journal=Astrobiology |language=en |volume=6 |issue=3 |pages=490–520 |doi=10.1089/ast.2006.6.490 |pmid=16805704 |bibcode=2006AsBio...6..490E |issn=1531-1074}}</ref> In [[graphene]] the PAH motif is extended to large 2D sheets.<ref>{{Cite journal |last1=Wang |first1=Xiao-Ye |last2=Yao |first2=Xuelin |last3=Müllen |first3=Klaus |date=2019-09-01 |title=Polycyclic aromatic hydrocarbons in the graphene era |journal=Science China Chemistry |language=en |volume=62 |issue=9 |pages=1099–1144 |doi=10.1007/s11426-019-9491-2 |s2cid=198333072 |issn=1869-1870|doi-access=free |hdl=21.11116/0000-0004-B547-0 |hdl-access=free }}</ref>

==Reactions==
Aromatic ring systems participate in many organic reactions.

===Substitution ===
In aromatic [[Substitution reaction|substitution]], one [[substituent]] on the arene ring, usually hydrogen, is replaced by another reagent.<ref name=":43" /> The two main types are [[electrophilic aromatic substitution]], when the active reagent is an electrophile, and [[nucleophilic aromatic substitution]], when the reagent is a nucleophile. In [[radical-nucleophilic aromatic substitution]], the active reagent is a [[Free radical|radical]].<ref>{{Cite web |date=2014-11-26 |title=22.4: Electrophilic Aromatic Substitution |url=https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Basic_Principles_of_Organic_Chemistry_(Roberts_and_Caserio)/22%3A_Arenes_Electrophilic_Aromatic_Substitution/22.04%3A_Electrophilic_Aromatic_Substitution |access-date=2023-11-29 |website=Chemistry LibreTexts |language=en}}</ref><ref name=":7">{{Cite web |date=2015-05-03 |title=16.7: Nucleophilic Aromatic Substitution |url=https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_(Morsch_et_al.)/16%3A_Chemistry_of_Benzene_-_Electrophilic_Aromatic_Substitution/16.07%3A_Nucleophilic_Aromatic_Substitution |access-date=2023-11-29 |website=Chemistry LibreTexts |language=en}}</ref>

An example of [[electrophilic aromatic substitution]] is the nitration of [[salicylic acid]], where a nitro group is added para to the hydroxide substituent:
:[[File:Synthesis 5-Nitrosalicylic acid.svg|class=skin-invert-image|400px|Nitration of salicylic acid]]

[[File:Aromatic nucleophilic substitution.svg|class=skin-invert-image|right|300px|Aromatic nucleophilic substitution]]
[[Nucleophilic aromatic substitution]] involves displacement of a [[leaving group]], such as a [[halide]], on an [[aromatic ring]]. Aromatic rings usually nucleophilic, but in the presence of [[electron-withdrawing group]]s aromatic compounds undergo nucleophilic substitution. Mechanistically, this reaction differs from a common [[SN2 reaction|S<sub>N</sub>2 reaction]], because it occurs at a trigonal carbon atom (sp<sup>2</sup> [[Hybridization (chemistry)|hybridization]]).<ref>{{Cite book|last1=Clayden|first1=Jonathan|url=https://global.oup.com/academic/product/organic-chemistry-9780199270293?cc=it&lang=en&#.UKpuU4VnJAw|title=Organic Chemistry|last2=Greeves|first2=Nick|last3=Warren|first3=Stuart|date=2012-03-15|publisher=Oxford University Press|isbn=978-0-19-927029-3|edition=Second|location=Oxford, New York|pages=514–515}}</ref>

===Hydrogenation===
[[Hydrogenation]] of arenes create saturated rings. The compound [[1-naphthol]] is completely reduced to a mixture of [[decalin]]-ol [[isomer]]s.<ref>{{OrgSynth|title=1-Naphthol |last1=Meyers |first1=A. I. |last2=Beverung |first2=W. N. |last3=Gault|first3=R. |collvol=6 |collvolpage=371 |volume=51 |page=103 |date=1971 |prep=CV6P0371}}</ref>
:[[Image:NaphtolHydrogenation.svg|class=skin-invert-image|500px|1-naphthol hydrogenation]]
The compound [[resorcinol]], hydrogenated with [[Raney nickel]] in presence of aqueous [[sodium hydroxide]] forms an [[enolate]] which is alkylated with [[methyl iodide]] to 2-methyl-1,3-cyclohexandione:<ref>{{OrgSynth|title=Ethyl Indole-2-carboxylate |last1=Noland |first1=Wayland E. |last2=Baude |first2=Frederic J. |collvol=5 |collvolpage=743 |volume=41 |page=56 |date=1961 |prep=CV5P0567}}</ref>
:[[Image:ResorcinolHydrogenation.svg|class=skin-invert-image|600px|Resorcinol hydrogenation]]

===Dearomatization===
In [[dearomatization reaction]]s the aromaticity of the reactant is lost. In this regard, the dearomatization is related to hydrogenation. A classic approach is [[Birch reduction]]. The methodology is used in synthesis.<ref>{{Cite journal |last1=Roche |first1=Stéphane P. |last2=Porco |first2=John A. |date=2011-04-26 |title=Dearomatization Strategies in the Synthesis of Complex Natural Products |journal=Angewandte Chemie International Edition |language=en |volume=50 |issue=18 |pages=4068–4093 |doi=10.1002/anie.201006017 |issn=1433-7851 |pmc=4136767 |pmid=21506209}}</ref>
[[File:Dearomatization.png|class=skin-invert-image|none|thumb|356x356px|Dearomatization of benzene through the Birch reduction<ref>{{Cite journal |last1=Zheng |first1=Chao |last2=You |first2=Shu-Li |date=2021-03-24 |title=Advances in Catalytic Asymmetric Dearomatization |journal=ACS Central Science |language=en |volume=7 |issue=3 |pages=432–444 |doi=10.1021/acscentsci.0c01651 |issn=2374-7943 |pmc=8006174 |pmid=33791426}}</ref>]]

== See also ==
* Aromatic substituents: [[Aryl]], [[Aryloxy]] and [[Arenediyl]] 
* [[Asphaltene]]
* [[Hydrodealkylation]]
* [[Simple aromatic rings]]

==References==
{{Reflist}}

==External links==
*{{commons category-inline|Aromatics|aromatic compounds}}

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