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Aromatic compound

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File:Benzene Structural diagram.svg
2D model of a benzene molecule. The carbon "ring" is what makes benzene "aromatic".

Aromatic compounds or arenes are organic compounds "with a chemistry typified by benzene" and "cyclically conjugated."<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</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:

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.

HeteroarenesEdit

Heteroarenes are aromatic compounds, where at least one methine or 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. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook.</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. Approximately half of compounds known in 2000 are described as aromatic to some extent.<ref>Template:Cite journal</ref>

File:Orbital overlap of furan.png
Electron flow through p orbitals for the heterocycle furan<ref name=":43">Template:Cite book</ref>
File:Pyridine line bond structure.png
Line bond structure of the heterocycle pyridine<ref name=":43" />
File:Furan line bond structure.png
Line bond structure of the heterocycle furan<ref name=":43" />

ApplicationsEdit

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 acids.

Benzene ring modelEdit

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File:Benzene with hydrogens.png
Line bond structure of benzene<ref name=":43" />
File:Benzene orbitals.png
Electron flow through p orbitals showing the aromatic nature of benzene<ref name=":43" />

Benzene, C6H6, is the least complex aromatic hydrocarbon, and it was the first one defined as such.<ref name=":2">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Its bonding nature was first recognized independently by 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 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 Sir Robert Robinson and his student James Armit in 1925 and popularized starting in 1959 by the Morrison & Boyd textbook on organic chemistry.<ref>Template:Cite journal</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">Template:Cite journal</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 benzeneEdit

File:Ortho meta para.png
Substitution nomenclature of benzene<ref name=":43" />

Benzene derivatives have from one to six substituents 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">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> When reacting to form more complex benzene derivatives, the substituents on a benzene ring can be described as either activated or 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 isomers 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">Template:Cite book</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.Template:Citation needed

Arene rings can stabilize charges, as seen in, for example, phenol (C6H5–OH), which is acidic at the hydroxyl (OH), as charge on the oxygen (alkoxide –O−) is partially delocalized into the benzene ring.

Non-benzylic arenesEdit

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 arenesEdit

Of annulenes 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>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Another example of a non-benzylic monocyclic arene is the cyclopropenyl (cyclopropenium cation), which satisfies Hückel's rule with an n equal to 0.<ref name=":4">{{#invoke:citation/CS1|citation |CitationClass=web }}</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>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Polycyclic aromatic hydrocarbonsEdit

File:Hexabenzocoronene-3D-balls.png
Hexabenzocoronene is a large polycyclic aromatic hydrocarbon.

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Polycyclic aromatic hydrocarbons, also known as polynuclear aromatic compounds (PAHs) are aromatic hydrocarbons that consist of fused aromatic rings and do not contain heteroatoms or carry substituents.<ref>Template:Cite journal</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 carcinogenic, mutagenic, and teratogenic.<ref name=":6">Template:Cite journal</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>Template:Cite journal</ref><ref>Template:Cite journal</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 candidate molecule to act as a basis for the earliest forms of life.<ref>Template:Cite journal</ref> In graphene the PAH motif is extended to large 2D sheets.<ref>Template:Cite journal</ref>

ReactionsEdit

Aromatic ring systems participate in many organic reactions.

SubstitutionEdit

In aromatic 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 radical.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=":7">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

An example of electrophilic aromatic substitution is the nitration of salicylic acid, where a nitro group is added para to the hydroxide substituent:

Nitration of salicylic acid
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 groups aromatic compounds undergo nucleophilic substitution. Mechanistically, this reaction differs from a common SN2 reaction, because it occurs at a trigonal carbon atom (sp2 hybridization).<ref>Template:Cite book</ref>

HydrogenationEdit

Hydrogenation of arenes create saturated rings. The compound 1-naphthol is completely reduced to a mixture of decalin-ol isomers.<ref>Template:OrgSynth</ref>

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>Template:OrgSynth</ref>

Resorcinol hydrogenation

DearomatizationEdit

In dearomatization reactions 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>Template:Cite journal</ref>

File:Dearomatization.png
Dearomatization of benzene through the Birch reduction<ref>Template:Cite journal</ref>

See alsoEdit

ReferencesEdit

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External linksEdit

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