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{{Short description|Chemical property}}
{{Redirect|Aromatic|meanings related to odor|aroma compound|the lack of romantic attraction|aromantic}}
{{Use dmy dates|date=December 2022}}
[[File:Benzene resonance structures.png|right|thumb|Two different [[Resonance (chemistry)|resonance]] forms of benzene (top) combine to produce an average structure (bottom)]]
In [[organic chemistry]], '''aromaticity''' is a chemical property describing the way in which a [[conjugated system|conjugated]] ring of [[unsaturated bond]]s, [[lone pair]]s, or [[empty orbital]]s exhibits a stabilization stronger than would be expected from conjugation alone.  The earliest use of the term was in an article by [[August Wilhelm Hofmann]] in 1855.<ref name="Hofmann1855">{{Cite journal |last=A. W. Hofmann |year=1855 |title=On Insolinic Acid |journal=Proceedings of the Royal Society |volume=8 |pages=1–3 |doi=10.1098/rspl.1856.0002}}</ref>  There is no general relationship between aromaticity as a chemical property and the [[olfaction|olfactory]] properties of such compounds.

Aromaticity can also be considered a manifestation of cyclic [[delocalization]] and of [[Resonance (chemistry)|resonance]].<ref>{{cite journal | last1 = Schleyer | first1 = Paul von Ragué | title = Introduction: Aromaticity | journal = Chemical Reviews | volume = 101 | issue = 5 | pages = 1115–8 | year = 2001 | pmid = 11749368 | doi = 10.1021/cr0103221 }}</ref><ref>{{cite journal | doi = 10.1021/cr0300946 | author = A. T. Balaban, P. v. R. Schleyer and H. S. Rzepa | title = Crocker, Not Armit and Robinson, Begat the Six Aromatic Electrons | journal = [[Chemical Reviews]] | year = 2005 | volume = 105 | pages = 3436–3447 | issue = 10 | pmid = 16218557 }}</ref><ref>{{cite journal | last1 = Schleyer | first1 = Paul von Ragué | title = Introduction: Delocalization Pi and Sigma | journal = Chemical Reviews | volume = 105 | pages = 3433–3435 | year = 2005 | doi = 10.1021/cr030095y | issue = 10| doi-access = free }}</ref> This is usually considered to be because [[electron]]s are free to cycle around circular arrangements of [[atom]]s that are alternately single- and double-[[covalent bond|bonded]] to one another. This commonly seen model of aromatic rings, namely the idea that [[benzene]] was formed from a six-membered carbon ring with alternating single and double bonds (cyclohexatriene), was developed by [[Friedrich August Kekulé von Stradonitz|Kekulé]] (see [[#History|History]] section below). Each bond may be seen as a hybrid of a single bond and a double bond, every bond in the ring identical to every other. The model for benzene consists of two [[Resonance (chemistry)|resonance]] forms, which corresponds to the double and single bonds superimposing to give rise to six one-and-a-half bonds.  Benzene is a more stable molecule than would be expected without accounting for charge delocalization.

==Theory==
[[File:Benzene-6H-delocalized.svg|right|100px|Modern depiction of benzene]]
As is standard for [[resonance (chemistry)|resonance diagrams]], a double-headed arrow is used to indicate that the two structures are not distinct entities, but merely hypothetical possibilities. Neither is an accurate representation of the ''actual'' compound, which is best represented by a hybrid (average) of these structures, which can be seen at right.  A C=C bond is shorter than a C−C bond, but benzene is perfectly hexagonal—all six carbon-carbon bonds have the same [[bond length|length]], intermediate between that of a [[single bond|single]] and that of a [[double bond]].

A better representation is that of the circular π bond (Armstrong's ''inner cycle''), in which the electron density is evenly distributed through a [[pi bond|π-bond]] above and below the ring. This model more correctly represents the location of electron density within the aromatic ring.

The single bonds are formed with electrons in line between the carbon nuclei — these are called [[sigma bond|σ-bonds]]. Double bonds consist of a σ-bond and a π-bond. The π-bonds are formed from overlap of [[Atomic orbital|atomic p-orbitals]] above and below the plane of the ring. The following diagram shows the positions of these p-orbitals:

[[File:Benzene-orbitals.png|330px|Benzene electron orbitals]]

Since they are out of the plane of the atoms, these orbitals can interact with each other freely, and become delocalized. This means that, instead of being tied to one atom of carbon, each electron is shared by all six in the ring. Thus, there are not enough electrons to form double bonds on all the carbon atoms, but the "extra" electrons strengthen all of the bonds on the ring equally. The resulting [[molecular orbital]] has π symmetry.

[[File:Benzene Orbitals.svg|330px|Benzene orbital delocalization]]

==History==

===Etymology===
The first known use of the word "aromatic" as a ''chemical'' term — namely, to apply to compounds that contain the [[phenyl]] [[Radical (chemistry)|radical]] — occurs in an article by [[August Wilhelm Hofmann]] in 1855.<ref name="Hofmann1855" />  If this is indeed the earliest introduction of the term, it is curious that Hofmann says nothing about why he introduced an adjective indicating [[olfactory]] character to apply to a group of chemical substances only some of which have notable [[aroma]]s. Also, many of the most odoriferous organic substances known are [[terpenes]], which are not aromatic in the chemical sense. But terpenes and benzenoid substances do have a chemical characteristic in common, namely higher unsaturation indices than many [[aliphatic compound]]s, and Hofmann may not have been making a distinction between the two categories.

===The structure of the benzene ring===
[[File:Historic Benzene Formulae Kekulé (original).png|thumb|300px|Historic benzene formulae as proposed by Kekulé.<ref name="August Kekulé">{{cite journal|author=August Kekulé|title=Ueber einige Condensationsproducte des Aldehyds|journal=Liebigs Ann. Chem. |year=1872|volume=162|issue=1|pages= 77–124|doi=10.1002/jlac.18721620110}}</ref>]]
In the 19th century, chemists found it puzzling that benzene could be so unreactive toward addition reactions, given its presumed high degree of unsaturation.  The cyclohexatriene structure for [[benzene]] was first proposed by [[August Kekulé]] in 1865. Over the next few decades, most chemists readily accepted this structure, since it accounted for most of the known isomeric relationships of aromatic chemistry.

Between 1897 and 1906, [[J. J. Thomson]], the discoverer of the electron, proposed three equivalent electrons between each carbon atom in benzene.

An explanation for the exceptional stability of benzene is conventionally attributed to [[Robert Robinson (organic chemist)|Sir Robert Robinson]], who was apparently the first (in 1925)<ref>{{cite journal | last1 = Armit | first1 = James Wilson | last2 = Robinson | first2 = Robert | title = CCXI.?Polynuclear heterocyclic aromatic types. Part II. Some anhydronium bases | journal = Journal of the Chemical Society, Transactions | volume = 127 | pages = 1604–1618 | year = 1925 | doi = 10.1039/CT9252701604 }}</ref> to coin the term ''aromatic sextet'' as a group of six electrons that resists disruption.

In fact, this concept can be traced further back, via Ernest Crocker in 1922,<ref>{{cite journal | title = Application Of The Octet Theory To Single-Ring Aromatic Compounds | author = Ernest C. Crocker | journal = [[J. Am. Chem. Soc.]] | year = 1922 | volume = 44 | issue = 8 | pages = 1618–1630 | doi = 10.1021/ja01429a002| bibcode = 1922JAChS..44.1618C }}</ref> to  [[Henry Edward Armstrong]],  who in 1890 wrote "the (six) centric affinities act within a cycle ... benzene may be represented by a '''double ring''' (''sic'') ... and when an additive compound is formed, the inner cycle of affinity suffers disruption, the contiguous carbon-atoms to which nothing has been attached of necessity acquire the ethylenic condition".<ref>{{cite journal | title = The structure of cycloid hydrocarbon | author = Henry Edward Armstrong  | journal = Proceedings of the Chemical Society (London) | volume = 6 | issue = 85 | pages = 95–106 | year = 1890 | doi = 10.1039/PL8900600095 }}</ref>{{verify source|date=January 2014}}<!-- is "double ring" bold? -->

Here, Armstrong is describing at least four modern concepts. First, his "affinity" is better known nowadays as the [[electron]], which was to be discovered only seven years later by J. J. Thomson.  Second, he is describing [[electrophilic aromatic substitution]],  proceeding (third) through a  [[Wheland intermediate]], in which  (fourth) the [[Conjugated system|conjugation]] of the ring is broken. He introduced the symbol '''C''' centered on the ring as a shorthand for the ''inner cycle'', thus anticipating [[Erich Clar]]'s notation. It is argued that he also anticipated the nature of [[Wave|wave mechanics]], since he recognized that his affinities had direction, not merely being point particles, and collectively having a distribution that could be altered by introducing substituents onto the benzene ring (''much as the distribution of the electric charge in a body is altered by bringing it near to another body'').

The [[quantum mechanical]] origins of this stability, or aromaticity,  were first modelled by [[Erich Huckel|Hückel]] in 1931. He was the first to separate the bonding electrons into sigma and pi electrons.

== Characteristics of aromatic (aryl) compounds ==
An aromatic (or [[aryl]]) compound contains a set of [[covalent bond|covalently bound]] atoms with specific characteristics:

# A [[Delocalized electron|delocalized]] conjugated [[pi electron|π]] system, most commonly an arrangement of alternating single and double [[Covalent bond|bonds]].
# [[Coplanar]] structure, with all the contributing atoms in the same plane.
# Contributing atoms arranged in one or more rings.
# A number of π delocalized electrons that is [[singly even|even, but not a multiple of 4]] (that is, 4n + 2 for some integer n). This is known as [[Hückel's rule]].

While benzene is aromatic, having 6 electrons from 3 double bonds, [[cyclobutadiene]] is not, since the number of π delocalized electrons is 4, which does not satisfy Hückel's rule. The cyclobutadienide(2−) ion, however, is aromatic, with 6 electrons. An atom in an aromatic system can have other electrons that are not part of the system, and which therefore do not affect Hückel's rule. In [[furan]], for example, the oxygen atom is sp{{sup|2}} hybridized. One lone pair is in the π system and the other in the plane of the ring (analogous to the C-H bond in the other positions). There are 6 π electrons, so furan is aromatic.

Aromatic molecules typically display enhanced chemical stability, compared to similar non-aromatic molecules. A molecule that can be aromatic will tend to alter its electronic or conformational structure to be in this situation.  This extra stability changes the chemistry of the molecule. Aromatic compounds undergo [[electrophilic aromatic substitution]] and [[nucleophilic aromatic substitution]] reactions, but not [[electrophilic addition]] reactions as happens with carbon-carbon double bonds.

Many of the earliest-known examples of aromatic compounds, such as benzene and toluene, have distinctive pleasant smells. This property led to the term "aromatic" for this class of compounds, and hence the term "aromaticity" for the eventually discovered electronic property.

The circulating π electrons in an aromatic molecule produce [[Aromatic ring current|ring currents]] that oppose the applied magnetic field in [[NMR]].<ref>{{cite journal | last1 = Merino | first1 = Gabriel | last2 = Heine | first2 = Thomas | last3 = Seifert | first3 = Gotthard | title = The Induced Magnetic Field in Cyclic Molecules | journal = Chemistry – A European Journal | volume = 10 | pages = 4367 | year = 2004 | doi = 10.1002/chem.200400457 | issue = 17| pmid = 15352120 }}</ref> The NMR signal of protons in the plane of an aromatic ring are shifted substantially further down-field than those on non-aromatic sp{{sup|2}} carbons.  This is an important way of detecting aromaticity.  By the same mechanism, the signals of protons located near the ring axis are shifted up-field. 
 
Aromatic molecules are able to interact with each other in so-called [[Pi stacking|π-π stacking]]: The π systems form two parallel rings overlap in a "face-to-face" orientation. Aromatic molecules are also able to interact with each other in an "edge-to-face" orientation: The slight positive charge of the substituents on the ring atoms of one molecule are attracted to the slight negative charge of the aromatic system on another molecule.

Planar monocyclic molecules containing 4n π electrons are called [[Antiaromaticity|antiaromatic]] and are, in general, destabilized.  Molecules that could be antiaromatic will tend to alter their electronic or conformational structure to avoid this situation, thereby becoming non-aromatic.  For example, [[cyclooctatetraene]] (COT) distorts itself out of planarity, breaking π overlap between adjacent double bonds. Relatively recently, [[cyclobutadiene]] was discovered to adopt an asymmetric, rectangular configuration in which single and double bonds indeed alternate; there is no resonance and the single bonds are markedly longer than the double bonds, reducing unfavorable p-orbital overlap. This reduction of symmetry lifts the degeneracy of the two formerly non-bonding molecular orbitals, which by [[Hund's rule]] forces the two unpaired electrons into a new, weakly bonding orbital (and also creates a weakly antibonding orbital). Hence, cyclobutadiene is non-aromatic; the strain of the asymmetric configuration outweighs the anti-aromatic destabilization that would afflict the symmetric, square configuration.

== Importance of aromatic compounds ==
Aromatic compounds play key roles in the biochemistry of all living things.  The four aromatic [[amino acid]]s [[histidine]], [[phenylalanine]], [[tryptophan]], and [[tyrosine]] each serve as one of the 20 basic building-blocks of proteins.  Further, all 5 [[nucleotide base]]s ([[adenine]], [[thymine]], [[cytosine]], [[guanine]], and [[uracil]]) that make up the sequence of the genetic code in DNA and RNA are aromatic [[purines]] or [[pyrimidines]].  The molecule [[heme]] contains an aromatic system with 22 π electrons. [[Chlorophyll]] also has a similar aromatic system.

Aromatic compounds are important in industry.  Key [[aromatic hydrocarbon]]s of commercial interest are [[benzene]], [[toluene]], [[xylene|''ortho''-xylene]] and [[xylene|''para''-xylene]].  About 35 million tonnes are produced worldwide every year.  They are extracted from complex mixtures obtained by the refining of oil or by distillation of coal tar, and are used to produce a range of important chemicals and polymers, including [[styrene]], [[phenol]], [[aniline]], [[polyester]] and [[nylon]].

== Types of aromatic compounds ==
The overwhelming majority of aromatic compounds are compounds of carbon, but they need not be hydrocarbons.

=== Neutral homocyclics ===
[[Benzene]], as well as most other [[annulene]]s ([[cyclodecapentaene]] excepted) with the formula C<sub>n</sub>H<sub>n</sub> where n ≥ 4  and is an even number, such as [[cyclotetradecaheptaene]].

=== Heterocyclics ===
In [[Heterocyclic compound|heterocyclic]] aromatics ('''heteroaromats'''), one or more of the atoms in the aromatic ring is of an element other than carbon. This can lessen the ring's aromaticity, and thus (as in the case of [[furan]]) increase its reactivity. Other examples include [[pyridine]], [[pyrazine]], [[imidazole]], [[pyrazole]], [[oxazole]], [[thiazole]], [[thiophene]], and their [[benzannulated]] analogs ([[benzimidazole]], for example).

=== Polycyclics ===
[[Polycyclic aromatic hydrocarbon]]s are molecules containing two or more simple aromatic rings fused together by sharing two neighboring carbon atoms (see also [[simple aromatic ring]]s). Examples are [[naphthalene]], [[anthracene]], and [[phenanthrene]].

=== Substituted aromatics ===
Many [[chemical compound]]s are aromatic rings with other [[functional group]]s attached.  Examples include [[trinitrotoluene]] (TNT), [[aspirin|acetylsalicylic acid]] (aspirin), [[paracetamol]], and the nucleotides of [[DNA]].

=== Atypical aromatic compounds ===
Aromaticity is found in [[ion]]s as well: the [[cyclopropenyl]] cation (2e system), the [[cyclopentadienyl]] anion (6e system), the [[tropylium]] ion (6e), and the [[cyclooctatetraene]] dianion (10e). Aromatic properties have been attributed to non-benzenoid compounds such as [[tropone]]. Aromatic properties are tested to the limit in a class of compounds called [[cyclophane]]s.

A special case of aromaticity is found in [[homoaromaticity]] where conjugation is interrupted by a single sp<sup>3</sup> [[orbital hybridisation|hybridized]] carbon atom.

When carbon in benzene is replaced by other elements in [[borabenzene]], [[silabenzene]], [[germanabenzene]], [[stannabenzene]], [[phosphorine]] or [[pyrylium salt]]s the aromaticity is still retained. Aromaticity also occurs in compounds that are not carbon-based at all.  Inorganic 6-membered-ring compounds analogous to benzene have been synthesized.  [[Hexasilabenzene]] (Si<sub>6</sub>H<sub>6</sub>) and [[borazine]] (B<sub>3</sub>N<sub>3</sub>H<sub>6</sub>) are structurally analogous to benzene, with the carbon atoms replaced by another element or elements.  In borazine, the boron and nitrogen atoms alternate around the ring. Quite recently, the aromaticity of planar Si<sub>5</sub><sup>6-</sup> rings occurring in the Zintl phase Li<sub>12</sub>Si<sub>7</sub> was experimentally evidenced by Li solid state NMR.<ref>{{cite journal | title = Li NMR Spectroscopy on Crystalline Li12Si7: Experimental Evidence for the Aromaticity of the Planar Cyclopentadienyl-Analogous Si56− Rings | author = Alexander Kuhn, Puravankara Sreeraj, Rainer Pöttgen, Hans-Dieter Wiemhöfer, Martin Wilkening,Paul Heitjans | journal = Angew. Chem. Int. Ed. | year = 2011 | volume = 50 | issue = 50 | pages = 12099–12102 | doi = 10.1002/anie.201105081| pmid = 22025292 }}</ref>

[[Metal aromaticity]] is believed to exist in certain metal clusters of aluminium.{{fact|date=December 2023}}

[[Möbius aromaticity]] occurs when a cyclic system of molecular orbitals, formed from  p<sub>π</sub> [[atomic orbitals]] and populated in a [[closed shell]] by 4n (n is an integer) electrons, is given a single half-twist to correspond to a [[Möbius strip]]. A π system with 4n electrons in a flat (non-twisted) ring would be anti-aromatic, and therefore highly unstable, due to the symmetry of the combinations of p atomic orbitals. By twisting the ring, the symmetry of the system changes and becomes allowed (see also [[Möbius–Hückel concept]] for details). Because the twist can be [[left-handed]] or  [[right-handed]], the resulting  Möbius aromatics are ''dissymmetric'' or [[Chirality (chemistry)|chiral]]. 
As of 2012, there is no proof that a Möbius aromatic molecule was synthesized.<ref>{{cite journal | doi = 10.1038/nature02224 | title = Synthesis of a Möbius aromatic hydrocarbon | author = D. Ajami, O. Oeckler, A. Simon, R. Herges | journal = Nature | year = 2003 | volume = 426 | issue = 6968 | pages = 819–21 | pmid = 14685233| bibcode = 2003Natur.426..819A }}</ref><ref>{{cite journal | title = Investigation of a Putative Möbius Aromatic Hydrocarbon. The Effect of Benzannelation on Möbius [4 n]Annulene Aromaticity | author = Claire Castro, Zhongfang Chen, Chaitanya S. Wannere, Haijun Jiao, William L. Karney, Michael Mauksch, Ralph Puchta, Nico J. R. van Eikema Hommes, [[Paul von Rague Schleyer|Paul von R. Schleyer]] | journal = [[J. Am. Chem. Soc.]] | year = 2005 | volume = 127 | issue = 8 | pages = 2425–2432 | doi = 10.1021/ja0458165 | pmid = 15724997 | bibcode = 2005JAChS.127.2425C }}</ref>
Aromatics with two half-twists corresponding to the [[paradromic]] topologies were first suggested by  [[Johann Benedict Listing|Johann Listing]].<ref>{{cite journal | last1 = Rzepa | first1 = Henry S. | title = A Double-Twist Möbius-Aromatic Conformation of [14]Annulene | journal = Organic Letters | volume = 7 | issue = 21 | pages = 4637–9 | year = 2005 | pmid = 16209498 | doi = 10.1021/ol0518333 }}</ref> In [[carbo-benzene]] the ring bonds are extended with alkyne and allene groups.

==== Y-aromaticity ====

Y-aromaticity is a concept which was developed to explain the extraordinary stability and high basicity of the [[Guanidine|guanidinium]] cation. Guanidinium does not have a ring structure but has six π-electrons which are delocalized over the molecule. However, this concept is controversial and some authors have stressed different effects.<ref>{{cite journal | author = Alberto Gobbi, Gemot Frenking | journal = Journal of the American Chemical Society | year = 1993 | volume = 115 | pages = 2362–2372 | doi = 10.1021/ja00059a035 | title = Y-Conjugated compounds: the equilibrium geometries and electronic structures of guanidine, guanidinium cation, urea, and 1,1-diaminoethylene | issue = 6 | bibcode = 1993JAChS.115.2362G }}</ref><ref>{{cite journal | author = Kenneth B. Wiberg | journal = Journal of the American Chemical Society | year = 1990 | volume = 112 | pages = 4177–4182 | doi = 10.1021/ja00167a011 | title = Resonance interactions in acyclic systems. 2. Y-Conjugated anions and cations | issue = 11 | bibcode = 1990JAChS.112.4177W }}</ref><ref>{{cite journal | author = R. Caminiti, A. Pieretti, L. Bencivenni, F. Ramondo, N. Sanna | journal = The Journal of Physical Chemistry | year = 1996 | volume = 100 | pages = 10928–10935 | doi = 10.1021/jp960311p | title = Amidine N−C(N)−N Skeleton: Its Structure in Isolated and Hydrogen-Bonded Guanidines from ab Initio Calculations | issue = 26 }}</ref>

== Global vs. local ==
[[File:Phenanthrene.svg|thumb|Phenanthrene]]
Researchers distinguish global and local aromaticity:<ref>{{cite journal |last1=Krygowski |first1=Tadeusz M. |last2=Szatyłowicz |first2=Halina |date=2015 |title=Aromaticity: what does it mean? |journal=ChemTexts |volume=1 |doi=10.1007/s40828-015-0012-2|pmc=6313370 }}</ref>
* Global aromaticity refers to measures that characterize the entire molecule. These indices assess the collective π-electron system and are useful for comparing the overall stability of different aromatic compounds. Global aromaticity measurements include energetic measures of aromaticity (resonance energy, later aromatic stabilization energy), anisotropy of magnetic susceptibility and magnetic susceptibility exaltation.
* Local aromaticity is used to assess the aromatic character of individual rings within a polycyclic system, which is crucial because different rings can have vastly different levels of stability and reactivity. For example, in [[phenanthrene]] the central ring is known to be less aromatic and more reactive than the two outer rings. The local aromaticity is evaluated through the geometry-based [[harmonic oscillator model of aromaticity]] (HOMA), calculated [[nucleus-independent chemical shift]] (NICS), measured [[proton NMR]] chemical shifts.

==See also==
* [[Aromatic hydrocarbon]]
* [[Aromatic amine]]
* [[BTX (chemistry)]]
* [[Polycyclic aromatic hydrocarbon|PAH]]
* [[Saturate, Aromatic, Resin and Asphaltene|SARA]]
* [[Simple aromatic ring]]
* [[Pi interaction]]
* [[Avoided crossing]]

== References ==
{{Reflist}}

{{Chemical bonds}}
{{Organic chemistry}}
{{Organic reactions}}


[[Category:Aromatic compounds| ]]
[[Category:Physical organic chemistry]]