Aromaticity
Inorganic chemistry,aromaticityis a chemical property describing the way in which aconjugatedring ofunsaturated bonds,lone pairs,orempty orbitalsexhibits a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article byAugust Wilhelm Hofmannin 1855.[1]There is no general relationship between aromaticity as a chemical property and theolfactoryproperties of such compounds.
Aromaticity can also be considered a manifestation of cyclicdelocalizationand ofresonance.[2][3][4]This is usually considered to be becauseelectronsare free to cycle around circular arrangements ofatomsthat are alternately single- and double-bondedto one another. These bonds may be seen as a hybrid of a single bond and a double bond, each bond in the ring identical to every other. This commonly seen model of aromatic rings, namely the idea thatbenzenewas formed from a six-membered carbon ring with alternating single and double bonds (cyclohexatriene), was developed byKekulé(seeHistorysection below). The model for benzene consists of tworesonanceforms, 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
[edit]
As is standard forresonance 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 theactualcompound, 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 samelength,intermediate between that of asingleand that of adouble bond.
A better representation is that of the circular π bond (Armstrong'sinner cycle), in which the electron density is evenly distributed through aπ-bondabove 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 calledσ-bonds.Double bonds consist of a σ-bond and a π-bond. The π-bonds are formed from overlap ofatomic p-orbitalsabove and below the plane of the ring. The following diagram shows the positions of these p-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 resultingmolecular orbitalhas π symmetry.
History
[edit]The term "aromatic"
[edit]The first known use of the word "aromatic" as achemicalterm — namely, to apply to compounds that contain thephenylradical— occurs in an article byAugust Wilhelm Hofmannin 1855.[1]If this is indeed the earliest introduction of the term, it is curious that Hofmann says nothing about why he introduced an adjective indicatingolfactorycharacter to apply to a group of chemical substances only some of which have notable aromas. Also, many of the most odoriferous organic substances known areterpenes,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 manyaliphatic compounds,and Hofmann may not have been making a distinction between the two categories.
The structure of the benzene ring
[edit]
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 forbenzenewas first proposed byAugust Kekuléin 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 toSir Robert Robinson,who was apparently the first (in 1925)[6]to coin the termaromatic sextetas a group of six electrons that resists disruption.
In fact, this concept can be traced further back, via Ernest Crocker in 1922,[7]toHenry Edward Armstrong,who in 1890 wrote "the (six) centric affinities act within a cycle...benzene may be represented by adouble 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 ".[8][verification needed]
Here, Armstrong is describing at least four modern concepts. First, his "affinity" is better known nowadays as theelectron,which was to be discovered only seven years later by J. J. Thomson. Second, he is describingelectrophilic aromatic substitution,proceeding (third) through aWheland intermediate,in which (fourth) theconjugationof the ring is broken. He introduced the symbolCcentered on the ring as a shorthand for theinner cycle,thus anticipatingErich Clar's notation. It is argued that he also anticipated the nature ofwave 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).
Thequantum mechanicalorigins of this stability, or aromaticity, were first modelled byHückelin 1931. He was the first to separate the bonding electrons into sigma and pi electrons.
Characteristics of aromatic (aryl) compounds
[edit]An aromatic (oraryl) compound contains a set ofcovalently boundatoms with specific characteristics:
- Adelocalizedconjugatedπsystem, most commonly an arrangement of alternating single and doublebonds
- Coplanarstructure, with all the contributing atoms in the same plane
- Contributing atoms arranged in one or more rings
- A number of π delocalized electrons that iseven, but not a multiple of 4.That is, 4n + 2 number of π electrons, where n=0, 1, 2, 3, and so on. This is known asHückel's Rule.
Whereas benzene is aromatic (6 electrons, from 3 double bonds),cyclobutadieneis not, since the number of π delocalized electrons is 4, which of course is a multiple of 4. The cyclobutadienide (2−) ion, however, is aromatic (6 electrons). An atom in an aromatic system can have other electrons that are not part of the system, and are therefore ignored for the 4n + 2 rule. Infuran,the oxygen atom is sp² hybridized. One lone pair is in the π system and the other in the plane of the ring (analogous to C-H bond on 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 undergoelectrophilic aromatic substitutionandnucleophilic aromatic substitutionreactions, but notelectrophilic additionreactions 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 producering currentsthat oppose the applied magnetic field inNMR.[9]The NMR signal of protons in the plane of an aromatic ring are shifted substantially further down-field than those on non-aromatic sp² 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-calledπ-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 calledantiaromaticand 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,cyclobutadienewas 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 byHund's ruleforces 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
[edit]Aromatic compounds play key roles in the biochemistry of all living things. The four aromatic amino acidshistidine,phenylalanine,tryptophan,andtyrosineeach serve as one of the 20 basic building-blocks of proteins. Further, all 5nucleotides(adenine,thymine,cytosine,guanine,anduracil) that make up the sequence of the genetic code in DNA and RNA are aromaticpurinesorpyrimidines.The moleculehemecontains an aromatic system with 22 π electrons.Chlorophyllalso has a similar aromatic system.
Aromatic compounds are important in industry. Keyaromatic hydrocarbonsof commercial interest arebenzene,toluene,ortho-xyleneandpara-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, includingstyrene,phenol,aniline,polyesterandnylon.
Types of aromatic compounds
[edit]The overwhelming majority of aromatic compounds are compounds of carbon, but they need not be hydrocarbons.
Neutral homocyclics
[edit]Benzene,as well as most otherannulenes(cyclodecapentaeneexcepted) with the formula CnHnwhere n ≥ 4 and is an even number, such ascyclotetradecaheptaene.
Heterocyclics
[edit]Inheterocyclicaromatics (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 offuran) increase its reactivity. Other examples includepyridine,pyrazine,imidazole,pyrazole,oxazole,thiophene,and theirbenzannulatedanalogs (benzimidazole,for example).
Polycyclics
[edit]Polycyclic aromatic hydrocarbonsare molecules containing two or more simple aromatic rings fused together by sharing two neighboring carbon atoms (see alsosimple aromatic rings). Examples arenaphthalene,anthracene,andphenanthrene.
Substituted aromatics
[edit]Manychemical compoundsare aromatic rings with otherfunctional groupsattached. Examples includetrinitrotoluene(TNT),acetylsalicylic acid(aspirin),paracetamol,and the nucleotides ofDNA.
Atypical aromatic compounds
[edit]Aromaticity is found inionsas well: thecyclopropenylcation (2e system), thecyclopentadienylanion (6e system), thetropyliumion (6e), and thecyclooctatetraenedianion (10e). Aromatic properties have been attributed to non-benzenoid compounds such astropone.Aromatic properties are tested to the limit in a class of compounds calledcyclophanes.
A special case of aromaticity is found inhomoaromaticitywhere conjugation is interrupted by a singlesp³hybridizedcarbon atom.
When carbon in benzene is replaced by other elements inborabenzene,silabenzene,germanabenzene,stannabenzene,phosphorineorpyrylium saltsthe 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(Si6H6) andborazine(B3N3H6) 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 Si56-rings occurring in the Zintl phase Li12Si7was experimentally evidenced by Li solid state NMR.[10]
Metal aromaticityis believed to exist in certain metal clusters of aluminium.[citation needed]
Möbius aromaticityoccurs when a cyclic system of molecular orbitals, formed from pπatomic orbitalsand populated in aclosed shellby 4n (n is an integer) electrons, is given a single half-twist to correspond to aMöbius 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 alsoMöbius–Hückel conceptfor details). Because the twist can beleft-handedorright-handed,the resulting Möbius aromatics aredissymmetricorchiral. As of 2012, there is no proof that a Möbius aromatic molecule was synthesized.[11][12] Aromatics with two half-twists corresponding to theparadromictopologies were first suggested byJohann Listing.[13]Incarbo-benzenethe ring bonds are extended with alkyne and allene groups.
Y-aromaticity
[edit]Y-aromaticity is a concept which was developed to explain the extraordinary stability and high basicity of theguanidiniumcation. 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.[14][15][16]
See also
[edit]- Aromatic hydrocarbon
- Aromatic amine
- BTX (chemistry)
- PAH
- SARA
- Simple aromatic ring
- Pi interaction
- Avoided crossing
References
[edit]- ^abA. W. Hofmann (1855). "On Insolinic Acid".Proceedings of the Royal Society.8:1–3.doi:10.1098/rspl.1856.0002.
- ^Schleyer, Paul von Ragué (2001). "Introduction: Aromaticity".Chemical Reviews.101(5): 1115–8.doi:10.1021/cr0103221.PMID11749368.
- ^A. T. Balaban, P. v. R. Schleyer and H. S. Rzepa (2005). "Crocker, Not Armit and Robinson, Begat the Six Aromatic Electrons".Chemical Reviews.105(10): 3436–3447.doi:10.1021/cr0300946.
- ^Schleyer, Paul von Ragué (2005)."Introduction: Delocalization Pi and Sigma".Chemical Reviews.105(10): 3433.doi:10.1021/cr030095y.
- ^August Kekulé (1872). "Ueber einige Condensationsproducte des Aldehyds".Liebigs Ann. Chem.162(1): 77–124.doi:10.1002/jlac.18721620110.
- ^Armit, James Wilson; Robinson, Robert (1925). "CCXI.?Polynuclear heterocyclic aromatic types. Part II. Some anhydronium bases".Journal of the Chemical Society, Transactions.127:1604.doi:10.1039/CT9252701604.
- ^Ernest C. Crocker (1922). "Application Of The Octet Theory To Single-Ring Aromatic Compounds".J. Am. Chem. Soc.44(8): 1618–1630.doi:10.1021/ja01429a002.
- ^Henry Edward Armstrong (1890). "The structure of cycloid hydrocarbon".Proceedings of the Chemical Society (London).6(85): 95–106.doi:10.1039/PL8900600095.
- ^Merino, Gabriel; Heine, Thomas; Seifert, Gotthard (2004). "The Induced Magnetic Field in Cyclic Molecules".Chemistry – A European Journal.10(17): 4367.doi:10.1002/chem.200400457.
- ^Alexander Kuhn, Puravankara Sreeraj, Rainer Pöttgen, Hans-Dieter Wiemhöfer, Martin Wilkening,Paul Heitjans (2011). "Li NMR Spectroscopy on Crystalline Li12Si7: Experimental Evidence for the Aromaticity of the Planar Cyclopentadienyl-Analogous Si56− Rings".Angew. Chem. Int. Ed.50(50): 12099.doi:10.1002/anie.201105081.
{{cite journal}}:CS1 maint: multiple names: authors list (link) - ^D. Ajami, O. Oeckler, A. Simon, R. Herges (2003). "Synthesis of a Möbius aromatic hydrocarbon".Nature.426(6968): 819–21.doi:10.1038/nature02224.PMID14685233.
{{cite journal}}:CS1 maint: multiple names: authors list (link) - ^Claire Castro, Zhongfang Chen, Chaitanya S. Wannere, Haijun Jiao, William L. Karney, Michael Mauksch, Ralph Puchta, Nico J. R. van Eikema Hommes,Paul von R. Schleyer(2005). "Investigation of a Putative Möbius Aromatic Hydrocarbon. The Effect of Benzannelation on Möbius [4 n]Annulene Aromaticity".J. Am. Chem. Soc.127(8): 2425–2432.doi:10.1021/ja0458165.
{{cite journal}}:CS1 maint: multiple names: authors list (link) - ^Rzepa, Henry S. (2005). "A Double-Twist Möbius-Aromatic Conformation of [14]Annulene".Organic Letters.7(21): 4637–9.doi:10.1021/ol0518333.PMID16209498.
- ^Alberto Gobbi, Gemot Frenking (1993). "Y-Conjugated compounds: the equilibrium geometries and electronic structures of guanidine, guanidinium cation, urea, and 1,1-diaminoethylene".Journal of the American Chemical Society.115:2362–2372.doi:10.1021/ja00059a035.
- ^Kenneth B. Wiberg (1990). "Resonance interactions in acyclic systems. 2. Y-Conjugated anions and cations".Journal of the American Chemical Society.112:4177–4182.doi:10.1021/ja00167a011.
- ^R. Caminiti, A. Pieretti, L. Bencivenni, F. Ramondo, N. Sanna (1996). "Amidine N−C(N)−N Skeleton: Its Structure in Isolated and Hydrogen-Bonded Guanidines from ab Initio Calculations".The Journal of Physical Chemistry.100:10928–10935.doi:10.1021/jp960311p.
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