Aromaticity
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π: This article is about a chemical property of molecules. For meanings related to odor see aroma compound.
Image:Benzene resonance.pngIn chemistry, an aromatic molecule is one in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. It can also be considered a manifestation of cyclic delocalization and of resonance Template:Ref Template:Ref Template:Ref.
This is usually considered to be because electrons are free to cycle around circular arrangements of atoms, which are alternately single- and double-bonded to 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 was developed by Kekulé. The model for benzene consists of two resonance forms, which corresponds to the double and single bonds' switching positions. Benzene is a more stable molecule than would be expected of cyclohexatriene, which is a theoretical molecule.
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Theory
Image:Benz4.pngBy convention, the double-headed arrow indicates that the two structures are simply hypothetical, since neither is an accurate representation of the actual compound. The actual molecule is best represented by a hybrid (average) of these structures, which can be seen above. A C=C bond is shorter than a C−C bond, but benzene is perfectly hexagonal--all six carbon-carbon bonds have the same length, intermediate between that of a 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 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 bonds. Double bonds consist of a sigma bond and another bond--a π bond. The π-bonds are formed from overlap of atomic p-orbitals above 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 delocalised. 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.
History
Image:Benzene circle.png The attribution of this exceptional stability is conventionally to Sir Robert Robinson, who was apparently the first, in 1925) Template: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 Template:Ref, to Henry Edward Armstrong, who in 1890, in an article entitled The structure of cycloid hydrocarbons, 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 Template:Ref.
He here is describing at least four modern concepts; firstly his affinity is better known nowadays as the electron, which was only to be discovered seven years later by J. J. Thomson. Secondly, he is describing electrophilic aromatic substitution proceeding (thirdly) through a Wheland intermediate, in which (fourthly) the conjugation of the ring is broken. He introduced the symbol C centered on the ring as a shorthand for the inner cycle, thus anticipating Eric Clar's notation. Arguably, he also anticipated the nature of 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 Hŭckel in 1931.
Characteristics of aromatic compounds
An aromatic compound contains a set of covalently-bound atoms with specific characteristics:
- A delocalized conjugated pi system, most commonly an arrangement of alternating single and double bonds
- Coplanar structure, with all the contributing atoms in the same plane
- Contributing atoms arranged in one or more rings
- A number of pi delocalized electrons that is even but not a multiple of 4. This is known as Hückel's rule. Permissible numbers of pi electrons include 6, 10, 14, and so on.
- special reactivity in organic reactions for example electrophilic aromatic substitution and nucleophilic aromatic substitution
Whereas benzene is aromatic (6 electrons, from 3 double bonds), Cyclobutadiene is not, since the number of π delocalized electron is 4, which is not satisfied by any n integer value. 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. In furan, the oxygen atom is sp2 hybridized. One lone pair is in the pi 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. The circulating π electrons in an aromatic molecule generate significant local magnetic fields that can be detected by NMR techniques. NMR experiments show that protons on the aromatic ring are shifted substantially further down-field than those on aliphatic carbons. Planar monocyclic molecules containing 4n π electrons are called 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. Möbius aromaticity describes a special case of aromaticity.
Aromatic molecules are able to interact with each other in so-called 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 anther molecule.
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 to "aromaticity" being the eventually-discovered electronic property of them.
Aromatic compound classifications
The key aromatic hydrocarbons of commercial interest are benzene, toluene, ortho-xylene and 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.
Heterocycles
In heterocyclic aromatics, one or more of the atoms in the aromatic ring is of an element other than carbon. Pyridine is used as a solvent and chemical intermediate. Furan is aromatic, but not as aromatic as benzene, and therefore is more reactive. Furan is used as a chemical intermediate. Furan is also a carcinogen. Other examples include imidazole, pyrazole, oxazole, thiophene, and their benzannulated analogs (benzimidazole, for example).
Polycyclics
Polycyclic aromatic hydrocarbons (PAH) are molecules containing two or more simple aromatic rings fused together by sharing two neighboring carbon atoms (see also simple aromatic rings). Examples are naphthalene, anthracene, phenanthrene, indole, quinoline and isoquinoline.
Substituted aromatics
Many chemical compounds contain simple aromatic rings in their structure. Examples are DNA which contains purine and pyrimidine, trinitrotoluene (TNT), acetylsalicylic acid (aspirin) and paracetamol.
Aromaticity in other systems
A special case of aromaticity is found in homoaromaticity where conjugation is interrupted by a single sp3 hybridized carbon atom. When carbon in benzene is replaced by other elements in borabenzene, silabenzene, germanabenzene, stannabenzene, phosphorine or pyrylium salts the aromaticity is still retained. Aromaticity is also not limited to compounds of carbon, oxygen and nitrogen. Metal aromaticity is believed to exist in certain metal clusters of aluminium. Möbius aromaticity occurs when a cyclic system of molecular orbitals formed from pπ 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 topology. Because the twist can be left-handed or right-handed, the resulting Möbius aromatics are dissymmetric or chiral. Aromatics with two half-twists corresponding to the paradromic topologies first suggested by Johann Listing have been proposed by Rzepa in 2005 Template:Ref.
References
- Template:Note P. v. R. Schleyer, "Aromaticity (Editorial)", Chemical Reviews, 2001, 101, 1115-1118. DOI: 10.1021/cr0103221 Abstract.
- Template:Note A. T. Balaban, P. v. R. Schleyer and H. S. Rzepa, "Crocker, Not Armit and Robinson, Begat the Six Aromatic Electrons", Chemical Reviews, 2005, 105, 3436-3447. DOI: 10.1021/cr0103221 Abstract.
- Template:Note P. v. R. Schleyer, "Introduction: Delocalization-π and σ (Editorial)", Chemical Reviews, 2005, 105, 3433-3435. DOI: 10.1021/cr030095y Abstract.
- Template:Note CCXI.—Polynuclear heterocyclic aromatic types. Part II. Some anhydronium bases James Wilson Armit and Robert Robinson Journal of the Chemical Society, Transactions, 1925, 127, 1604 - 1618 Abstract
- Template:Note APPLICATION OF THE OCTET THEORY TO SINGLE-RING AROMATIC COMPOUNDS Ernest C. Crocker J. Am. Chem. Soc.; 1922; 44(8) pp 1618 - 1630; Abstract
- Template:Note The structure of cycloid hydrocarbons Henry Edward Armstrong Proceedings of the Chemical Society (London), 1890, 6, 95 - 106 Abstract
- Template:Note A Double-Twist Möbius-Aromatic Conformation of [14]Annulene Henry S. Rzepa Org. Lett.; 2005; 7(21) pp 4637 Abstractde:Aromaten
es:Hidrocarburo aromático fr:Hydrocarbure aromatique he:ארומטיות ja:芳香族炭化水素 nl:Aromatische verbinding pl:Węglowodór aromatyczny zh:芳香性