Aromaticity

Theory

By 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 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 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 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

The first known use of the word "aromatic" as a chemical term -- namely, to apply to compounds that contain the phenyl aliphatic compounds, and Hofmann may not have been making a distinction between the two categories.

The cyclohexatriene structure for 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. However, it was always puzzling that this purportedly highly-unsaturated molecule was so unreactive toward addition reactions.

The discoverer of the electron J. J. Thomson, in 1921 placed three equivalent electrons between each carbon atom in benzene.

An explanation for the exceptional stability of benzene is conventionally attributed to Sir Robert Robinson, who was apparently the first (in 1925)^[5] 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,^[6] 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.^[7]

Here, Armstrong is describing at least four modern concepts. First, his "affinity" is better known nowadays as 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. It is argued that 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 Hückel in 1931. He was the first to separate the bonding electrons in sigma and pi electrons.

Characteristics of aromatic (Aryl) compounds

An aromatic compound contains a set of covalently-bound atoms with specific characteristics:

1. A bonds
2. Coplanar structure, with all the contributing atoms in the same plane
3. Contributing atoms arranged in one or more rings
4. A number of π delocalized electrons that is even, but not a multiple of 4. This is known as Hückel's rule. Permissible numbers of π electrons include 2, 6, 10, 14, and so on
5. Special reactivity in nucleophilic aromatic substitution

Whereas benzene is aromatic (6 electrons, from 3 double bonds), furan, 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. The circulating π electrons in an aromatic molecule produce cyclooctatetraene (COT) distorts itself out of planarity, breaking π overlap between adjacent double bonds. 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 another 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 nylon.

Heterocyclics

In benzimidazole, for example).

Polycyclics

phenanthrene.

Substituted aromatics

Many pyrimidine.

Aromaticity in other systems

Aromaticity is found in cyclophanes.

A special case of aromaticity is found in pyrylium salts the aromaticity is still retained. Aromaticity is also not limited to compounds of carbon, oxygen and nitrogen.

chiral. Up to now there is no doubtless proof, that a Möbius aromatic molecule was synthesized.^[8][9] Aromatics with two half-twists corresponding to the paradromic topologies first suggested by Johann Listing have been proposed by Rzepa in 2005.^[10] In carbo-benzene the ring bonds are extended with alkyne and allene groups.

See also

• Aromatic hydrocarbons
• Aromatic amines
• Hückel's rule
• PAH
• Simple aromatic ring

References

1. ^ P. v. R. Schleyer, "Aromaticity (Editorial)", Chemical Reviews, 2001, 101, 1115-1118. DOI: 10.1021/cr0103221 Abstract.
2. ^ 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.
3. ^ P. v. R. Schleyer, "Introduction: Delocalization-π and σ (Editorial)", Chemical Reviews, 2005, 105, 3433-3435. DOI: 10.1021/cr030095y Abstract.
4. ^ A. W. Hofmann, "On Insolinic Acid," Proceedings of the Royal Society, 8 (1855), 1-3.
5. ^ 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
6. ^ APPLICATION OF THE OCTET THEORY TO SINGLE-RING AROMATIC COMPOUNDS Ernest C. Crocker J. Am. Chem. Soc.; 1922; 44(8) pp 1618 - 1630; Abstract
7. ^ The structure of cycloid hydrocarbons Henry Edward Armstrong Proceedings of the Chemical Society (London), 1890, 6, 95 - 106 Abstract
8. ^ Synthesis of a Möbius aromatic hydrocarbon D. Ajami, O. Oeckler, A. Simon, R. Herges, Nature; 2003; 426 pp 819.
9. ^ Investigation of a Putative Möbius Aromatic Hydrocarbon. The Effect of Benzannelation on Möbius [4 n]Annulene Aromaticity Claire Castro, Zhongfang Chen, Chaitanya S. Wannere, Haijun Jiao, William L. Karney, Michael Mauksch, Ralph Puchta, Nico J. R. van Eikema Hommes, J. Am. Chem. Soc.; 2005; 127(8) pp 2425-2432 Abstract
10. ^ A Double-Twist Möbius-Aromatic Conformation of [14]Annulene Henry S. Rzepa Org. Lett.; 2005; 7(21) pp 4637 Abstract