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14-September-2008 18:38:39 - Aromaticity Aromatic compound redirects here. For meanings related to odor, see aroma compound. Benzene resonance Aromaticity is a chemical property 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.123 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, namely the idea that benzene was formed from a six-membered carbon ring with alternating single and double bonds cyclohexatriene, was developed by Kekulé see History section below. 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 without accounting for charge delocalization. Contents 1 Theory 2 History 3 Characteristics of aromatic Aryl compounds 4 Importance of aromatic compounds 5 Types of aromatic compounds 5.1 Heterocyclics 5.2 Polycyclics 5.3 Substituted aromatics 5.4 Inorganic aromatic compounds 6 Aromaticity in other systems 7 See also 8 References Theory Modern depiction of benzene As is standard for 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 length, intermediate between that of a single and that of a double bond. A better representation is that of the circular Ï€ bond Armstrong's inner cycle, in which the electron density is evenly distributed through a Ï€ 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 a Ï€ bond. The Ï€-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: Benzene electron 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 Ï€ symmetry. Benzene orbital delocalisation History The first known use of the word aromatic as a chemical term-namely, to apply to compounds that contain the phenyl radical-occurs in an article by August Wilhelm Hofmann in 1855.4 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 aromas. It is the case, however, that 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 indexes than many aliphatic compounds, and Hofmann may not have been making a distinction between the two categories. 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. 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. alternative representation An explanation for the exceptional stability of benzene is conventionally attributed to Sir Robert Robinson, who was apparently the first in 19255 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 electron, which was only to be discovered seven years later by J. J. Thomson. Second, he is describing electrophilic aromatic substitution, proceeding third through a Wheland intermediate, in which fourth 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 into sigma and pi electrons. Characteristics of aromatic Aryl compounds An aromatic compound contains a set of covalently-bound atoms with specific characteristics: A delocalized conjugated Ï€ 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 Ï€ delocalized electrons that is even, but not a multiple of 4. That is, 4n + 2 number of Ï€ electrons, where n=0, 1, 2, 3, and so on. This is known as Hückel's Rule. Whereas benzene is aromatic 6 electrons, from 3 double bonds, cyclobutadiene is not, since the number of Ï€ 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. In furan, the oxygen atom is sp² hybridized. One lone pair is in the Ï€ system and the other in the plane of the ring analogous to C-H bond on the other positions. There are 6 Ï€ 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 Ï€ electrons in an aromatic molecule produce ring currents that oppose the applied magnetic field in NMR. 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-called Ï€-Ï€ stacking: the Ï€ 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 Ï€ 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 Ï€ overlap between adjacent double bonds. Importance of aromatic compounds Aromatic compounds are important in industry. 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. Other aromatic compounds play key roles in the biochemistry of all living things. Three aromatic amino acids phenylalanine, tryptophan, and tyrosine, each serve as one the 20 basic building blocks of proteins. Further, all 5 nucleotides that make up the sequence of the genetic code in DNA are aromatic purines or pyrimidines. As well as that, the molecule haem contains an aromatic system with 22 Ï€ electrons. Chlorophyll also has a similar aromatic system. Types of aromatic compounds The overwhelming majority of aromatic compounds are compounds of carbon, but they need not be hydrocarbons. Heterocyclics In 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, imidazole, pyrazole, oxazole, thiophene, and their benzannulated analogs benzimidazole, for example. Polycyclics Polycyclic aromatic hydrocarbons 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 and phenanthrene. Substituted aromatics Many chemical compounds are aromatic rings with other things attached. Examples include trinitrotoluene TNT, acetylsalicylic acid aspirin, paracetamol, and the nucleotides of DNA. Inorganic aromatic compounds Aromaticity occurs in compounds not made of carbon as well. Inorganic 6 membered ring compounds anlogous to benzene have been synthesized. Silicazine Si6H6 and borazine 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. Aromaticity in other systems Aromaticity is found in ions 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 cyclophanes. 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 pÏ€ 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. Up to now there is no doubtless proof, that a Möbius aromatic molecule was synthesized.89 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 ^ P. v. R. Schleyer, Aromaticity orial, Chemical Reviews, 2001, 101, 1115-1118. DOI: 10.1021/cr0103221 Abstract. ^ 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. ^ P. v. R. Schleyer, Introduction: Delocalization-Ï€ and σ orial, Chemical Reviews, 2005, 105, 3433-3435. DOI: 10.1021/cr030095y Abstract. ^ A. W. Hofmann, On Insolinic Acid, Proceedings of the Royal Society, 8 1855, 1-3. ^ 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. ^ APPLICATION OF THE OCTET THEORY TO SINGLE-RING AROMATIC COMPOUNDS Ernest C. Crocker J. Am. Chem. Soc.; 1922; 448 pp 1618-1630; Abstract ^ The structure of cycloid hydrocarbons Henry Edward Armstrong Proceedings of the Chemical Society London, 1890, 6, 95 - 106 Abstract ^ Synthesis of a Möbius aromatic hydrocarbon D. Ajami, O. Oeckler, A. Simon, R. Herges, Nature; 2003; 426 pp 819. ^ Investigation of a Putative Möbius Aromatic Hydrocarbon. The Effect of Benzannelation on Möbius 4 nAnnulene Aromaticity 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 J. Am. Chem. Soc.; 2005; 1278 pp 2425-2432 Abstract ^ A Double-Twist Möbius-Aromatic Conformation of 14Annulene Henry S. Rzepa Org. Lett.; 2005; 721 pp 4637 Abstract v d e Chemical bonds Strong Covalent bonds Antibonding Sigma bonds: 3c-2e bent bond · 3c-4e Hydrogen bond, Dihydrogen bond, Agostic interaction · 4c-2e Pi bonds: Ï€ backbonding · Conjugation · Hyperconjugation · Aromaticity · Metal aromaticity Delta bond: Quadruple bond · Quintuple bond · Sextuple bond Coordinate covalent bond · Hapticity Ionic bonds Cation-pi interaction · Salt bridge Metallic bonds Metal aromaticity Weak Hydrogen bond Dihydrogen bond · Dihydrogen complex · Low-barrier hydrogen bond · Symmetric hydrogen bond · Hydrophile Other noncovalent van der Waals force · Mechanical bond · Halogen bond · Aurophilicity · Intercalation · Stacking · Entropic force · Chemical polarity other Disulfide bond · Peptide bond · Phosphodiester bond Note: the weakest strong bonds are not necessarily stronger than the strongest weak bonds v d e Concepts in organic chemistry Aromaticity, Covalent bonding, Functional groups, Nomenclature, Organic compounds, Organic reactions, Organic synthesis, Publications, Spectroscopy, Stereochemistry, List of organic compounds Retrieved from http://en..org/wiki/Aromaticity Categories: Chemical bonding | Aromatic compounds | Physical organic chemistry Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages العربية ÄŒesky Dansk Español Ù?ارسی Gaeilge МакедонÑ?ки 日本語 ‪Norsk bokmÃ¥l‬ Português РуÑ?Ñ?кий ä¸æ–‡ This page was last modified on 11 September 2008, at 04:46
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