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. A chemically conjugated system is a system of atoms Covalently bonded with alternating single and multiple (e In Organic chemistry, an unsaturated compound is a Chemical compound that contains carbon - carbon Pi bonds such as an Alkene or an Alkyne lone pair is a (valence electron pair without bonding or sharing with other Atoms They are found in the outermost Electron shell of an atom so lone pairs An atomic orbital is a Mathematical function that describes the wave-like behavior of an electron in an atom It can also be considered a manifestation of cyclic delocalization and of resonance. In chemistry delocalized electrons are Electrons in a Molecule that are not associated with a single Atom or to a Covalent bond. Resonance in Chemistry is a theory used to represent and model certain types of non-classical Molecular structures Resonance is a key component ^[1][2][3]

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. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny 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). Friedrich August Kekule von Stradonitz (also August Kekulé) (7 September 1829 &ndash 13 July 1896 was a German organic chemist. The model for benzene consists of two resonance forms, which corresponds to the double and single bonds' switching positions. Benzene, or benzol, is an organic Chemical compound and a known Carcinogen with the molecular formula C 6 H 6 Resonance in Chemistry is a theory used to represent and model certain types of non-classical Molecular structures Resonance is a key component 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 Aromatic compound classifications 4.1 Heterocyclics 4.2 Polycyclics 4.3 Substituted aromatics 5 Aromaticity in other systems 6 See also 7 References

Theory

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. Resonance in Chemistry is a theory used to represent and model certain types of non-classical Molecular structures Resonance is a key component 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. In Molecular geometry, bond length or bond distance is the average distance between nuclei of two bonded Atoms in a Molecule.

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. In Chemistry, pi bonds ( π bonds) are covalent Chemical bonds where two lobes of one involved electron orbital overlap two lobes 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. In Chemistry, sigma bonds ( σ bonds) are the strongest type of covalent Chemical bond. Double bonds consist of a sigma bond and a π bond. The π-bonds are formed from overlap of atomic p-orbitals above and below the plane of the ring. An atomic orbital is a Mathematical function that describes the wave-like behavior of an electron in an atom 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. In Chemistry, a molecular orbital (or MO) is a region in which an Electron may be found in a Molecule.

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. In Organic chemistry, the phenyl group or phenyl ring (often abbreviated as -Ph) is the Functional group with the formula - In Chemistry, radicals (often referred to as free radicals) are atoms molecules or ions with Unpaired electrons on an otherwise Open shell August Wilhelm von Hofmann ( April 8, 1818 &ndash May 5, 1892) was a German Chemist. ^[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. Olfaction (also known as olfactics or smell) refers to the Sense of smell. It is the case, however, that many of the most odoriferous organic substances known are terpenes, which are not aromatic in the chemical sense. Terpenes are a large and varied class of Hydrocarbons, produced primarily by a wide variety of plants particularly Conifers though also by some insects such 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. In Organic chemistry, compounds composed of Carbon and Hydrogen are divided into two classes Aromatic compounds which contain Benzene rings

The cyclohexatriene structure for benzene was first proposed by August Kekulé in 1865. Benzene, or benzol, is an organic Chemical compound and a known Carcinogen with the molecular formula C 6 H 6 Friedrich August Kekule von Stradonitz (also August Kekulé) (7 September 1829 &ndash 13 July 1896 was a German organic chemist. 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. Sir Joseph John “JJ” Thomson, OM, FRS (18 December 1856 &ndash 30 August 1940 was a British Physicist and Nobel laureate

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. Sir Robert Robinson OM, PRS ( 13 September 1886 – 8 February 1975) was an English chemist and Nobel laureate

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. Henry Edward Armstrong FRS ( 6 May 1848 – 13 July 1937) was an English chemist . . 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. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J Sir Joseph John “JJ” Thomson, OM, FRS (18 December 1856 &ndash 30 August 1940 was a British Physicist and Nobel laureate Second, he is describing electrophilic aromatic substitution, proceeding (third) through a Wheland intermediate, in which (fourth) the conjugation of the ring is broken. Electrophilic aromatic substitution or EAS is an Organic reaction in which an atom usually Hydrogen, appended to an aromatic system is replaced An arenium ion in Organic chemistry is the cyclohexadienyl Cation that appears as a reactive intermediate in Electrophilic aromatic substitution. A chemically conjugated system is a system of atoms Covalently bonded with alternating single and multiple (e 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). In Physics, especially Quantum mechanics, the Schrödinger equation is an equation that describes how the Quantum state of a Physical system

The quantum mechanical origins of this stability, or aromaticity, were first modelled by Hückel in 1931. Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons Erich Armand Arthur Joseph Hückel ( August 9, 1896 - February 16, 1980) was a German physicist and physical chemist 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:

1. A delocalized conjugated π system, most commonly an arrangement of alternating single and double 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. In chemistry delocalized electrons are Electrons in a Molecule that are not associated with a single Atom or to a Covalent bond. In Chemistry, pi bonds ( π bonds) are covalent Chemical bonds where two lobes of one involved electron orbital overlap two lobes In Geometry, a set of points in space is coplanar if the points all lie in the same geometric plane. In Mathematics an even Integer, that is a number that is divisible by 2 is called evenly even or doubly even if it is a multiple This is known as Hückel's rule. In Organic chemistry, Hückel's rule estimates whether a Planar ring Molecule will have Aromatic properties Permissible numbers of π electrons (4n+2), π electrons where n=0,1,2,3,4,. . . . and so on
5. Special reactivity in organic reactions such as 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 electrons is 4, which of course is a multiple of 4. Organic reactions are Chemical reactions involving Organic compounds The basic Organic chemistry reaction types are Addition reactions Elimination Electrophilic aromatic substitution or EAS is an Organic reaction in which an atom usually Hydrogen, appended to an aromatic system is replaced nucleophilic aromatic substitution is a Substitution reaction in Organic chemistry in which the Nucleophile displaces a good Leaving group, Cyclobutadiene is the smallest - Annulene (-annulene an extremely unstable Hydrocarbon having a lifetime shorter than five seconds in the free state 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. Furan, also known as furane and furfuran, is a heterocyclic Organic compound. 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 ring currents that oppose the applied magnetic field in NMR. If you are looking for the meteorological effect it is at Ring current. 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. Planar monocyclic molecules containing 4n π electrons are called antiaromatic and are, in general, destabilized. Antiaromatic Molecules are Cyclic systems containing alternating single and Double bonds where the Pi electron energy of antiaromatic compounds Molecules that could be antiaromatic will tend to alter their electronic or conformational structure to avoid this situation, thereby becoming non-aromatic. Antiaromatic Molecules are Cyclic systems containing alternating single and Double bonds where the Pi electron energy of antiaromatic compounds For example, cyclooctatetraene (COT) distorts itself out of planarity, breaking π overlap between adjacent double bonds. 1357-Cyclooctatetraene ( COT) is an unsaturated Derivative of Cyclooctane, with the formula C8H8 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. Stacking in Supramolecular chemistry refers to a stacked arrangement of Aromatic Molecules which interact through aromatic interactions 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 benzene, toluene, ortho-xylene and para-xylene. Benzene, or benzol, is an organic Chemical compound and a known Carcinogen with the molecular formula C 6 H 6 Toluene, also known as methylbenzene or phenylmethane, is a clear water -insoluble liquid with the typical smell of Paint thinners redolent of o-Xylene|m-Xylene|p-XyleneThe term xylene or xylol refers to a mixture of three Benzene derivatives which is used as a Solvent in the Printing o-Xylene|m-Xylene|p-XyleneThe term xylene or xylol refers to a mixture of three Benzene derivatives which is used as a Solvent in the Printing 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. Styrene, also known as vinyl benzene as well as many other names (see table is an Organic compound with the Chemical formula C6H5CH=CH2 Phenol, is a toxic colourless Crystalline Solid with a sweet tarry odor commonly referred to as a "hospital smell" Aniline, phenylamine or aminobenzene is an Organic compound with the formula C6H7N Polyester is a category of Polymers which contain the Ester Functional group in their main chain Overview Nylon is a Thermoplastic silky material first used commercially in a nylon- Bristled Toothbrush (1938 followed more famously by

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. Furan, also known as furane and furfuran, is a heterocyclic Organic compound. Other examples include pyridine, imidazole, pyrazole, oxazole, thiophene, and their benzannulated analogs (benzimidazole, for example). Pyridine is a Chemical compound with the formula C5[[Hydrogen H5]] N. Imidazole is a Organic compound with the formula HC3H3N2 This Aromatic Heterocyclic is classified as an Alkaloid Pyrazole refers both to the class of Simple aromatic ring Organic compounds of the Heterocyclic series characterized by a 5-membered Ring structure Oxazole is the parent compound for a vast class of heterocyclic Aromatic Organic compounds Thse are Azoles with an oxygen and a nitrogen Thiophene is the Heterocyclic compound with the formula C4H4S Annulation (derived from Annular, occasionally annelation) in Organic chemistry is a Chemical reaction in which a new ring is constructed on Benzimidazole is a Heterocyclic Aromatic Organic compound. This bicyclic compound consists of the fusion of Benzene and Imidazole

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). Polycyclic aromatic hydrocarbons ( PAH s are Chemical compounds that consist of fused Aromatic rings and do not contain Heteroatoms or Simple aromatic rings, also known as simple arenes or simple aromatics, are Aromatic Organic compounds that consist only of a conjugated Examples are naphthalene, anthracene and phenanthrene. Naphthalene, also known as naphthalin, naphthaline, tar camphor, white tar, albocarbon, or antimite and not to be Anthracene is a solid Polycyclic aromatic hydrocarbon consisting of three fused Benzene rings derived from Coal-tar. Phenanthrene is a Polycyclic aromatic hydrocarbon composed of three fused Benzene rings

Substituted aromatics

Many chemical compounds contain simple aromatic rings in their structure. A chemical compound is a substance consisting of two or more different elements chemically bonded together in a fixed proportion by Mass. Simple aromatic rings, also known as simple arenes or simple aromatics, are Aromatic Organic compounds that consist only of a conjugated Examples include trinitrotoluene (TNT), acetylsalicylic acid (aspirin), paracetamol, and DNA, which contains both purine and pyrimidine. Trinitrotoluene ( TNT) is a Chemical compound with the formula C6H2(NO23CH3 Aspirin, or acetylsalicylic acid (ASA (əˌsɛtɨlsælɨˌsɪlɨk ˈæsɨd is a Salicylate drug, often used as an Analgesic to relieve Paracetamol ( INN) (ˌpærəˈsiːtəmɒl -ˈsɛtə- or acetaminophen ( USAN) is a widely-used Analgesic and Antipyretic Medication Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known Purine ( 1) is a heterocyclic Aromatic Organic compound, consisting of a Pyrimidine ring fused to an Imidazole ring Pyrimidine is a Heterocyclic Aromatic Organic compound similar to Benzene and Pyridine, containing two Nitrogen Atoms

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). An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge Cyclopropene is an Organic compound with the formula 34 It is the simplest isolable Cycloalkene. A cyclopentadienyl complex is a Metal complex with one or more cyclopentadienyl groups (C5H5&minus abbreviated as Cp In Organic chemistry, the tropylium ion is an Aromatic species with a formula of + 1357-Cyclooctatetraene ( COT) is an unsaturated Derivative of Cyclooctane, with the formula C8H8 Aromatic properties have been attributed to non-benzenoid compounds such as tropone. Tropone or 246-cycloheptatrien-l-one is an Organic compound with some importance in Organic chemistry as a non-benzenoid aromatic. Aromatic properties are tested to the limit in a class of compounds called cyclophanes. cyclophane is a Hydrocarbon consisting of an Aromatic unit (typically a Benzene ring and an Aliphatic chain that forms a Bridge

A special case of aromaticity is found in homoaromaticity where conjugation is interrupted by a single sp³ hybridized carbon atom. Homoaromaticity in Organic chemistry is found in conjugated cyclic systems that are able to skip a part of the ring as opposed to regular Aromaticity. -->In Chemistry When carbon in benzene is replaced by other elements in borabenzene, silabenzene, germanabenzene, stannabenzene, phosphorine or pyrylium salts the aromaticity is still retained. borabenzene is a heteroaromatic compound that has a Boron Atom instead of the Carbon Atom of a Benzene Molecule Silabenzene is a heteroaromatic compound containing a Silicon atom instead of the Carbon atom in Benzene, so silabenzene has been one of the Germanabenzene (C5H6Ge is the parent representative of a group of Chemical compounds containing in their molecular structure a Benzene ring with Stannabenzene (C5H6Sn is the parent representative of a group of Chemical compounds containing in their molecular structure a Benzene ring with Phosphorine is a heavy Benzene analog containing a Phosphorus atom instead of a CH moiety so it is considered to be a heavier element analog of Pyridine A pyrylium compound is a conjugated 6 membered Carbon ring system with one carbon atom replaced by a positively charged Oxygen atom forming a Salt Aromaticity is also not limited to compounds of carbon, oxygen and nitrogen.

Metal aromaticity is believed to exist in certain metal clusters of aluminium. In metal aromaticity the concept of Aromaticity found in many Hydrocarbons is extended to Metals. Möbius aromaticity occurs when a cyclic system of molecular orbitals formed from p_π 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. Organic chemistry, Möbius aromaticity is a special type of Aromaticity believed to exist in a number of Organic molecules In terms of MO theory An atomic orbital is a Mathematical function that describes the wave-like behavior of an electron in an atom In the context of Atomic orbitals, an open shell is a Valence shell which is not completely filled with Electrons or that has not given all of its valence Topology ( Greek topos, "place" and logos, "study" is the branch of Mathematics that studies the properties of Because the twist can be left-handed or right-handed, the resulting Möbius aromatics are dissymmetric or chiral. Someone who is right-handed will prefer to use this hand for everyday activities such as writing, maintaining personal hygiene, Cooking and so forth The term chiral (pronounced /ˈkaɪɹ(əl̩/ is used to describe an object that is non- superimposable on its mirror image 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. Johann Benedict Listing ( July 25, 1808 &ndash December 24 1882) was a German Mathematician. Henry S Rzepa is a contemporary computational organic chemist ^[10] In carbo-benzene the ring bonds are extended with alkyne and allene groups. In Organic chemistry, a carbomer is an expanded Molecule obtained by insertion of a C2 unit in a given molecule.

See also

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

References

Organic Letters (usually abbreviated as Org Lett) is a peer-reviewed Scientific journal, published since 1999 by the

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