Aromaticity

In organic chemistry, the term aromaticity is used to describe a cyclic (ring-shaped), planar (flat) molecule with a ring of resonance bonds that exhibits more stability than other geometric or connective arrangements with the same set of atoms. Aromatic molecules are very stable, and do not break apart easily to react with other substances. Organic compounds that are not aromatic are classified as aliphatic compounds—they might be cyclic, but only aromatic rings have special stability (low reactivity).

Since the most common aromatic compounds are derivatives of benzene (an aromatic hydrocarbon common in petroleum and its distillates), the word aromatic occasionally refers informally to benzene derivatives, and so it was first defined. Nevertheless, many non-benzene aromatic compounds exist. In living organisms, for example, the most common aromatic rings are the double-ringed bases in RNA and DNA. An aromatic functional group or other substituent is called an aryl group.

The earliest use of the term aromatic was in an article by August Wilhelm Hofmann in 1855.[1] Hofmann used the term for a class of benzene compounds, many of which have odors (aromas), unlike pure saturated hydrocarbons. Aromaticity as a chemical property bears no general relationship with the olfactory properties of such compounds (how they smell), although in 1855, before the structure of benzene or organic compounds was understood, chemists like Hofmann were beginning to understand that odiferous molecules from plants, such as terpenes, had chemical properties that we recognize today are similar to unsaturated petroleum hydrocarbons like benzene.

In terms of the electronic nature of the molecule, aromaticity describes a conjugated system often made of alternating single and double bonds in a ring. This configuration allows for the electrons in the molecule's pi system to be delocalized around the ring, increasing the molecule's stability. The molecule cannot be represented by one structure, but rather a resonance hybrid of different structures, such as with the two resonance structures of benzene. These molecules cannot be found in either one of these representations, with the longer single bonds in one location and the shorter double bond in another (see Theory below). Rather, the molecule exhibits bond lengths in between those of single and double bonds. 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 August Kekulé (see History below). The model for benzene consists of two resonance forms, which corresponds to the double and single bonds superimposing to produce six one-and-a-half bonds. Benzene is a more stable molecule than would be expected without accounting for charge delocalization.

Benzene resonance structures
Two different resonance forms of benzene (top) combine to produce an average structure (bottom)

Theory

Modern depiction of benzene

As it is a standard for resonance diagrams, the use of a double-headed arrow indicates that 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. A C=C bond is shorter than a C−C bond. Benzene is a regular hexagon—it is planar and all six carbon–carbon bonds have the same length, which is intermediate between that of a single and that of a double bond.

In a cyclic molecule with three alternating double bonds, cyclohexatriene, the bond length of the single bond would be 1.54 Å and that of the double bond would be 1.34 Å. However, in a molecule of benzene, the length of each of the bonds is 1.40 Å, indicating it to be the average of single and double bond.[2][3]

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 from overlap of hybridized atomic sp2-orbitals in line between the carbon nuclei—these are called σ-bonds. Double bonds consist of a σ-bond and 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:

Benzene electron orbitals
Benzene electron 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 resulting molecular orbital is considered to have π symmetry.

Benzene orbital delocalization
Benzene orbital delocalization

History

The term "aromatic"

The first known use of the word "aromatic" as a chemical term—namely, to apply to compounds that contain the phenyl group—occurs in an article by August Wilhelm Hofmann in 1855.[1][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. Also, 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 than many aliphatic compounds, and Hofmann may not have been making a distinction between the two categories. 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.[5]

The structure of the benzene ring

Historic Benzene Formulae Kekulé (original)
Historic benzene formulae as proposed by August Kekulé in 1865.[6]
Ouroboros-benzene
The ouroboros, Kekulė's inspiration for the structure of benzene.

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 for benzene was first proposed by August Kekulé in 1865.[7][8] Most chemists were quick to accept this structure, since it accounted for most of the known isomeric relationships of aromatic chemistry. The hexagonal structure explains why only one isomer of benzene exists and why disubstituted compounds have three isomers.[4]

Between 1897 and 1906, J. J. Thomson, the discoverer of the electron, proposed three equivalent electrons between each pair of carbon atoms in benzene. An explanation for the exceptional stability of benzene is conventionally attributed to Sir Robert Robinson, who was apparently the first (in 1925)[9] 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,[10] to Henry Edward Armstrong, who in 1890 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".[11]

Here, Armstrong is describing at least four modern concepts. First, his "affinity" is better known nowadays as the electron, which was to be discovered only 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 Erich 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.

Aromaticity of an arbitrary aromatic compound can be measured quantitatively by the nucleus-independent chemical shift (NICS) computational method[12] and aromaticity percentage[13] methods.

Characteristics of aromatic (aryl) compounds

An aromatic (or aryl) 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. That is, 4n + 2 π-electrons, where n = 0, 1, 2, 3, and so on. This is known as Hückel's rule.

According to Hückel's rule, if a molecule has 4n + 2 π-electrons, it is aromatic, but if it has 4n π-electrons and has characteristics 1–3 above, the molecule is said to be antiaromatic. Whereas benzene is aromatic (6 electrons, from 3 double bonds), cyclobutadiene is antiaromatic, 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. In 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 the C–H bond in 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 change toward aromaticity, and the added 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.

In the presence of a magnetic field, the circulating π-electrons in an aromatic molecule produce an aromatic ring current that induces an additional magnetic field, an important effect in nuclear magnetic resonance.[14] 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 upfield.

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.

Planar monocyclic molecules containing 4n π-electrons are called antiaromatic and are, in general, unstable. Molecules that could be antiaromatic will tend to change from this electronic or conformation, thereby becoming non-aromatic. For example, cyclooctatetraene (COT) distorts out of planarity, breaking π overlap between adjacent double bonds. Recent studies have determined that cyclobutadiene adopts an asymmetric, rectangular configuration in which single and double bonds indeed alternate, with no resonance; 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 by Hund's rule forces 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

Aromatic compounds play key roles in the biochemistry of all living things. The four aromatic amino acids histidine, phenylalanine, tryptophan, and tyrosine each serve as one of the 20 basic building-blocks of proteins. Further, all 5 nucleotides (adenine, thymine, cytosine, guanine, and uracil) that make up the sequence of the genetic code in DNA and RNA are aromatic purines or pyrimidines. The molecule heme contains an aromatic system with 22 π-electrons. Chlorophyll also has a similar aromatic system.

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.

Types of aromatic compounds

The overwhelming majority of aromatic compounds are compounds of carbon, but they need not be hydrocarbons.

Neutral homocyclics

Benzene, as well as most other annulenes (with the exception of cyclodecapentaene, because it is non-planar) with the formula C4n+2H4n+2 where n is a natural number, such as cyclotetradecaheptaene (n=3).

Heterocyclics

In heterocyclic aromatics (heteroaromatics), 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, pyrazine, pyrrole, imidazole, pyrazole, oxazole, thiophene, and their benzannulated analogs (benzimidazole, for example). In all these examples, the number of π-electrons is 6, due to the π-electrons from the double bonds as well as the two electrons from any lone pair that is in the p-orbital that is in the plane of the aromatic π system. For example, in pyridine, the five sp2-hybridized carbons each have a p-orbital that is perpendicular to the plane of the ring, and each of these p-orbitals contains one π-electron. Additionally, the nitrogen atom is also sp2-hybridized and has one electron in a p-orbital, which adds up to 6 p-electrons, thus making pyridine aromatic. The lone pair on the nitrogen is not part of the aromatic π system. Pyrrole and imidazole are both five membered aromatic rings that contain heteroatoms. In pyrrole, each of the four sp2-hybridized carbons contributes one π-electron, and the nitrogen atom is also sp2-hybridized and contributes two π-electrons from its lone pair, which occupies a p-orbital. In imidazole, both nitrogens are sp2-hybridized; the one in the double bond contributes one electron and the one which is not in the double bond and is in a lone pair contributes two electrons to the π system.[15]

Fused aromatics and 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. In fused aromatics, not all carbon–carbon bonds are necessarily equivalent, as the electrons are not delocalized over the entire molecule. The aromaticity of these molecules can be explained using their orbital picture. Like benzene and other monocyclic aromatic molecules, polycyclics have a cyclic conjugated pi system with p-orbital overlap above and below the plane of the ring.[15]

Substituted aromatics

Many chemical compounds are aromatic rings with other functional groups attached. Examples include trinitrotoluene (TNT), acetylsalicylic acid (aspirin), paracetamol, and the nucleotides of DNA.

Aromatic ions

Aromatic molecules need not be neutral molecules. Ions that satisfy Huckel's rule of 4n + 2 π-electrons in a planar, cyclic, conjugated molecule are considered to be aromatic ions. For example, the cyclopentadienyl anion and the cycloheptatrienylium cation are both considered to be aromatic ions, and the azulene molecule can be approximated as a combination of both.

In order to convert the atom from sp3 to sp2, a carbocation, carbanion, or carbon radical must be formed. These leave sp2-hybridized carbons that can partake in the π system of an aromatic molecule. Like neutral aromatic compounds, these compounds are stable and form easily. The cyclopentadienyl anion is formed very easily and thus 1,3-cyclopentadiene is a very acidic hydrocarbon with a pKa of 16.[15] Other examples of aromatic ions include the cyclopropenium cation (2 π-electrons) and cyclooctatetraenyl dianion (10 π electrons).

Atypical aromatic compounds

Aromaticity also occurs in compounds that are not carbocyclic or heterocyclic; inorganic six-membered-ring compounds analogous to benzene have been synthesized. For example, borazine is a six-membered ring composed of alternating boron and nitrogen atoms, each with one hydrogen attached. It has a delocalized π system and undergoes electrophilic substitution reactions appropriate to aromatic rings rather than reactions expected of non-aromatic molecules.[16]

Quite recently, the aromaticity of planar Si6−
5
rings occurring in the Zintl phase Li12Si7 was experimentally evinced by Li solid-state NMR.[17] Metal aromaticity is believed to exist in certain clusters of aluminium, for example.

Homoaromaticity is the term used to describe systems where conjugation is interrupted by a single sp3 hybridized carbon atom.[18]

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 form a Möbius strip. A π system with 4n electrons in a flat (non-twisted) ring would be antiaromatic, 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 also Möbius–Hückel concept for details). Because the twist can be left-handed or right-handed, the resulting Möbius aromatics are dissymmetric or chiral. But as of 2012, no Möbius aromatic molecules had been synthesized.[19][20] Aromatics with two half-twists corresponding to the paradromic topologies were first suggested by Johann Listing.[21] In one form of carbo-benzene, the ring is expanded and contains alkyne and allene groups.

Y-aromaticity is used to describe a Y-shaped, planar (flat) molecule with resonance bonds. The concept was developed to explain the extraordinary stability and high basicity of the guanidinium cation. Guanidinium is not a ring molecule, and is cross-conjugated rather than a linear π system, but is reported to have its six π-electrons delocalized over the whole molecule. The concept is controversial and some authors emphasize different effects.[22][23][24] This has also been suggested as the reason that the trimethylenemethane dication is more stable than the butadienyl dication.[25]

σ-aromaticity refers to stabilization arising from the delocalization of sigma bonds. It is often invoked in cluster chemistry and is closely related to Wade's Rule.

See also

References

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  4. ^ a b Rocke, A. J. (2015). "It Began with a Daydream: The 150th Anniversary of the Kekulé Benzene Structure". Angew. Chem. Int. Ed. 54: 46–50. doi:10.1002/anie.201408034.
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  9. ^ Armit, James Wilson; Robinson, Robert (1925). "CCXI. Polynuclear heterocyclic aromatic types. Part II. Some anhydronium bases". J. Chem. Soc. Trans. 127: 1604. doi:10.1039/CT9252701604.
  10. ^ Crocker, Ernest C. (1922). "Application Of The Octet Theory To Single-Ring Aromatic Compounds". J. Am. Chem. Soc. 44 (8): 1618–1630. doi:10.1021/ja01429a002.
  11. ^ Armstrong, Henry Edward (1890). "The structure of cycloid hydrocarbon". Proc. Chem. Soc. 6 (85): 95–106. doi:10.1039/PL8900600095.
  12. ^ Schleyer, Paul von Ragué; Maerker, Christoph; Dransfeld, Alk; Jiao, Haijun; Van Eikema Hommes, Nicolaas J. R. (1996). "Nucleus-Independent Chemical Shifts: A Simple and Efficient Aromaticity Probe". J. Am. Chem. Soc. 118 (26): 6317–6318. doi:10.1021/ja960582d.
  13. ^ Mucsi, Z.; Viskolcz, B.; Csizmadia, I. G. (2007). "A Quantitative Scale for the Degree of Aromaticity and Antiaromaticity". J. Phys. Chem. A. 111: 1123–1132. Bibcode:2007JPCA..111.1123M. doi:10.1021/jp0657686.
  14. ^ 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.
  15. ^ a b c McMurry, John (2011). Organic Chemistry (8th ed.). Brooks-Cole. p. 544. ISBN 0-8400-5444-0.
  16. ^ "Borazine: to be or not to be aromatic". Structural Chemistry. 18 (6): 833–839. 2007. doi:10.1007/s11224-007-9229-z.
  17. ^ Kuhn, Alexander; Sreeraj, Puravankara; Pöttgen, Rainer; Wiemhöfer, Hans-Dieter; Wilkening, Martin; Heitjans, Paul (2011). "Li NMR Spectroscopy on Crystalline Li12Si7: Experimental Evidence for the Aromaticity of the Planar Cyclopentadienyl-Analogous Si6−
    5
    Rings". Angew. Chem. Int. Ed. 50 (50): 12099. doi:10.1002/anie.201105081.
  18. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "Homoaromatic".
  19. ^ Ajami, D.; Oeckler, O.; Simon, A.; Herges, R. (2003). "Synthesis of a Möbius aromatic hydrocarbon". Nature. 426 (6968): 819–21. Bibcode:2003Natur.426..819A. doi:10.1038/nature02224. PMID 14685233.
  20. ^ Castro, Claire; Chen, Zhongfang; Wannere, Chaitanya S.; Jiao, Haijun; Karney, William L.; Mauksch, Michael; Puchta, Ralph; Van Eikema Hommes, Nico J. R.; Schleyer, Paul von R. (2005). "Investigation of a Putative Möbius Aromatic Hydrocarbon. The Effect of Benzannulation on Möbius [4n]Annulene Aromaticity". J. Am. Chem. Soc. 127 (8): 2425–2432. doi:10.1021/ja0458165.
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  22. ^ Gobbi, Alberto; Frenking, Gemot (1993). "Y-Conjugated compounds: the equilibrium geometries and electronic structures of guanidine, guanidinium cation, urea, and 1,1-diaminoethylene". J. Am. Chem. Soc. 115: 2362–2372. doi:10.1021/ja00059a035.
  23. ^ Wiberg, Kenneth B. (1990). "Resonance interactions in acyclic systems. 2. Y-Conjugated anions and cations". J. Am. Chem. Soc. 112: 4177–4182. doi:10.1021/ja00167a011.
  24. ^ Caminiti, R.; Pieretti, A.; Bencivenni, L.; Ramondo, F.; Sanna, N. (1996). "Amidine N−C(N)−N Skeleton:  Its Structure in Isolated and Hydrogen-Bonded Guanidines from ab Initio Calculations". J. Phys. Chem. 100: 10928–10935. doi:10.1021/jp960311p.
  25. ^ Dworkin, Amy; Naumann, Rachel; Seigfredi, Christopher; Karty, Joel M.; Mo, Yirong (2005). "Y-aromaticity: Why is the trimethylenemethane dication more stable than the butadienyl dication?". J. Org. Chem. 70 (19): 7605–7616. doi:10.1021/jo0508090.
Annulene

Annulenes are completely conjugated monocyclic hydrocarbons. They have the general formula CnHn (when n is an even number) or CnHn+1 (when n is an odd number). The IUPAC naming conventions are that annulenes with 7 or more carbon atoms are named as [n]annulene, where n is the number of carbon atoms in their ring, though sometimes the smaller annulenes are referred to using the same notation, and benzene is sometimes referred to simply as annulene.The first three annulenes are cyclobutadiene, benzene, and cyclooctatetraene ([8]annulene). Some annulenes, namely cyclobutadiene, cyclodecapentaene or [10]annulene, cyclododecahexaene or [12]annulene and cyclotetradecaheptaene ([14]annulene), are unstable, with cyclobutadiene extremely so.

Annulenes may be aromatic (benzene), non-aromatic ([10]annulene), or anti-aromatic (cyclobutadiene, [12]annulene). Only cyclobutadiene and benzene are fully planar, though [18]annulene (and [14]annulene, to some extent) can achieve planarity with a combination of cis and trans double bonds (placing some of the hydrogens inside the ring) can achieve the planar conformation needed for aromaticity, with [14] and [18]annulene following Hückel's rule with 4n+2 π electrons. [10]Annulene is too small to achieve a planar structure. In a planar conformation, ring strain due to either steric hindrance of internal hydrogens or bond angle distortion is unavoidable. Thus, it does not exhibit appreciable aromaticity.

Many of the larger annulenes, [18]annulene for example, are large enough to minimize the van der Waals strain of internal hydrogens and thermodynamically qualify as aromatic. However, none of the larger annulenes are as stable as benzene, as their reactivity more closely resembles a conjugated polyene than an aromatic hydrocarbon.

In general, charged annulene species of the form [C(4n+2)+qH(4n+2)+q]q (n = 0, 1, 2, ..., q = 0, ±1, ±2) are aromatic, provided a planar conformation can be achieved. For instance, C5H5–, C3H3+, and C8H82– are all known aromatic species.

In annulynes, one double bond is replaced by a triple bond.

Aromatic hydrocarbon

An aromatic hydrocarbon or arene (or sometimes aryl hydrocarbon) is a hydrocarbon with sigma bonds and delocalized pi electrons between carbon atoms forming a circle. In contrast, aliphatic hydrocarbons lack this delocalization. The term "aromatic" was assigned before the physical mechanism determining aromaticity was discovered; the term was coined as such simply because many of the compounds have a sweet or pleasant odour. The configuration of six carbon atoms in aromatic compounds is known as a benzene ring, after the simplest possible such hydrocarbon, benzene. Aromatic hydrocarbons can be monocyclic (MAH) or polycyclic (PAH).

Some non-benzene-based compounds called heteroarenes, which follow Hückel's rule (for monocyclic rings: when the number of its π electrons equals 4n + 2, where n = 0, 1, 2, 3, ...), are also called aromatic compounds. In these compounds, at least one carbon atom is replaced by one of the heteroatoms oxygen, nitrogen, or sulfur. Examples of non-benzene compounds with aromatic properties are furan, a heterocyclic compound with a five-membered ring that includes a single oxygen atom, and pyridine, a heterocyclic compound with a six-membered ring containing one nitrogen atom.

Aromatic ring current

An aromatic ring current is an effect observed in aromatic molecules such as benzene and naphthalene. If a magnetic field is directed perpendicular to the plane of the aromatic system, a ring current is induced in the delocalized π electrons of the aromatic ring. This is a direct consequence of Ampère's law; since the electrons involved are free to circulate, rather than being localized in bonds as they would be in most non-aromatic molecules, they respond much more strongly to the magnetic field.

The ring current creates its own magnetic field. Outside the ring, this field is in the same direction as the externally applied magnetic field; inside the ring, the field counteracts the externally applied field. As a result, the net magnetic field outside the ring is greater than the externally applied field alone, and is less inside the ring.

Aromatic ring currents are relevant to NMR spectroscopy, as they dramatically influence the chemical shifts of 1H nuclei in aromatic molecules. The effect helps distinguish these nuclear environments and is therefore of great use in molecular structure determination. In benzene, the ring protons experience deshielding because the induced magnetic field has the same direction outside the ring as the external field and their chemical shift is 7.3 ppm compared to 5.6 for the vinylic proton in cyclohexene. In contrast any proton inside the aromatic ring experiences shielding because both fields are in opposite direction. This effect can be observed in cyclooctadecanonaene ([18]annulene) with 6 inner protons at −3 ppm.

The situation is reversed in antiaromatic compounds. In the dianion of [18]annulene the inner protons are strongly deshielded at 20.8 ppm and 29.5 ppm with the outer protons significantly shielded (with respect to the reference) at −1.1 ppm. Hence a diamagnetic ring current or diatropic ring current is associated with aromaticity whereas a paratropic ring current signals antiaromaticity.

A similar effect is observed in three-dimensional fullerenes; in this case it is called a sphere current.

Arsole

Arsole, also called arsenole or arsacyclopentadiene, is an organoarsenic compound with the formula C4H4AsH. It is classified as a metallole and is isoelectronic to and related to pyrrole except that an arsenic atom is substituted for the nitrogen atom. Whereas the pyrrole molecule is planar, the arsole molecule is not, and the hydrogen atom bonded to arsenic extends out of the molecular plane. Arsole is only moderately aromatic, with about 40% the aromaticity of pyrrole. Arsole itself has not been reported in pure form, but several substituted analogs called arsoles exist. Arsoles and more complex arsole derivatives have similar structure and chemical properties to those of phosphole derivatives. When arsole is fused to a benzene ring, this molecule is called arsindole, or benzarsole.

Benzisoxazole

1,2-Benzisoxazole is an aromatic organic compound with a molecular formula C7H5NO containing a benzene-fused isoxazole ring structure. The compound itself has no common applications; however, functionalized benzisoxazoles and benzisoxazoyls have a variety of uses, including pharmaceutical drugs such as some antipsychotics (including risperidone, paliperidone, ocaperidone, and iloperidone) and the anticonvulsant zonisamide.

Its aromaticity makes it relatively stable; however, it is only weakly basic.

Bicycloaromaticity

Bicycloaromaticity in chemistry is an extension of the concept of homoaromaticity with two aromatic ring currents situated in a non-planar molecule and sharing the same electrons. The concept originates with Melvin Goldstein who first reported about it in 1967. It is of some importance in academic research. Using MO theory the bicyclo[3.2.2]nonatrienyl cation was predicted to be destabilised and the corresponding anion predicted to be stabilised by bicycloaromaticity.

Bicycloaromaticity has been studied by others in relation to the bicyclo[3.2.2]nonatrienyl cation and in relation to specific carbanions . In 2017 experimental evidence was reported for bicycloaromaticity (dual aromaticity) to exist in a bicyclic porphyrinoid. This system has been described as aromatic with two ring systems of 26 (n=6) and 34 (n=8) electrons. By oxidation, another system was described as a triplet-state biradical, again considered aromatic by application of Baird's rule.

Borazine

Borazine is an inorganic compound with the chemical formula (BH)3(NH)3. In this cyclic compound, the three BH units and three NH units alternate. The compound is isoelectronic and isostructural with benzene. For this reason borazine is sometimes referred to as “inorganic benzene”. Like benzene, borazine is a colourless liquid.

Histidine

Histidine (symbol His or H) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH3+ form under biological conditions), a carboxylic acid group (which is in the deprotonated –COO− form under biological conditions), and an imidazole side chain (which is partially protonated), classifying it as a positively charged amino acid at physiological pH. Initially thought essential only for infants, longer-term studies have shown it is essential for adults also. It is encoded by the codons CAU and CAC.

Histidine was first isolated by German physician Albrecht Kossel and Sven Hedin in 1896. It is also a precursor to histamine, a vital inflammatory agent in immune responses. The acyl radical is histidyl.

Homa

Homa may refer to:

Homa (ritual), a religious practice in Hinduism, Buddhism and Jainism, involving making offerings into a consecrated fire

Homa (leafhopper), an insect genus in the tribe Empoascini

Homa (mythology), a creature of Persian mythology

Another spelling for Haoma in Zoroastrianism

A popular female Persian name

Homa F.C., a professional league football club based in Tehran, Iran

Homeostatic model assessment, a medical formula for quantifying insulin resistance

Homa Bay, a town and a bay on the shore of Lake Victoria in Kenya

Homa Mountain, a volcano near Homa Bay

Harmonic Oscillator Model of Aromaticity, a method for quantifying aromaticity

Homa, Ethiopia

Homa, Iran, a village in Lorestan Province, Iran

Homa, North Khorasan, a village in North Khorasan Province, Iran

Homa-ye Bala (disambiguation), places in Iran

Homag (disambiguation), various places in Iran

Homay, Iran (disambiguation), various places in Iran

Homa TV, an Iranian TV channel

Iran Air's acronym, in Persian-language

Homa Darabi foundation, founded by Parvin Darabi

Homoaromaticity

Homoaromaticity, in organic chemistry, refers to a special case of aromaticity in which conjugation is interrupted by a single sp3 hybridized carbon atom. Although this sp3 center disrupts the continuous overlap of p-orbitals, traditionally thought to be a requirement for aromaticity, considerable thermodynamic stability and many of the spectroscopic, magnetic, and chemical properties associated with aromatic compounds are still observed for such compounds. This formal discontinuity is apparently bridged by p-orbital overlap, maintaining a contiguous cycle of π electrons that is responsible for this preserved chemical stability.

The concept of homoaromaticity was pioneered by Saul Winstein in 1959, prompted by his studies of the “tris-homocyclopropenyl” cation. Since the publication of Winstein's paper, much research has been devoted to understanding and classifying these molecules, which represent an additional class of aromatic molecules included under the continuously broadening definition of aromaticity. To date, homoaromatic compounds are known to exist as cationic and anionic species, and some studies support the existence of neutral homoaromatic molecules, though these are less common. The 'homotropylium' cation (C8H9+) is perhaps the best studied example of a homoaromatic compound.

Metal aromaticity

Metal aromaticity is the concept of aromaticity found in many organic compounds is extended to metals. The first experimental evidence for the existence of aromaticity in metals was found in aluminium cluster compounds of the type MAl4− where M stands for lithium, sodium or copper. These anions can be generated in a helium gas by laser vaporization of an aluminium / lithium carbonate composite or a copper or sodium / aluminium alloy, separated and selected by mass spectrometry and analyzed by photoelectron spectroscopy. The evidence for aromaticity in these compounds is based on several considerations. Computational chemistry shows that these aluminium clusters consist of a tetranuclear Al42− plane and a counterion at the apex of a square pyramid. The Al42− unit is perfectly planar and is not perturbed the presence of the counterion or even the presence of two counterions in the neutral compound M2Al4. In addition its HOMO is calculated to be a doubly occupied delocalized pi system making it obey Hückel's rule. Finally a match exists between the calculated values and the experimental photoelectron values for the energy required to remove the first 4 valence electrons.

D-orbital aromaticity is found in trinuclear tungsten W3O9− and molybdenum Mo3O9− metal clusters generated by laser vaporization of the pure metals in the presence of oxygen in a helium stream. In these clusters the three metal centers are bridged by oxygen and each metal has two terminal oxygen atoms. The first signal in the photoelectron spectrum corresponds to the removal of the valence electron with the lowest energy in the anion to the neutral M3O9 compound. This energy turns out to be comparable to that of bulk tungsten trioxide and molybdenum trioxide. The photoelectric signal is also broad which suggests a large difference in conformation between the anion and the neutral species. Computational chemistry shows that the M3O9− anions and M3O92− dianions are ideal hexagons with identical metal-to-metal bond lengths.

The molecules discussed thus far only exist diluted in the gas phase. A study exploring the properties of a compound formed in water from sodium molybdate (Na2MoO4.2H2O) and iminodiacetic acid also revealed evidence of aromaticity, but this compound has actually been isolated. X-ray crystallography showed that the sodium atoms are arranged in layers of hexagonal clusters akin to pentacenes. The sodium-to-sodium bond lengths are unusually short (327 pm versus 380 pm in elemental sodium) and, like benzene, the ring is planar. In this compound each sodium atom has a distorted octahedral molecular geometry with coordination to molybdenum atoms and water molecules. The experimental evidence is supported by computed NICS aromaticity values.

Möbius aromaticity

In organic chemistry, Möbius aromaticity is a special type of aromaticity believed to exist in a number of organic molecules. In terms of molecular orbital theory these compounds have in common a monocyclic array of molecular orbitals in which there is an odd number of out-of-phase overlaps, the opposite pattern compared to the aromatic character to Hückel systems. The nodal plane of the orbitals, viewed as a ribbon, is a Möbius strip, rather than a cylinder, hence the name. The pattern of orbital energies is given by a rotated Frost circle (with the edge of the polygon on the bottom instead of a vertex), so systems with 4n electrons are aromatic, while those with 4n + 2 electrons are anti-aromatic/non-aromatic. Due to incrementally twisted nature of the orbitals of a Möbius aromatic system, stable Möbius aromatic molecules need to contain at least 8 electrons, although 4 electron Möbius aromatic transition states are well known in the context of the Dewar-Zimmerman framework for pericyclic reactions. Möbius molecular systems were considered in 1964 by Edgar Heilbronner by application of the Hückel method, but the first such isolable compound was not synthesized until 2003 by the group of Rainer Herges. However, the fleeting trans-C9H9+ cation, one conformation of which is shown on the right, was proposed to be Möbius aromatic in 1998 based on computational and experimental data.

Nitrogenous base

A nitrogenous base, or nitrogen-containing base, is an organic molecule with a nitrogen atom that has the chemical properties of a base. The main biological function of a nitrogenous base is to bond nucleic acids together. A nitrogenous base owes its basic properties to the lone pair of electrons of a nitrogen atom.

Nitrogenous bases are typically classified as the derivatives of two parent compounds, pyrimidine and purine. They are non-polar and due to their aromaticity, planar. Both pyrimidines and purines resemble pyridine and are thus weak bases and relatively unreactive towards electrophilic aromatic substitution.

Silabenzene

A silabenzene is a heteroaromatic compound containing one or more silicon atoms instead of carbon atoms in benzene. A single substitution gives silabenzene proper; additional substitutions give a disilabenzene (3 theoretical isomers), trisilabenzene (3 isomers), etc.

Silabenzenes have been the targets of many theoretical and synthetic studies by organic chemists interested in the question of whether analogs of benzene with Group IV elements heavier than carbon, e.g., silabenzene, stannabenzene and germabenzene—so-called "heavy benzenes"—exhibit aromaticity.

Although several heteroaromatic compounds bearing nitrogen, oxygen, and sulfur atoms have been known since the early stages of organic chemistry, silabenzene had been considered to be a transient, un-isolable compound and was detected only in low-temperature matrices or as its Diels-Alder adduct for a long time. In recent years, however, a kinetically stabilized silabenzene and other heavy aromatic compounds with silicon or germanium atoms have been reported.

Simple aromatic ring

Simple aromatic rings, also known as simple arenes or simple aromatics, are aromatic organic compounds that consist only of a conjugated planar ring system. Many simple aromatic rings have trivial names. They are usually found as substructures of more complex molecules ("substituted aromatics"). Typical simple aromatic compounds are benzene, indole, and pyridine.Simple aromatic rings can be heterocyclic if they contain non-carbon ring atoms, for example, oxygen, nitrogen, or sulfur. They can be monocyclic as in benzene, bicyclic as in naphthalene, or polycyclic as in anthracene. Simple monocyclic aromatic rings are usually five-membered rings like pyrrole or six-membered rings like pyridine. Fused aromatic rings consist of monocyclic rings that share their connecting bonds.

Spherical aromaticity

In organic chemistry, spherical aromaticity is formally used to describe an unusually stable nature of some spherical compounds such as fullerenes, polyhedral boranes.

In 2000, Andreas Hirsch and coworkers in Erlangen, Germany, formulated a rule to determine when a fullerene would be aromatic. They found that if there were 2(n+1)2 π-electrons, then the fullerene would display aromatic properties. This follows from the fact that an aromatic fullerene must have full icosahedral (or other appropriate) symmetry, so the molecular orbitals must be entirely filled. This is possible only if there are exactly 2(n+1)2 electrons, where n is a nonnegative integer. In particular, for example, buckminsterfullerene, with 60 π-electrons, is non-aromatic, since 60/2 = 30, which is not a perfect square.In 2011, Jordi Poater and Miquel Solà, expanded the rule to determine when a fullerene species would be aromatic. They found that if there were 2n2+2n+1 π-electrons, then the fullerene would display aromatic properties. This follows from the fact that a spherical species having a same-spin half-filled last energy level with the whole inner levels being fully filled is also aromatic. It is similar to Baird's rule.

Spiro compound

A spiro compound, or spirane, from the Latin spīra, meaning a twist or coil, is a chemical compound, typically an organic compound, that presents a twisted structure of two or more rings (a ring system), in which 2 or 3 rings are linked together by one common atom, examples of which are shown at right.

The simplest spiro compounds are bicyclic (having just two rings), or have a bicyclic portion as part of the larger ring system, in either case with the two rings connected through the defining single common atom. The one common atom connecting the participating rings distinguishes spiro compounds from other bicyclics: from isolated ring compounds like biphenyl that have no connecting atoms, from fused ring compounds like decalin having two rings linked by two adjacent atoms, and from bridged ring compounds like norbornane with two rings linked by two non-adjacent atoms.

Spiro compounds may be fully carbocyclic (all carbon) or heterocyclic (having one or more non-carbon atom). One common type of spiro compound encountered in educational settings is a heterocyclic one— the acetal formed by reaction of a diol with a cyclic ketone. The common atom that connects the two (or sometimes three) rings is called the spiro atom; in carbocyclic spiro compounds like spiro[5.5]undecane (see image at right), the spiro-atom is a quaternary carbon, and as the -ane ending implies, these are the types of molecules to which the name spirane was first applied (though it is now used general of all spiro compounds). Likewise, a tetravalent neutral silicon or positively charged quaternary nitrogen atom (ammonium cation) can be the spiro center in these compounds, and many of these have been prepared and described. The 2-3 rings being joined are most often different in nature, though they, on occasion, be identical [e.g., spiro[5.5]undecane, just shown, and spirapentadiene, at right]. Although sketches of organic structures makes spiro compounds appear planar, they are not; for instance, a spiro compound with a pair of three-membered cyclopropene rings connected in spiro fashion (image below) has been given the popular misnomer of being a bow tie structure, when it is not flat or planar like a bow tie. This can be stated another way, saying that the best-fit planes to each ring are often perpendicular or are otherwise non-coplanar to one another.Spiro compounds are present throughout the natural world, some cases of which have been exploited to provide tool compounds for biomedical study and to serve as scaffolds for the design of therapeutic agents with novel shapes. As well, the spiro motif is present in various practical compound types (such as dyes), as well as in a wide variety of oligo- and polymeric materials designs, for the unique shapes and properties the spiro center imparts, e.g., in the design of electronically active materials in particular. In both cases, the presence of the spiro center, often with four distinct groups attached, and with its unique aspects of chirality, adds unique challenges to the chemical synthesis of each compound type.

Stacking (chemistry)

In chemistry, pi stacking (also called π–π stacking) refers to attractive, noncovalent interactions between aromatic rings, since they contain pi bonds. These interactions are important in nucleobase stacking within DNA and RNA molecules, protein folding, template-directed synthesis, materials science, and molecular recognition, although new research suggests that pi stacking may not be operative in some of these applications. Despite intense experimental and theoretical interest, there is no unified description of the factors that contribute to pi stacking interactions.

Tapis crude

Tapis crude is a Malaysian crude oil used as a pricing benchmark in Singapore. Tapis is very light, with an API gravity of 43°-45°, and very sweet, with only about 0.04% sulfur. While it is not traded on a market like Brent Crude or West Texas Intermediate (WTI), it is often used as an oil marker for Asia and Australia.The price of Tapis in Singapore is often considerably higher than the price of benchmark crude oils such as Brent or WTI (those commonly referenced in market commentaries). This is because its greater aromaticity (i.e. higher ° API) allows for greater production of higher-value products, such as petrol, than from Brent or WTI. Its high price is also due to the purity of the blend. Because it contains less sulfur it requires less refinery processing than sourer crude oils such as Brent Oil and WTI.Malaysia is the only country which has significant amounts of oil produced as Tapis, from fields located in the South China Sea.

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