Aromatic compound

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2D model of a benzene molecule. The carbon "ring" is what makes benzene "aromatic". Benzene Structural diagram.svg
2D model of a benzene molecule. The carbon "ring" is what makes benzene "aromatic".

Aromatic compounds or arenes usually refers to organic compounds "with a chemistry typified by benzene" and "cyclically conjugated." [1] The word "aromatic" originates from the past grouping of molecules based on odor, before their general chemical properties were understood. The current definition of aromatic compounds does not have any relation to their odor. Aromatic compounds are now defined as cyclic compounds satisfying Hückel's Rule. [2] Aromatic compounds have the following general properties:

Contents

Arenes are typically split into two categories - benzoids, that contain a benzene derivative and follow the benzene ring model, and non-benzoids that contain other aromatic cyclic derivatives. [2] Aromatic compounds are commonly used in organic synthesis and are involved in many reaction types, following both additions and removals, as well as saturation and dearomatization.

Heteroarenes

Heteroarenes are closely related aromatic compounds, where at least one methine or vinylene (-C= or -CH=CH-) group is replaced by a heteroatom: oxygen, nitrogen, or sulfur. [4] 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. Hydrocarbons without an aromatic ring are called aliphatic. Approximately half of compounds known in the year 2000 are described as aromatic to some extent. [5]

Electron flow through p orbitals for the heterocycle furan Orbital overlap of furan.png
Electron flow through p orbitals for the heterocycle furan
Line bond structure of the heterocycle pyridine Pyridine line bond structure.png
Line bond structure of the heterocycle pyridine
Line bond structure of the heterocycle furan Furan line bond structure.png
Line bond structure of the heterocycle furan

Applications

Aromatic compounds are pervasive in nature and industry. Key industrial aromatic hydrocarbons are benzene, toluene, Xylene called BTX. Many biomolecules have phenyl groups including the so-called aromatic amino acids.

Benzene ring model

Line bond structure of benzene Benzene with hydrogens.png
Line bond structure of benzene
Electron flow through p orbitals showing the aromatic nature of benzene Benzene orbitals.png
Electron flow through p orbitals showing the aromatic nature of benzene

Benzene, C6H6, is the least complex aromatic hydrocarbon, and it was the first one defined as such. [7] Its bonding nature was first recognized independently by Joseph Loschmidt and August Kekulé in the 19th century. [7] Each carbon atom in the hexagonal cycle has four electrons to share. One electron forms a sigma bond with the hydrogen atom, and one is used in covalently bonding to each of the two neighboring carbons. This leaves six electrons, shared equally around the ring in delocalized pi molecular orbitals the size of the ring itself. [6] This represents the equivalent nature of the six carbon-carbon bonds all of bond order 1.5. This equivalency can also explained by resonance forms. [6] The electrons are visualized as floating above and below the ring, with the electromagnetic fields they generate acting to keep the ring flat. [6]

The circle symbol for aromaticity was introduced by Sir Robert Robinson and his student James Armit in 1925 and popularized starting in 1959 by the Morrison & Boyd textbook on organic chemistry. [8] The proper use of the symbol is debated: some publications use it to any cyclic π system, while others use it only for those π systems that obey Hückel's rule. Some argue that, in order to stay in line with Robinson's originally intended proposal, the use of the circle symbol should be limited to monocyclic 6 π-electron systems. [9] In this way the circle symbol for a six-center six-electron bond can be compared to the Y symbol for a three-center two-electron bond. [9]

Benzene and derivatives of benzene

Substitution nomenclature of benzene Ortho meta para.png
Substitution nomenclature of benzene

Benzene derivatives have from one to six substituents attached to the central benzene core. Examples of benzene compounds with just one substituent are phenol, which carries a hydroxyl group, and toluene with a methyl group. When there is more than one substituent present on the ring, their spatial relationship becomes important for which the arene substitution patterns ortho, meta, and para are devised. [10] When reacting to form more complex benzene derivatives, the substituents on a benzene ring can be described as either activated or deactivated, which are electron donating and electron withdrawing respectively. [10] Activators are known as ortho-para directors, and deactivators are known as meta directors. [10] Upon reacting, substituents will be added at the ortho, para or meta positions, depending on the directivity of the current substituents to make more complex benzene derivatives, often with several isomers. Electron flow leading to re-aromatization is key in ensuring the stability of such products. [10]

For example, three isomers exist for cresol because the methyl group and the hydroxyl group (both ortho para directors) can be placed next to each other (ortho), one position removed from each other (meta), or two positions removed from each other (para). [11] Given that both the methyl and hydroxyl group are ortho-para directors, the ortho and para isomers are typically favoured. [11] Xylenol has two methyl groups in addition to the hydroxyl group, and, for this structure, 6 isomers exist.[ citation needed ]

Arene rings can stabilize charges, as seen in, for example, phenol (C6H5–OH), which is acidic at the hydroxyl (OH), as charge on the oxygen (alkoxide –O−) is partially delocalized into the benzene ring.

Non-benzylic arenes

Although benzylic arenes are common, non-benzylic compounds are also exceedingly important. Any compound containing a cyclic portion that conforms to Hückel's rule and is not a benzene derivative can be considered a non-benzylic aromatic compound. [6]

Monocyclic arenes

Of annulenes larger than benzene, [12]annulene and [14]annulene are weakly aromatic compounds and [18]annulene, Cyclooctadecanonaene, is aromatic, though strain within the structure causes a slight deviation from the precisely planar structure necessary for aromatic categorization. [12] Another example of a non-benzylic monocyclic arene is the cyclopropenyl (cyclopropenium cation), which satisfies Hückel's rule with an n equal to 0. [13] Note, only the cationic form of this cyclic propenyl is aromatic, given that neutrality in this compound would violate either the octet rule or Hückel's rule. [13]

Other non-benzylic monocyclic arenes include the aforementioned heteroarenes that can replace carbon atoms with other heteroatoms such as N, O or S. [6] Common examples of these are the six-membered pyrrole and five-membered pyridine, both of which have a substituted nitrogen [14]

Polycyclic aromatic hydrocarbons

Hexabenzocoronene is a large polycyclic aromatic hydrocarbon. Hexabenzocoronene-3D-balls.png
Hexabenzocoronene is a large polycyclic aromatic hydrocarbon.

Polycyclic aromatic hydrocarbons, also known as polynuclear aromatic compounds (PAHs) are aromatic hydrocarbons that consist of fused aromatic rings and do not contain heteroatoms or carry substituents. [15] Naphthalene is the simplest example of a PAH. PAHs occur in oil, coal, and tar deposits, and are produced as byproducts of fuel burning (whether fossil fuel or biomass). [16] As pollutants, they are of concern because some compounds have been identified as carcinogenic, mutagenic, and teratogenic. [17] [18] [19] [20] PAHs are also found in cooked foods. [16] Studies have shown that high levels of PAHs are found, for example, in meat cooked at high temperatures such as grilling or barbecuing, and in smoked fish. [16] [17] They are also a good candidate molecule to act as a basis for the earliest forms of life. [21] In graphene the PAH motif is extended to large 2D sheets. [22]

Reactions

Aromatic ring systems participate in many organic reactions.

Substitution

In aromatic substitution, one substituent on the arene ring, usually hydrogen, is replaced by another reagent. [6] The two main types are electrophilic aromatic substitution, when the active reagent is an electrophile, and nucleophilic aromatic substitution, when the reagent is a nucleophile. In radical-nucleophilic aromatic substitution, the active reagent is a radical. [23] [24]

An example of electrophilic aromatic substitution is the nitration of salicylic acid, where a nitro group is added para to the hydroxide substituent:

Synthesis 5-Nitrosalicylic acid.svg
Aromatic nucleophilic substitution Aromatic nucleophilic substitution.svg
Aromatic nucleophilic substitution

Nucleophilic aromatic substitution involves displacement of a leaving group, such as a halide, on an aromatic ring. Aromatic rings usually nucleophilic, but in the presence of electron-withdrawing groups aromatic compounds undergo nucleophilic substitution. Mechanistically, this reaction differs from a common SN2 reaction, because it occurs at a trigonal carbon atom (sp2 hybridization). [25]

Hydrogenation

Hydrogenation of arenes create saturated rings. The compound 1-naphthol is completely reduced to a mixture of decalin-ol isomers. [26]

NaphtolHydrogenation.svg

The compound resorcinol, hydrogenated with Raney nickel in presence of aqueous sodium hydroxide forms an enolate which is alkylated with methyl iodide to 2-methyl-1,3-cyclohexandione: [27]

ResorcinolHydrogenation.svg

Dearomatization

In dearomatization reactions the aromaticity of the reactant is lost. In this regard, the dearomatization is related to hydrogenation. A classic approach is Birch reduction. The methodology is used in synthesis. [28]

Dearomatization of benzene through the Birch reduction Dearomatization.png
Dearomatization of benzene through the Birch reduction

See also

Related Research Articles

<span class="mw-page-title-main">Phenyl group</span> Cyclic chemical group (–C₆H₅)

In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula C6H5, and is often represented by the symbol Ph. The phenyl group is closely related to benzene and can be viewed as a benzene ring, minus a hydrogen, which may be replaced by some other element or compound to serve as a functional group. A phenyl group has six carbon atoms bonded together in a hexagonal planar ring, five of which are bonded to individual hydrogen atoms, with the remaining carbon bonded to a substituent. Phenyl groups are commonplace in organic chemistry. Although often depicted with alternating double and single bonds, the phenyl group is chemically aromatic and has equal bond lengths between carbon atoms in the ring.

<span class="mw-page-title-main">Aromaticity</span> Chemical property

In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August Wilhelm Hofmann in 1855. There is no general relationship between aromaticity as a chemical property and the olfactory properties of such compounds.

<span class="mw-page-title-main">Aryl group</span> Molecular groups or substituents derived from an aromatic ring

In organic chemistry, an aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used as a placeholder for the aryl group in chemical structure diagrams, analogous to “R” used for any organic substituent. “Ar” is not to be confused with the elemental symbol for argon.

<span class="mw-page-title-main">Nitration</span> Chemical reaction which adds a nitro (–NO₂) group onto a molecule

In organic chemistry, nitration is a general class of chemical processes for the introduction of a nitro group into an organic compound. The term also is applied incorrectly to the different process of forming nitrate esters between alcohols and nitric acid. The difference between the resulting molecular structures of nitro compounds and nitrates is that the nitrogen atom in nitro compounds is directly bonded to a non-oxygen atom, whereas in nitrate esters, the nitrogen is bonded to an oxygen atom that in turn usually is bonded to a carbon atom.

In electrophilic aromatic substitution reactions, existing substituent groups on the aromatic ring influence the overall reaction rate or have a directing effect on positional isomer of the products that are formed. An electron donating group (EDG) or electron releasing group is an atom or functional group that donates some of its electron density into a conjugated π system via resonance (mesomerism) or inductive effects —called +M or +I effects, respectively—thus making the π system more nucleophilic. As a result of these electronic effects, an aromatic ring to which such a group is attached is more likely to participate in electrophilic substitution reaction. EDGs are therefore often known as activating groups, though steric effects can interfere with the reaction.

<span class="mw-page-title-main">Hückel's rule</span> Method of determining aromaticity in organic molecules

In organic chemistry, Hückel's rule predicts that a planar ring molecule will have aromatic properties if it has 4n + 2 π electrons, where n is a non-negative integer. The quantum mechanical basis for its formulation was first worked out by physical chemist Erich Hückel in 1931. The succinct expression as the 4n + 2 rule has been attributed to W. v. E. Doering (1951), although several authors were using this form at around the same time.

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. Typical simple aromatic compounds are benzene, indole, and pyridine.

<span class="mw-page-title-main">Triazine</span> Aromatic, heterocyclic compound

Triazines are a class of nitrogen-containing heterocycles. The parent molecules' molecular formula is C3H3N3. They exist in three isomeric forms, 1,3,5-triazines being common.

Arene substitution patterns are part of organic chemistry IUPAC nomenclature and pinpoint the position of substituents other than hydrogen in relation to each other on an aromatic hydrocarbon.

<span class="mw-page-title-main">Nucleophilic aromatic substitution</span> Chemical reaction mechanism

A nucleophilic aromatic substitution is a substitution reaction in organic chemistry in which the nucleophile displaces a good leaving group, such as a halide, on an aromatic ring. Aromatic rings are usually nucleophilic, but some aromatic compounds do undergo nucleophilic substitution. Just as normally nucleophilic alkenes can be made to undergo conjugate substitution if they carry electron-withdrawing substituents, so normally nucleophilic aromatic rings also become electrophilic if they have the right substituents.

<span class="mw-page-title-main">Cyclodecapentaene</span> Chemical compound

Cyclodecapentaene or [10]annulene is an annulene with molecular formula C10H10. This organic compound is a conjugated 10 pi electron cyclic system and according to Huckel's rule it should display aromaticity. It is not aromatic, however, because various types of ring strain destabilize an all-planar geometry.

<span class="mw-page-title-main">Prelog strain</span> Interactions between atomic groups on different parts of a ring molecule

In organic chemistry, transannular strain is the unfavorable interactions of ring substituents on non-adjacent carbons. These interactions, called transannular interactions, arise from a lack of space in the interior of the ring, which forces substituents into conflict with one another. In medium-sized cycloalkanes, which have between 8 and 11 carbons constituting the ring, transannular strain can be a major source of the overall strain, especially in some conformations, to which there is also contribution from large-angle strain and Pitzer strain. In larger rings, transannular strain drops off until the ring is sufficiently large that it can adopt conformations devoid of any negative interactions.

<span class="mw-page-title-main">Cyclic compound</span> Molecule with a ring of bonded atoms

A cyclic compound is a term for a compound in the field of chemistry in which one or more series of atoms in the compound is connected to form a ring. Rings may vary in size from three to many atoms, and include examples where all the atoms are carbon, none of the atoms are carbon, or where both carbon and non-carbon atoms are present. Depending on the ring size, the bond order of the individual links between ring atoms, and their arrangements within the rings, carbocyclic and heterocyclic compounds may be aromatic or non-aromatic; in the latter case, they may vary from being fully saturated to having varying numbers of multiple bonds between the ring atoms. Because of the tremendous diversity allowed, in combination, by the valences of common atoms and their ability to form rings, the number of possible cyclic structures, even of small size numbers in the many billions.

In organic chemistry, the Hammett equation describes a linear free-energy relationship relating reaction rates and equilibrium constants for many reactions involving benzoic acid derivatives with meta- and para-substituents to each other with just two parameters: a substituent constant and a reaction constant. This equation was developed and published by Louis Plack Hammett in 1937 as a follow-up to qualitative observations in his 1935 publication.

<span class="mw-page-title-main">Vicarious nucleophilic substitution</span>

In organic chemistry, the vicarious nucleophilic substitution is a special type of nucleophilic aromatic substitution in which a nucleophile replaces a hydrogen atom on the aromatic ring and not leaving groups such as halogen substituents which are ordinarily encountered in SNAr. This reaction type was reviewed in 1987 by Polish chemists Mieczysław Mąkosza and Jerzy Winiarski.

<span class="mw-page-title-main">Birch reduction</span> Organic reaction used to convert arenes to cyclohexadienes

The Birch reduction is an organic reaction that is used to convert arenes to 1,4-cyclohexadienes. The reaction is named after the Australian chemist Arthur Birch and involves the organic reduction of aromatic rings in an amine solvent with an alkali metal and a proton source. Unlike catalytic hydrogenation, Birch reduction does not reduce the aromatic ring all the way to a cyclohexane.

Benzylic activation and stereocontrol in tricarbonyl(arene)chromium complexes refers to the enhanced rates and stereoselectivities of reactions at the benzylic position of aromatic rings complexed to chromium(0) relative to uncomplexed arenes. Complexation of an aromatic ring to chromium stabilizes both anions and cations at the benzylic position and provides a steric blocking element for diastereoselective functionalization of the benzylic position. A large number of stereoselective methods for benzylic and homobenzylic functionalization have been developed based on this property.

Heteroatom-promoted lateral lithiation is the site-selective replacement of a benzylic hydrogen atom for lithium for the purpose of further functionalization. Heteroatom-containing substituents may direct metalation to the benzylic site closest to the heteroatom or increase the acidity of the ring carbons via an inductive effect.

Electrophilic aromatic substitution is an organic reaction in which an atom that is attached to an aromatic system is replaced by an electrophile. Some of the most important electrophilic aromatic substitutions are aromatic nitration, aromatic halogenation, aromatic sulfonation, alkylation and acylation Friedel–Crafts reaction.

Ortho effect is a organic chemistry phenomenon where the presence of an chemical group at the at ortho position or the 1 and 2 position of a phenyl ring, relative to the carboxylic compound changes the chemical properties of the compound. This is caused by steric effects and bonding interactions along with polar effects caused by the various substituents which are in a given molecule, resulting in changes in its chemical and physical properties. The ortho effect is associated with substituted benzene compounds.

References

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