An aromatic hydrocarbon or arene is a hydrocarbon with alternating double and single bonds between
carbon atoms forming rings.
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 odor.
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 or polycyclic.
Some non-benzene-based compounds called heteroarenes, which follows Hückel's rule, are also called
as 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 an oxygen atom, and pyridine, a heterocyclic
compound with a six-membered ring containing one nitrogen atom.
Benzene ring model
Benzene, C6H6, is the simplest aromatic hydrocarbon, and it was the first one recognized.
The nature of its bonding was first recognized by August Kekulé in the 19th century.
Each carbon atom in the hexagonal cycle has four electrons to share.
One goes to the hydrogen atom, and one each to the two neighboring carbons.
This leaves one to share with one of its two neighboring carbon atoms, which is why the
benzene molecule is drawn with alternating single and double bonds around the hexagon.
The structure is also illustrated as a circle around the inside of the ring to show six
electrons floating around in delocalized molecular orbitals the size of the ring itself.
This also represents the equivalent nature of the six carbon-carbon bonds all of bond
order ~1.5.
This equivalency is well explained by resonance forms.
The electrons are visualized as floating above and below the ring with the electromagnetic
fields they generate acting to keep the ring flat.
General properties: Display aromaticity.
The carbon-hydrogen ratio is high.
They burn with a sooty yellow flame because of the high carbon-hydrogen ratio.
They undergo electrophilic substitution reactions and nucleophilic aromatic substitutions.
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.
The proper use of the symbol is debated; it is used to describe any cyclic pi system in
some publications, or only those pi systems that obey Hückel's rule on others.
Jensen argues that in line with Robinson's original proposal, the use of the circle symbol
should be limited to monocyclic 6 pi-electron systems.
In this way the circle symbol for a 6c–6e bond can be compared to the Y symbol for a
3c–2e bond.
Arene synthesis A reaction that forms an arene compound from
an unsaturated or partially unsaturated cyclic precursor is simply called an aromatization.
Many laboratory methods exist for the organic synthesis of arenes from non-arene precursors.
Many methods rely on cycloaddition reactions.
Alkyne trimerization describes the [2+2+2] cyclization of three alkynes, in the Dötz
reaction an alkyne, carbon monoxide and a chromium carbene complex are the reactants.Diels-Alder
reactions of alkynes with pyrone or cyclopentadienone with expulsion of carbon dioxide or carbon
monoxide also form arene compounds.
In Bergman cyclization the reactants are an enyne plus a hydrogen donor.
Another set of methods is the aromatization of cyclohexanes and other aliphatic rings:
reagents are catalysts used in hydrogenation such as platinum, palladium and nickel, quinones
and the elements sulfur and selenium.
Arene reactions Arenes are reactants in many organic reactions.
Aromatic substitution In aromatic substitution one substituent on
the arene ring, usually hydrogen, is replaced by another substituent.
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.
An example of electrophilic aromatic substitution is the nitration of salicylic acid:
Coupling reactions In coupling reactions a metal catalyses a
coupling between two formal radical fragments.
Common coupling reactions with arenes result in the formation of new carbon–carbon bonds
e.g., alkylarenes, vinyl arenes, biraryls, new carbon–nitrogen bonds or new carbon–oxygen
bonds.
An example is the direct arylation of perfluorobenzenes
Hydrogenation Hydrogenation of arenes create saturated rings.
The compound 1-naphthol is completely reduced to a mixture of decalin-ol isomers.
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:
Cycloadditions Cycloaddition reaction are not common.
Unusual thermal Diels-Alder reactivity of arenes can be found in the Wagner-Jauregg
reaction.
Other photochemical cycloaddition reactions with alkenes occur through excimers.
Benzene and derivatives 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.
For example, three isomers exist for cresol because the methyl group and the hydroxyl
group can be placed next to each other, one position removed from each other, or two positions
removed from each other.
Xylenol has two methyl groups in addition to the hydroxyl group, and, for this structure,
6 isomers exist.
Representative arene compounds
The arene ring has an ability to stabilize charges.
This is seen in, for example, phenol, which is acidic at the hydroxyl, since a charge
on this oxygen is partially delocalized into the benzene ring.
Polycyclic aromatic hydrocarbons
Polycyclic aromatic hydrocarbons are aromatic hydrocarbons that consist of fused aromatic
rings and do not contain heteroatoms or carry substituents.
Naphthalene is the simplest example of a PAH.
PAHs occur in oil, coal, and tar deposits, and are produced as byproducts of fuel burning.
As pollutants, they are of concern because some compounds have been identified as carcinogenic,
mutagenic, and teratogenic.
PAHs are also found in cooked foods.
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.
They are also found in the interstellar medium, in comets, and in meteorites and are a candidate
molecule to act as a basis for the earliest forms of life.
In graphene the PAH motif is extended to large 2D sheets.
See also Aromatic substituents: Aryl, Aryloxy and Arenediyl
Asphaltene Hydrodealkylation
Simple aromatic rings References
External links Media related to Aromatic hydrocarbons at
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