Aromatization is a chemical reaction in which an aromatic system is
formed. It can also refer to the production of a new aromatic moiety
in a molecule which is already aromatic. Theoretically, this can be
achieved by dehydrogenation of existing cyclic compounds (such as in
converting cyclohexane into benzene) or by formation of new cyclic
system (such as in the cyclotrimerization of acetylene to benzene);
practically, other moieties are typically required to carry out such
conversion, and other approaches like applying condensation reactions
Aromatization includes the formation of any aromatic
system (including heterocyclic systems), and is not restricted to
benzene and its derivatives.
1.3 Characteristics of aromatic systems
2 Significance of aromatic systems
3 Formation of new cyclic systems
3.2 Other cyclizations
4 From existing cyclic systems
4.1 Ionic systems
5 See also
Main article: benzene
The substance now called benzene, C6H6, was known as a component of
Southeast Asian aromatic resins used in perfumery since the 15th
century. It was first isolated and identified by
Michael Faraday in
1825 who named it bicarburet of hydrogen. In 1845, Charles
Mansfield, working under August Wilhelm von Hofmann, isolated benzene
from coal tar and began the industrial-scale production based on this
method four years later. Over time, consensus developed that
other substances were chemically related to benzene, comprising a
diverse chemical family; Hofmann used the word "aromatic" for this
family for the first time in 1856.
Historic benzene formulae (from left to right) by Claus (1867), Dewar
(1867), Ladenburg (1869), Armstrong (1887), Thiele (1899), and Kekulé
Dewar benzene and prismane are different chemicals that have
Dewar's and Ladenburg's structures. Thiele and Kekulé's structures
are used today.
The various representations of benzene
Historic interest in benzene arose from the uncertainty over its
structure and reactivity. It was well established that it had a carbon
to hydrogen ratio of 1:1 which suggested the presence of double or
triple bond carbon-to-carbon bonds, yet such unsaturated compounds
typically undergo addition reactions, which benzene does not. Numerous
potential structures were proposed, including by Claus, Dewar,
Ladenburg, Armstrong, and Thiele. The most influential
proposal was that of Kekulé who in 1865 suggested a cyclic structure
of six carbon atoms with alternating single and double bonds
Kekulé based his argument on the now well-known observations of arene
substitution patterns, namely that there are always only one isomer of
any monoderivative of benzene and three isomers of every disubstituted
derivative. Critics noted that Kekulé's proposal implied that
there should be two distinguishable isomers for an ortho-substitution
(a 1,2-disubstituion) depending on whether a single or double bond
joined these two carbons. Kekulé suggested in reply that benzene had
two complementary structures and that these forms rapidly
interconverted, which would also explain the lack of addition
reactions (as the valency of each carbon atom is not that expected of
a double bond). Though this interconversion is incorrect —in fact,
they are both resonance contributors to the actual structure (which is
closest to Thiele's structure)— the geometry and structure proposed
by Kekulé are correct.
Benzene is the prototypical example of an
aromatic compound, a hitherto unknown class of compounds, and its
synthesis was the first example of aromatization, the process and
chemical reactions whereby an aromatic system is formed. The new
understanding of benzene and all aromatic compounds was so important
for both pure and applied chemistry that in 1890 the German Chemical
Society organized an elaborate appreciation in Kekulé's honor,
celebrating the twenty-fifth anniversary of his first benzene
paper; its 150th anniversary was also marked. The cyclic
nature of benzene was confirmed crystallographically by Kathleen
Lonsdale in 1929.
X-ray diffraction confirms that all six carbon-carbon bonds in benzene
are the same length, 140 pm (1.40 Å) – intermediate
between the bond lengths of single (154 pm) and double
(134 pm) carbon-carbon bonds. This is consistent with
delocalization of the electrons from the double bonds in Kekulé's
structures equally between each of the six carbon atoms which form a
planar ring. The molecular orbital theory description involves the
formation of three delocalized π orbitals spanning all six carbon
atoms, while the valence bond theory description involves a
superposition of resonance structures. This electronic
structure provides benzene with unusual stability and contributes to
the molecular and chemical properties which are now known as
aromaticity. To accurately reflect the nature of the bonding as having
neither single nor double bonds, but rather something in between,
benzene is often depicted with a circle inside a hexagonal arrangement
of carbon atoms, similar to the structure first illustrated by
Main article: aromaticity
Crystal structure of a fullerene bound in a buckycatcher through
In organic chemistry, the term aromaticity has come to refer to cyclic
(ring-shaped), planar (flat) organic compounds with a ring of
resonance bonds that exhibit unusually high stability relative to
other geometric or connective arrangements with the same set of
Aromatic molecules consequently do not easily break apart
and react with other substances (they have low chemical reactivity)
and exhibit special physical characteristics such as π
stacking (the understanding of which is still the subject of
ongoing investigation). In terms of electronic structure,
aromaticity describes a conjugated system made of alternating
notionally single and double bonds, plus in some cases a lone pair
from an occupied p-orbital perpendicular to the plane of the ring. The
term "aromatic sextet" for the electronic system which resists
disruption (and hence possesses unusual stability) is attributed to
Sir Robert Robinson though the idea can be traced further to
Crocker and Armstrong. This configuration allows for the
delocalization of π electrons around the ring, increasing the
molecule's stability. Such molecules cannot be accurately represented
by a single structure and are understood to exist as a combination of
resonance hybrids, though single representations are frequently chosen
as a short-hand for convenience. As an example, the cyclohexatriene
structures Kekulé proposed should have alternating single and double
bonds of different lengths, yet each is the same and has a length
consistent with benzene having six "one-and-a-half" bonds which is
logically the superposition of each resonance contributor. A more
accurate representation of the electrons following the merging of p
orbitals into π-bonds (see illustration above) is Armstrong's inner
cycle model as the electron density is evenly distributed around the
aromatic ring in molecular orbitals considered to have π
symmetry, though this representation is inconvenient in arrow
pushing in mechanistic organic chemistry. The quantum mechanical
origins of aromaticity were first modelled by Hückel in
Characteristics of aromatic systems
An aromatic (or aryl) compound contains a set of covalently bound
atoms with specific characteristics:
Pyrrole (left) is a much weaker base than pyridine (right), despite
both heterocycles having a lone pair on the nitrogen atom
A delocalized conjugated π system, most commonly an arrangement of
alternating single and double bonds
Coplanar structure, with all the contributing atoms in the same plane
Contributing atoms arranged in one or more rings
A number of 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.
Systems following Hückel's rule, having 4n + 2
π-electrons, are aromatic, but if there are 4n π-electrons along
with characteristics 1–3 above, the molecule is said to be
Antiaromatic systems are destabilized and sometimes
adopt distorted structures to avoid planarity. 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. For example, in
furan, the oxygen atom is sp2 hybridized. One lone pair (in the pz
orbital) contributes to the π system and the other in the plane of
the ring (in the non-bonding sp2 orbital and analogous to the C–H
bond in the other positions) does not; consequently, there are 6
π-electrons in furan and it is aromatic. Comparing the nitrogen
heterocycles pyrrole and pyridine, each has a 6 π-electron system:
the lone pair on the nitrogen atom is required to complete the
aromatic sextet in the case of pyrrole while the lone pair of in
pyridine is not required. The difference can be seen in the basicity
of the two systems, as pyridine can be easily protonated (its lone
pair is available for reaction) whilst pyrrole does not react
readily. This can be seen in the pKa values of the respective
conjugate acids – 8.25 for pyridine compared with −0.27 for
pyrrole. In the case of oxazole and isoxazole, the lone pairs
contributed to the aromatic sextet come from the occupied pz orbital
of oxygen atom rather than the nitrogen lone pairs in hybridized sp2
Electrostatic potential surfaces illustrate the difference
in electronegativity of the heteroatom bearing an electron pair
depending on whether or not it forms part of the aromatic system. In
the diagram below, the nitrogen atom in pyridine is markedly more
electronegative than nitrogen atom in indole, as indicated by the red
regions on the surfaces, because the pyridine nitrogen is free to act
Lewis base whereas the indole nitrogen is not. The surfaces also
illustrate that heteroatoms influence the ability to form cation–pi
interactions as cations will be attracted to the aromatic system much
more strongly in indole and in benzene than will be the case in
pyridine where a direct interaction with the lone pair is more
Electrostatic potential surfaces for indole, benzene, and pyridine
(left to right), available from the Spartan software suite of
computational chemistry tools. Shading colors vary from blue
(electropositive areas) to red (electronegative areas) and are
determined by calculating the energy of interaction of a positive
spherical point charge (a proton) as it moves over the surface of the
An aromatic ring current (orange) interacts with an applied magnetic
field, B0 (red arrow) and induces a magnetic field (purple) which
changes the chemical shifts in
Molecules which can possess an aromatic system will tend to undergo
any electronic or conformational structure changes needed to attain
aromaticity. Chemical changes which occur as a consequence include a
tendency to undergo electrophilic aromatic substitution and
nucleophilic aromatic substitution reactions, but not electrophilic
addition reactions as happens with carbon–carbon double bonds.
This chemical behaviour offers one way to confirm experimentally that
an aromatic system is present. The nuclear magnetic resonance (NMR)
phenomenon offers a spectroscopic method for demonstrating and
investigating aromatic systems as the circulating π-electrons produce
ring currents which oppose the applied magnetic field and lead to
changes in the measured chemical shifts for both 1H and 13C
nuclei. X-ray crystal structures confirm the existence of
π-π interactions in aromatic systems, such as the dimer of benzene;
Perpendicular and offset parallel configurations can be observed in
the crystal structures of many simple aromatic compounds. More
recently, direct measures of electron delocalization allows direct
quantification of the extent of aromaticity.
Selection of simple hydrocarbon and heterocyclic aromatic systems
Significance of aromatic systems
Zwitterionic form of histidine, with the aromatic side chain including
an imidazole functionality shown in blue
Research work undertaken to understand the benzene system opened a new
field in organic chemistry leading to the modern understanding of
aromatic systems as common throughout the chemical world. Benzene
itself is a useful solvent as well as a gasoline additive due to its
high octane number and ability to reduce engine knocking, although
methylbenzene is now being used as an alternative.
Benzene is also a
precursor used in the manufacture of styrene, nylon, and epoxy resins.
Naphthalene is the active ingredient in traditional mothballs and is
used to make the insecticide carbaryl.
Aromatic systems appear in many
other important industrial products including Bakelite, polystyrene,
and PET plastics.
Aromatic systems are common in biological systems. Amongst the 20
standard naturally occurring amino acids necessary for protein
synthesis, four are aromatic – histidine, phenylalanine, tryptophan,
and tyrosine. (
Histidine is aromatic at all pH levels due to its
imidazole side-chain but is also basic and so is often grouped
with the two other basic amino acids arginine and lysine.)
Aromatic amino acids also serve as the starting materials for
biosynthesis of vitamins, neurotransmitters and hormones including
dopamine and norepinephrine from tyrosine, serotonin and niacin
from tryptophan, and tyrosine, thyroxine, and
N-methylphenethylamine from phenylalanine. Pharmaceuticals
with many different applications possess aromatic moieties, as
Selection of pharmaceuticals containing aromatic systems
Beta blocker and antihypertensive
Leukemia chemotherapeutic agent
All five of the bases found in
RNA are aromatic – adenine
and guanine (purine core structures), and cytosine, thyamine, and
uracil (pyrimidine core structures). Biologically active
substances including caffeine and uric acid (responsible for gout)
also possess purine structures.
RNA have polymeric structures
with repeating nucleotide units, each possessing a phosphate group, a
ribose or deoxyribose sugar, and a nucleobase. Nucleobases are
Watson-Crick base paired through hydrogen bonding, and these pairs
are stacked through the center of the double helix through π-π
interactions of their aromatic systems. The possibility for
extended planar aromatic systems such as acridine
(dibenzo[b,e]pyridine) to insert between base pairs was recognised by
Lerman in a process now known as intercalation.
Intercalating agents have applications in biochemical research as
stains (ethidium bromide) and in medicine as chemotherapeutic
agents (doxorubicin). Polycyclic aromatic hydrocarbons like
benzopyrenes and dibenz[a,h]anthracene are some of the 43 known
carcinogens in tobacco smoke (the Environment Protection Agency
classes tobacco smoke as a class A carcinogen). Benzo[a]pyrene has
been shown to form preferentially in lung cancer hotspots and is
known to be transformed into a metabolite,
(+)-Benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide, which forms mutagenic
adduct with DNA. The epoxide and group on the metabolite alkylate
the N2 of guanine bases while the pyrene system intercalates between
base pairs, disrupting the structure. Intercalation interferes with
DNA replication which can inhibit tumour growth but can also cause
changes in the base pair sequence which explains its potential for
(+)-Benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide, the metabolite of
benzo[a]pyrene which binds with DNA
(Left): A segment of
DNA with nucleobase pairs π-stacked through the
center of the double helix.
(Center): Intercalation distorts the
DNA structure by inserting
suitable extended aromatic systems between base pairs. Ethidium
bromide is shown along with an example of its intercalation.
(Right): Tobacco smoke carcinogen and mutagen benzo[a]pyrene
metabolises to (+)-Benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide which
reacts with a guanine base and intercalates the pyrene system between
Formation of new cyclic systems
Bönnemann cyclization to produce pyridine derivatives
Many aromatization processes start with two or three acyclic molecules
which are fused into a new aromatic ring.
Alkyne trimerization is one
simple case whereby the [2+2+2] cycloaddition of three alkynes yields
a substituted benzene – cyclization of
hexamethylbenzene, for example.
Benzene itself can be prepared by
Reppe synthesis with the ligands on the nickel catalyst dictating
whether production of benzene or cyclooctatetraene predominates. If
an unsymmetric alkyne is used, a variety of products is formed;
Rhodium-catalysed cyclotrimerization of phenylacetylene yields both
the 1,2,4- and 1,3,5- isomers of triphenylbenzene along with
substantial quantities of acyclic enyne products, though use of an
alternative catalyst system can produce 97% selectivity for the
Antiaromatic cyclobutadiene-derivatives and
non-aromatic cyclooctatetraene products are also observed
side-products of cyclotrimerizations, and when a mixture of
substituted acetylenes is used regioselectivity becomes especially
problematic. Replacement of one acetylene by a nitrile allows
formation of pyridine or one of its derivatives by Bönnemann
Alkyne trimerization involving an aryne generated in situ
Cyclisation carried out with a diyne and a separate alkyne affords
greater control as well as high atom economy. Using commercially
available cyclopentadienylcobalt dicarbonyl, CpCo(CO)2, as catalyst,
bis(trimethylsilyl)acetylene (BTMSA) will react with a
diyne-1,2-disubstituted benzene to form an anthroquinone aromatic
system as shown below: Benzyne, generated in situ from a bezene
ring bearing ortho-distributed triflate and trimethylsilyl
substituents, can be used to generate an aryne in place of an
acetylene and combined with a suitable diyne. Such a benzene
derivative reacts with 1,7-octadiyne in the presence of a suitable
catalyst to generate a naphthalene system. This is an example of a
hexadehydro Diels-Alder reaction.
If three alkyne moieties are tethered together, it is possible to
create three rings in a single step without the problems with side
products mentioned above. An example is seen in the synthesis of
Wilkinson's catalyst to achieve an
Bergman cyclization is conceptually similar, using an enediyne
with (Z)-stereochemistry plus a hydrogen donor. (Z)-hex-3-en-1,5-diyne
cyclizes to benzene after reduction of the intermediate diradical, for
example. The enediyne moiety can be included within an existing
ring, allowing access to a bicyclic system under mild conditions as a
consequence of the ring strain in the reactant.
Cyclodeca-3-en-1,5-diyne reacts with
1,3-cyclohexadiene to produce
benzene and tetralin at 37 °C, the reaction being highly
favorable owing to the formation of two new aromatic rings:
Wulff–Dötz reaction an alkyne, carbon monoxide and a
chromium carbene complex are the reactants and the product is a
chromium half-sandwich complex. The phenol ligand can be
isolated by mild oxidation with ceric ammonium nitrate.
Diels-Alder synthesis of hexaphenylbenzene from tetracyclone
Diels–Alder reactions of alkynes with 2-pyrone or
cyclopentadienone with expulsion of carbon dioxide or carbon monoxide,
respectively, also form arene compounds.
Tetracyclone reacts with
diphenylacetylene to form hexaphenylbenzene after loss of carbon
monoxide, for example. The
Wagner-Jauregg reaction involves a
double Diels-Alder addition of maleic anhydride to a
1,1-diarylethylene, resulting in an aryl-substituted naphthalene
product. The reaction is unusual in that the aromaticity of the
starting material is lost as the aryl-substituted styrene moiety is
sufficiently activated to act as a dienophile, but the intermediate
undergoes re-aromatization with sulfur.
In a Huisgen cycloaddition, an azide is combined with an alkyne to
generate a triazole, and is a specific example of a 1,3-dipolar
cycloaddition to a form a heterocyclic five-membered aromatic rings
with regio- and stereoselectivity. American chemist Barry
Sharpless has described this cycloaddition as "the cream of the crop"
of click chemistry and "the premier example of a click
Synthesis of 2,3-diphenylquinoxaline
Condensation reactions are another route to the formation of
heterocyclic aromatic systems. The Chichibabin pyridine synthesis
involves condensation of acrolein (from a Knoevenagel reaction of
acetaldehyde with formaldehyde) with acetaldehyde and ammonia,
followed by catalysed oxidation of dihydropyridine to
pyridine. Hantzsch's approach to pyridines dates from 1881 and
involves condensation of a β-keto acid or ester with an aldehyde and
a nitrogen donor (typicallyammonia or one of its salts.
Knoevenagel modified this process for asymmetrically substituted
Quinoxaline systems can be formed by the
condensation of 1,2-diketones with o-diaminobenzene, quinoxaline
itself forming from glyoxal and o-phenylenediamine. Combining
benzil with o-phenylenediamine in the presence of 2-iodobenzoic acid
(IBX, catalyst) yields 2,3-diphenylquinoxaline in high yield.
Synthesis of pentaphenylarsole from diphenylacetylene
Another effective means of generating heterocyclic systems is to
effect ring-closure using the heteroatom. "Moderately aromatic" arsole
derivatives can be formed by dimerising diphenylacetylene with lithium
to form 1,4-dilithiotetraphenyl-1,3-butadiene. Phenylarsenous
dichloride is added to close the ring to form pentaphenylarsole.
The diiodo analogue of the lithium salt can be used in its place, and
other pnictogen heterocycles can be produced by replacing the arsenic
compound as appropriate.
The high stability of aromatic systems was demonstrated as early as
1880 in a one-pot synthesis of hexamethylbenzene from methanol. Though
the authors did not understand what the process occurring, they
interpreted it as a condensation of methylene (CH2) units followed by
Friedel-Crafts alkylation with chloromethane (generated in situ) to
exhaustively methylate the resulting ring. At 283 °C, the
melting point of the zinc chloride catalyst, the reaction has a ΔG of
−1090 kJ mol−1.
3OH → C
6 + 3 CH
4 + 15 H
From existing cyclic systems
Semmler-Wolff synthesis of aniline
Non aromatic rings can be aromatized through a variety of simple
transformations. Dehydration allows the Semmler-Wolff transformation
2-cyclohexenone oxime to aniline under acidic conditions. In
applying redox processes, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) and an acid catalyst has been used to synthesise a steroid with
a phenanthrene core by oxidatation accompanied by a double methyl
migration. In the process, DDQ is itself reduced into an aromatic
The p-cymene ruthenium(II) dichloride dimer, [(η6-p-cymene)RuCl2]2
Redox reactions involving transition metals can also be used to
produce aromatic systems which then serve as ligands in organometallic
is oxidised to p-iso-propyltoluene with the reduction of ruthenium
trichloride. The resulting ruthenium(II) dimer
[(η6-cymene)RuCl2]2] can be cleaved to a monomeric adduct with a
Lewis base – [(η6-cymene)RuCl2(PPh3)] is formed with
triphenylphosphine, for example. Exchange reactions occur with other
arenes and so the p-cymene product can be recovered.
Ciamician-Dennstedt rearrangement of a pyrrole to a pyridine
The 10 π-electron system cyclodecapentaene is non-aromatic as it is
non-planar, but aromatic derivatives can be prepared. In the following
reaction sequence, naphthalene is converted through a series of
non-aromatic intermediates to 1,6-methanoannulene by Birch
reduction, carbene addition, and then DDQ oxidation. Many other
conversions of one aromatic system to another are known. One simple
example is the industrial synthesis of pyrrole by exchanging the
oxygen atom in furan for a nitrogen moiety.
Ammonia and solid acid
catalysts (like SiO2 and Al2O3) are required.
Pyrrole can be
converted to 3-chloropyridine by insertion of a carbene into the
Dichlorocarbene adds to form a strained bicyclic
cyclopropane system which then opens to form the six-member pyridine
product, a transformation known as the Ciamician-Dennstedt
Aromatase is an enzyme which converts some enones with a six membered
ring to aromatic rings - specifically testosterone to estradiol and
androstenedione to estrone. Each of these aromatizations involves the
oxidation of the C-19 methyl group to formic acid to allow for the
formation of the aromatic system, conversions which are necessary
parts of estrogen tumorogenesis in the development of breast cancer
and ovarian cancer in postmenopausal women and gynecomastia in
Aromatase inhibitors like exemestane (which forms a
permanent and deactivating bond with the aromatase enzyme) and
anastrozole and letrozole (which compete for the enzyme) have
been shown to be more effective than anti-estrogen medications such as
tamoxifen likely because they prevent the formation of estradiol.
Structural formula of the tropylium cation
Aromaticity can be found in individual ions which satisfy Hückel's
rule, which can be accessed by simple redox or acid-base reactions. A
Hückel n = 0 cation can be prepared from cyclopropene
systems, an example being the bromide salt of the
triphenylcycloprenium cation. Ionic Hückel n = 1
systems are known both for anions (cyclopentadienyl) and cations
(tropylium). Tropylium, C
7, was first reported in 1881 though its identity was not
recognized for over 60 years. Tropylium appears regularly in
mass spectrometry, given a signal at m/z = 91. Compounds
containing the benzyl moiety produce the molecular fragment PhCH+
2, which isomeizes to the stable tropylium cation. Cycloheptatriene
(non-aromatic) undergoes a redox reaction with bromine to produce
tropylium with the release of hydrogen bromide; it can also be
oxidised with chromic acid and a ring-contracting rearrangement occurs
in which benzaldehyde is produced:
8 + Br
2 → C
7 + Br− + HBr
7 + HCrO−
4 → C
5CHO + CrO
2 + H
Thallium cyclopentadienide, an organometallic polymer of the
Cyclopentadiene is more acidic than a typical hydrocarbon
(pKa ~ 15-16 compared with cyclopentane's
pKa ~ 45) and so can be readily deprotonated to its aromatic
conjugate base, an approach used to form compounds like thallium
cyclopentadienide, an example of an organometallic polymer.
A direct redox reaction of sodium metal with cyclopentadiene yields
sodium cyclopentadienide, though a convenient base like sodium
hydride can also be used. Compounds like these can be used to
prepare metallocene compounds such as ferrocene where two
cyclopentadienyl ligands coordinated to a metal centre.
Freshly-cracked cyclopentadiene will react with iron(II) chloride and
a base to effect deprotonation to produce ferrocene in good yield.
Even a weak bases such as amines are sufficient to deprotonate
cyclopentadiene for this reaction. Like most aromatic substances,
the product here undergoes substitution reactions rather than addition
reactions. As an example,
Friedel-Crafts acylation of ferrocene with
acetic anhydride yields acetylferrocene just as acylation of
benzene yields acetophenone under similar conditions.
Corannulene, a circulene, is a fragment of buckminsterfullerene
which is found in the buckycatcher shown above and often called a
buckybowl due to its bowl-like shape. Motivation for its synthesis was
the hypothesis that it would display annulene-within-an-annulene
aromaticity, with a 6 π-electron cyclopentadienyl anion core within a
14 π-electron annulenyl cation. Later work has questioned the
validity of this model though the molecule is definitely
aromatic. The dianion has been demonstrated to be
antiaromatic and the tetraanion is aromatic.
Sandwich compound uranocene, [U(η8-C8H8)2]
Cyclooctatetraene in its native "tub-shaped" conformation
Annulene-within-an-annulene model of corannulene aromaticity
Cyclooctatetraene has been shown to be non-aromatic and to
possess a bent "tub-shaped" conformation with alternating double and
single bonds. Its planar conformation would be antiaromatic
and is thus disfavoured. By contrast, its dianion C
8 is both planar and aromatic and has a Hückel electron count of 10.
It is formed readily by direct reaction with potassium metal and
uranocene, an analogue of ferrocene, has been prepared from
dipotassium cyclooctatetraenide and uranium tetrachloride in
tetrahydrofuran at 0 °C:
Equilibrium between phenol and its cyclohexadienone tautomer
1,4-Dioxotetralin in equilibrium with 1,4-naphthalenediol
An isomerization reaction involves the rearrangement of atoms within a
single molecule to one of its isomers. Some isomerization processes
may occur spontaneously, resulting in an equilibrium between the
isomers, and the position of the system generally favors an aromatic
product if the process is itself an aromatization. Amongst keto-enol
tautomers, the keto form is generally favoured on by thermodynamics,
but in the case of cyclohexa-2,4-dienone its aromatic isomer phenol is
strongly favoured. The equilibrium constant for this system has
been determined to be 10−13, meaning that in phenol there are 10
trillion molecules in the aromatic enol form for every molecule in the
Isomerization reactions are temperature dependent, for
example melting 1,4-naphthalenediol at 200 °C produces a 2:1
mixture with its keto form, 1,4-dioxotetralin. The conversion to
the 1,4-dioxotetralin tautomer can be driven to completion when bound
in a chromium tricarbonyl piano stool complex.
Representation of two resonance forms of azulene
A non-tautomeric example of isomerization can be seen in the
azulene–napthalene system, a thermal rearrangement in which
each of the compounds is stable at room temperature; such
interconversion aromatization reactions are rare. Carbon-13
isotope studies have shown scrambling of the α- and β- positions of
naphthalene through an azulene intermediate at elevated
temperatures though naphthalene has roughly double the aromatic
stabilization of azulene. The synthesis of the azulene system is an
example of a multi-step organic synthesis, in this case from
cycloheptatriene, as shown below.
Azulene is unusual in that it
is a polar hydrocarbon – the molecular dipole moment of azulene is
1.08 D compared with naphthalene's 0 D. This can be
understood by recognizing that one of the important resonance
contributors for azulene has the tropylium cation fused to the
cyclopentadienyl anion; reactivity studies confirm the electrophilic
and nucleophilic characteristics of these rings, respectively.
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