Azulene is an
organic compoundAn organic compound is any member of a large class of chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of compounds such as carbonates, simple oxides of carbon and cyanides, as well as the allotropes of carbon, are considered inorganic...
and a
isomerIn organic chemistry, two molecules are valence isomers when they are constitutional isomers that can interconvert through pericyclic reactions. For example, Dewar benzene transforms spontaneously into its valence isomer benzene. The compounds both can be described as 6 but the...
of
naphthaleneNaphthalene, also known as naphthalin, or antimite and not to be confused with naphtha, is a crystalline, aromatic, white, solid hydrocarbon with formula C
10H
8 and the structure of two fused benzene rings. It is best known as the traditional, primary ingredient of mothballs...
, but whereas naphthalene is a colourless, azulene is dark blue. Its name is derived from the Spanish word
azulAzul may refer to:* Azul... is a poetry collection by Rubén Darío* Azul, Buenos Aires, a town in Argentina* Operation Azul, the Argentine codename for the military landings that started the Falklands War*Azul , 2001 Cristian Castro album...
, meaning "blue". Two terpenoids,
vetivazuleneVetivazulene is an azulene derivate obtained from vetiver oil. It is a bicyclic sesquiterpene and an isomer of guaiazulene....
(4,8-dimethyl-2-isopropylazulene) and
guaiazuleneGuaiazulene, also azulon or 1,4-dimethyl-7-isopropylazulene, is a dark blue crystalline hydrocarbon. A derivative of azulene, guaiazulene is a bicyclic sesquiterpene that is a constituent of some essential oils, mainly oil of guaiac and chamomile oil, which also serve as its commercial sources....
(1,4-dimethyl-7-isopropylazulene), that feature the azulene skeleton are found in nature, e.g., as constituents of guaiac wood oil and some marine invertibrates.
Azulene has a long history, dating back to the 15th century as the azure-blue
chromophoreA chromophore is part of a molecule responsible for its color.When a molecule absorbs certain wavelengths of visible light and transmits or reflects others, the molecule has a color. A chromophore is a region in a molecule where the energy difference between two different molecular orbitals falls...
obtained by
steam distillationSteam distillation is a special type of distillation for temperature sensitive materials like natural aromatic compounds....
of German chamomile.
Azulene is an
organic compoundAn organic compound is any member of a large class of chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of compounds such as carbonates, simple oxides of carbon and cyanides, as well as the allotropes of carbon, are considered inorganic...
and a
isomerIn organic chemistry, two molecules are valence isomers when they are constitutional isomers that can interconvert through pericyclic reactions. For example, Dewar benzene transforms spontaneously into its valence isomer benzene. The compounds both can be described as 6 but the...
of
naphthaleneNaphthalene, also known as naphthalin, or antimite and not to be confused with naphtha, is a crystalline, aromatic, white, solid hydrocarbon with formula C
10H
8 and the structure of two fused benzene rings. It is best known as the traditional, primary ingredient of mothballs...
, but whereas naphthalene is a colourless, azulene is dark blue. Its name is derived from the Spanish word
azulAzul may refer to:* Azul... is a poetry collection by Rubén Darío* Azul, Buenos Aires, a town in Argentina* Operation Azul, the Argentine codename for the military landings that started the Falklands War*Azul , 2001 Cristian Castro album...
, meaning "blue". Two terpenoids,
vetivazuleneVetivazulene is an azulene derivate obtained from vetiver oil. It is a bicyclic sesquiterpene and an isomer of guaiazulene....
(4,8-dimethyl-2-isopropylazulene) and
guaiazuleneGuaiazulene, also azulon or 1,4-dimethyl-7-isopropylazulene, is a dark blue crystalline hydrocarbon. A derivative of azulene, guaiazulene is a bicyclic sesquiterpene that is a constituent of some essential oils, mainly oil of guaiac and chamomile oil, which also serve as its commercial sources....
(1,4-dimethyl-7-isopropylazulene), that feature the azulene skeleton are found in nature, e.g., as constituents of guaiac wood oil and some marine invertibrates.
Azulene has a long history, dating back to the 15th century as the azure-blue
chromophoreA chromophore is part of a molecule responsible for its color.When a molecule absorbs certain wavelengths of visible light and transmits or reflects others, the molecule has a color. A chromophore is a region in a molecule where the energy difference between two different molecular orbitals falls...
obtained by
steam distillationSteam distillation is a special type of distillation for temperature sensitive materials like natural aromatic compounds....
of German chamomile. The chromophore was discovered in in
yarrowAchillea millefolium or yarrow is a flowering plant in the family Asteraceae, native to the Northern Hemisphere...
and
wormwoodWormwood may refer to:*Various plants of the genus Artemisia but commonly Artemisia absinthium, also called grande wormwood or absinthe wormwood...
and named in 1863 by Septimus Piesse. Its structure and first
organic synthesisOrganic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
were reported by
Lavoslav RužičkaLeopold Ružička born as Lavoslav Stjepan Ružička was a Swiss-Croatian scientist, winner of the 1939 Nobel Prize in Chemistry...
, followed in 1937 by Placidus Plattner.
Structure and bonding
Azulene is usually viewed as resulting from fusion of
cyclopentadieneCyclopentadiene is a chemical compound with the formula C5H6. This colorless liquid organic chemical has a strong and unpleasant odor. At room temperature, this cyclic diene dimerizes over the course of hours to give dicyclopentadiene via a Diels-Alder reaction...
and
cycloheptatrieneCycloheptatriene is a colourless liquid that has been of recurring theoretical interest in organic chemistry. It is widely used as a ligand in organometallic chemistry and as a building block in organic synthesis....
rings. Like naphthalene nand
cyclodecapentaeneCyclodecapentaene or [10]annulene is an annulene with molecular formula C
10H
10. 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 of a combination of steric...
, it is a 10 pi electron system. It exhibits aromatic properties: (i) the
peripheralGenerally, a periphery is a boundary or outer part of any space or body.- In national divisions :* Periphery countries includes nations that are not core countries - capitalist or industrialized nations...
bonds have similar lengths and (ii) it undergoes
Friedel-CraftsThe Friedel-Crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877. There are two main types of Friedel-Crafts reactions: alkylation reactions and acylation reactions. This reaction type is part of electrophilic aromatic substitution...
-like substitutions. The stability gain from aromaticity is estimated to be half that of naphthalene.
Its
dipole momentDipole moment refers to the quality of a system to behave like a dipole. Dipole moment is the measured polarity of a polar covalent bond. It is defined as the product magnitude of charge on the atoms and the distance between the two bonded atoms...
is 0.8
debyeThe debye is a CGS unit of electric dipole moment
[Electric dipole moment is defined as charge times displacement:] Historically the debye was defined as the dipole moment resulting from two charges of opposite sign but an equal magnitude of 10
-10 statcoulomb
, in contrast with naphthalene, which has a dipole moment of zero. This polarity can be explained by regarding azulene as the fusion of the aromatic 6 π-electronIn chemistry, pi bonds are covalent chemical bonds where two lobes of one involved electron orbital overlap two lobes of the other involved electron orbital...
cyclopentadienyl anion and aromatic 6 π-electron tropylium cationIn organic chemistry, the tropylium ion is an aromatic species with a formula of [C
7H
7]
+. Its name derives from the molecule tropane ....
. In order to achieve the stable aromatic sextet in both rings, one electron from the seven-membered ring is transferred to the five-membered ring. The dipolar nature of the ground state is reflected in its deep colour, which is unusual for small unsaturated aromatic compounds. Reactivity studies confirm that seven-membered ring is electrophilic and the five-membered ring is nucleophilic.
Organic synthesis
Synthetic routes to azulene have long been of interest because of its unusual structure. An efficient one-pot route entails annulationAnnulation in organic chemistry is a chemical reaction in which a new ring is constructed on another molecule ....
of cyclopentadieneCyclopentadiene is a chemical compound with the formula C5H6. This colorless liquid organic chemical has a strong and unpleasant odor. At room temperature, this cyclic diene dimerizes over the course of hours to give dicyclopentadiene via a Diels-Alder reaction...
with unsaturated C5-synthons. The alternative approach from cycloheptatrieneCycloheptatriene is a colourless liquid that has been of recurring theoretical interest in organic chemistry. It is widely used as a ligand in organometallic chemistry and as a building block in organic synthesis....
has long been known, one illustrative method being shown below.
Organometallic complexes
In organometallic chemistryOrganometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal. Since many compounds without such bonds are chemically similar, an alternative may be compounds containing metal-element bonds of a largely covalent character...
, azulene serves as a ligand for low-valent metal centers, which otherwise are known to form π-complexes with both cyclopentadienyl and cycloheptatrienyl ligands. Illustrative complexes are (azulene)Mo2(CO)6 and (azulene)Fe2(CO)5.
Related compounds
In naphth[a]azulenes, a naphthaleneNaphthalene, also known as naphthalin, or antimite and not to be confused with naphtha, is a crystalline, aromatic, white, solid hydrocarbon with formula C
10H
8 and the structure of two fused benzene rings. It is best known as the traditional, primary ingredient of mothballs...
ring is condensed at the 1,2-positions of azulene. In one such system deformation from planarity is found similar to that of tetraheliceneHelicenes in organic chemistry are ortho-condensed polycyclic aromatic compounds in which benzene rings or other aromatic compounds are angularly annulated so as to give helically shaped molecules...
.
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