# Azulene

> Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Azulene
> Markdown URL: https://mediated.wiki/source/Azulene.md
> Source: https://en.wikipedia.org/wiki/Azulene
> Source revision: 1352858457
> License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/)

{{Use dmy dates|date=December 2023}}

{{chembox
| ImageFileL1_Ref = {{chemboximage|correct|??}}
| ImageFileL1 = Azulen_num.svg
| ImageSizeL1 = 120
| ImageFileR1 = Azulene_3d_structure.png
| ImageSizeR1 = 120
| PIN = Azulene<ref>{{cite book |author=[International Union of Pure and Applied Chemistry](/source/International_Union_of_Pure_and_Applied_Chemistry) |date=2014 |title=Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013 |publisher=[The Royal Society of Chemistry](/source/Royal_Society_of_Chemistry) |page=207 |doi=10.1039/9781849733069 |isbn=978-0-85404-182-4}}</ref>
| SystematicName = Bicyclo[5.3.0]deca-2,4,6,8,10-pentaene
| OtherNames = 
|Section1={{Chembox Identifiers
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = 82R6M9MGLP
| KEGG_Ref = {{keggcite|correct|kegg}}
| KEGG = C13392
| InChI = 1/C10H8/c1-2-5-9-7-4-8-10(9)6-3-1/h1-8H
| InChIKey = CUFNKYGDVFVPHO-UHFFFAOYAT
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/C10H8/c1-2-5-9-7-4-8-10(9)6-3-1/h1-8H
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = CUFNKYGDVFVPHO-UHFFFAOYSA-N
| CASNo_Ref = {{cascite|correct|CAS}}
| CASNo = 275-51-4
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID =8876
| PubChem = 9231
| ChEBI_Ref = {{ebicite|correct|EBI}}
| ChEBI = 31249
| SMILES = c1cccc2cccc2c1
}}
|Section2={{Chembox Properties
| C=10 | H=8
| Appearance = Dark blue crystalline solid
| Density = 
| MeltingPtC = 99 to 100
| MeltingPt_notes = 
| BoilingPtC = 242
| BoilingPt_notes = 
| Solubility =  
| MagSus = −98.5·10<sup>−6</sup> cm<sup>3</sup>/mol<ref>{{cite journal |doi=10.1007/s001289900330 |pmid=8975804|url=http://deepblue.lib.umich.edu/bitstream/2027.42/42354/1/128-58-2-268_58n2p268.pdf|title=Lethal and Sublethal Effects of Azulene and Longifolene to Microtox®, Ceriodaphnia dubia, Daphnia magna, and Pimephales promelas|journal=[Bulletin of Environmental Contamination and Toxicology](/source/Bulletin_of_Environmental_Contamination_and_Toxicology) |volume=58|issue=2|pages=268–274|year=1997|last1=Sweet|first1=L. I.|last2=Meier|first2=P. G.|bibcode=1997BuECT..58..268S |hdl=2027.42/42354|hdl-access=free}}</ref>}}
|Section3={{Chembox Thermochemistry
| DeltaHc = −1266.5 kcal/mol<ref>{{cite journal |last1=Salter |first1=Carl |last2=Foresman |first2=James B. |year=1998 |title=Naphthalene and Azulene I: Semimicro Bomb Calorimetry and Quantum Mechanical Calculations  |journal=[Journal of Chemical Education](/source/Journal_of_Chemical_Education) |volume=75 |issue=10 |page=1341 |bibcode=1998JChEd..75.1341S |doi=10.1021/ed075p1341}}</ref>
| DeltaHf =
| Entropy = 
| HeatCapacity = }}
|Section4={{Chembox Hazards
| MainHazards = 
| FlashPt = 
| AutoignitionPt = 
 }}
}}

'''Azulene''' is an aromatic [organic compound](/source/organic_compound) and an [isomer](/source/Valence_isomer) of [naphthalene](/source/naphthalene). Naphthalene is colourless, whereas azulene is dark blue. The compound is named after its colour, as "azul" is [Spanish](/source/Spanish_language) for blue. Two terpenoids, [vetivazulene](/source/vetivazulene) (4,8-dimethyl-2-isopropylazulene) and [guaiazulene](/source/guaiazulene) (1,4-dimethyl-7-isopropylazulene), that feature the azulene skeleton are found in nature as constituents of pigments in mushrooms, [guaiac wood oil](/source/Oil_of_guaiac), and some marine invertebrates.

Azulene has a long history, dating back to the 15th century as the azure-blue [chromophore](/source/chromophore) obtained by [steam distillation](/source/steam_distillation) of [German chamomile](/source/Matricaria_recutita). The chromophore was discovered in [yarrow](/source/yarrow) and [wormwood](/source/Artemisia_(genus)) and named in 1863 by Septimus Piesse. Its structure was first reported by [Lavoslav Ružička](/source/Lavoslav_Ru%C5%BEi%C4%8Dka), followed by its [organic synthesis](/source/organic_synthesis) in 1937 by Placidus Plattner.

== Structure and bonding==
{{Multiple image
| total_width = 380px
| align       = left
| header      = Blue azulene pigments found in mushrooms

| image1      = Lactarius indigo 48568 edit.jpg
| alt1        = Close-up of a large blue Lactarius indigo mushroom cap torn from its stem and upturned on mossy forest floor. The gills take on a darker blue coloration compared to that of the edges of the cap and stem.
| caption1    = ''[Lactarius indigo](/source/Lactarius_indigo)'': (7-isopropenyl-4-methylazulen-1-yl)methyl stearate.<ref>{{cite journal |last1=Harmon |first1=A. D. |last2=Weisgraber |first2=K. H. |last3=Weiss |first3=U. |s2cid=21207966 |year=1980 |title=Preformed azulene pigments of ''Lactarius indigo'' (Schw.) Fries (Russulaceae, Basidiomycetes) |journal=[Experientia](/source/Experientia) |volume=36 |pages=54–56 |doi=10.1007/BF02003967}}</ref>

| image2      = Entoloma hochstetteri.jpg
| alt2        = Close-up of a small, thin Entoloma mushroom growing from the mossy bark of a tree. The cap and gills are evenly pigmented a light blue colour and the stem takes on paler tones towards its base.
| caption2    = ''[Entoloma hochstetteri](/source/Entoloma_hochstetteri)'': 7-acetyl-1,4-dimethylazulene.<ref>{{cite thesis |last1=Nicholas |first1=Gillian May |title=Australasian fungi: a natural product study. |date=1998 |page=56 |doi=10.26021/9162 |doi-access=free }}</ref>
}}
Azulene is usually viewed as resulting from fusion of [cyclopentadiene](/source/cyclopentadiene) and [cycloheptatriene](/source/cycloheptatriene) rings. Like naphthalene and [cyclodecapentaene](/source/cyclodecapentaene), it is a 10 [pi electron](/source/pi_electron) system. It exhibits [aromatic](/source/aromatic) properties: (i) the peripheral bonds have similar lengths and (ii) it undergoes [Friedel-Crafts](/source/Friedel-Crafts_reaction)-like substitutions. The stability gain from aromaticity is estimated to be half that of naphthalene.

Its [dipole moment](/source/Molecular_dipole_moment) is {{val|1.08|u=[D](/source/debye)}},<ref>{{cite journal |last1=Anderson |first1=Arthur G. |last2=Steckler |first2=Bernard M. |year=1959 |title=Azulene. VIII. A Study of the Visible Absorption Spectra and Dipole Moments of Some 1- and 1,3-Substituted Azulenes |journal=[Journal of the American Chemical Society](/source/Journal_of_the_American_Chemical_Society) |volume=81 |issue=18 |pages=4941–4946 |doi=10.1021/ja01527a046}}</ref> in contrast with naphthalene, which has a dipole moment of zero. This polarity can be explained by regarding azulene as the fusion of a 6 [π-electron](/source/Pi_bond) [cyclopentadienyl anion](/source/cyclopentadienyl_anion) and a 6 π-electron [tropylium cation](/source/tropylium_cation): one electron from the seven-membered ring is transferred to the five-membered ring to give each ring aromatic stability by [Hückel's rule](/source/H%C3%BCckel's_rule). Reactivity studies confirm that seven-membered ring is electrophilic and the five-membered ring is [nucleophilic](/source/nucleophilic).

frameless|center|upright=1|class=skin-invert-image

The dipolar nature of the ground state is reflected in its deep colour, which is unusual for small unsaturated aromatic compounds.<ref>{{cite journal |last1=Michl |first1=Joseph |last2=Thulstrup |first2=E. W. |date=1976 |title=Why is azulene blue and anthracene white? A simple mo picture  |journal=[Tetrahedron](/source/Tetrahedron_(journal)) |volume=32 |issue=2 |page=205 |doi=10.1016/0040-4020(76)87002-0}}</ref> Another notable feature of azulene is that it violates [Kasha's rule](/source/Kasha's_rule) by exhibiting fluorescence from an upper-excited state (S<sub>2</sub> → S<sub>0</sub>).<ref>{{cite journal | last1 = Tétreault | first1 = N. | last2 = Muthyala | first2 = R. S. | last3 = Liu | first3 = R. S. H. | last4 = Steer | first4 = R.P. | year = 1999 | title = Control of the Photophysical Properties of Polyatomic Molecules by Substitution and Solvation: The Second Excited Singlet State of Azulene | journal = [Journal of Physical Chemistry A](/source/Journal_of_Physical_Chemistry_A) | volume = 103 | issue = 15| pages = 2524–31 | bibcode = 1999JPCA..103.2524T | doi=10.1021/jp984407q}}</ref>

==Organic synthesis==
Synthetic routes to azulene have long been of interest because of its unusual structure.<ref>{{cite journal |last1=Gordon |first1=Maxwell |title=The Azulenes |journal=[Chemical Reviews](/source/Chemical_Reviews) |date=1 February 1952 |volume=50 |issue=1 |pages=127–200 |doi=10.1021/cr60155a004}}</ref> In 1939 the first method was reported by St. Pfau and Plattner <ref>{{Cite journal |doi = 10.1002/hlca.19390220126|title = Zur Kenntnis der flüchtigen Pflanzenstoffe VIII. Synthese des Vetivazulens|journal = [Helvetica Chimica Acta](/source/Helvetica_Chimica_Acta)|volume = 22|pages = 202–208|year = 1939|last1 = St. Pfau|first1 = Alexander|last2 = Plattner|first2 = Pl. A.}}</ref> starting from [indane](/source/indane) and [ethyl diazoacetate](/source/ethyl_diazoacetate).

An efficient [one-pot](/source/one-pot_synthesis) route entails [annulation](/source/annulation) of [cyclopentadiene](/source/cyclopentadiene) with unsaturated C<sub>5</sub>-[synthon](/source/synthon)s.<ref>{{cite journal |last1=Hafner |first1=Klaus |last2=Meinhardt |first2=Klaus-Peter | doi= 10.15227/orgsyn.062.0134| title= Azulene| journal=[Organic Syntheses](/source/Organic_Syntheses) | volume= 62| page= 134| year= 1984}}</ref> The alternative approach from [cycloheptatriene](/source/cycloheptatriene) has long been known, one illustrative method being shown below.<ref>{{Cite journal | doi=10.1002/anie.200501276| pmid=16013070| title=Approach to the Blues: A Highly Flexible Route to the Azulenes| journal=[Angewandte Chemie International Edition](/source/Angewandte_Chemie_International_Edition)| volume=44| issue=32| pages=5130–5133| year=2005| last1=Carret| first1=Sébastien| last2=Blanc| first2=Aurélien| last3=Coquerel| first3=Yoann| last4=Berthod| first4=Mikaël| last5=Greene| first5=Andrew E.| last6=Deprés| first6=Jean-Pierre}}</ref><ref name="Lemal" >{{cite journal |journal=[Journal of Chemical Education](/source/Journal_of_Chemical_Education) |title=Synthesis of azulene, a blue hydrocarbon |volume=65 |issue=10 |year=1988 |page=923 |doi=10.1021/ed065p923|bibcode=1988JChEd..65..923L |last1=Lemal |first1=David M. |last2=Goldman |first2=Glenn D. }}</ref>

center|frameless|upright=2.8|class=skin-invert-image

Procedure:
# [cycloheptatriene](/source/cycloheptatriene) 2+2 [cycloaddition](/source/cycloaddition) with dichloro [ketene](/source/ketene)
# [diazomethane](/source/diazomethane) [insertion reaction](/source/insertion_reaction)
# [dehydrohalogenation](/source/dehydrohalogenation) reaction with [DMF](/source/dimethylformamide)
# [Luche reduction](/source/Luche_reduction) to alcohol with [sodium borohydride](/source/sodium_borohydride)
# [elimination reaction](/source/elimination_reaction) with [Burgess reagent](/source/Burgess_reagent)
# [oxidation](/source/organic_oxidation) with [p-chloranil](/source/p-chloranil)
# [dehalogenation](/source/dehalogenation) with [polymethylhydrosiloxane](/source/polymethylhydrosiloxane), [palladium(II) acetate](/source/palladium(II)_acetate), potassium [phosphate](/source/phosphate) and the [DPDB ligand](/source/DPDB_ligand) (BINAP)
Another synthesis route starts by treating [pyridinium](/source/pyridinium) or [pyrylium](/source/pyrylium) salts with [cyclopentadienyl anion](/source/cyclopentadienyl_anion):<ref name=":0">{{Cite journal |last1=Shoji |first1=Taku |last2=Ito |first2=Shunji |last3=Yasunami |first3=Masafumi |date=2021-10-01 |title=Synthesis of Azulene Derivatives from 2H-Cyclohepta[b]furan-2-ones as Starting Materials: Their Reactivity and Properties |journal=International Journal of Molecular Sciences |language=en |volume=22 |issue=19 |article-number=10686 |doi=10.3390/ijms221910686 |doi-access=free |issn=1422-0067 |pmc=8509482 |pmid=34639027}}</ref>
center|frameless|upright=2|class=skin-invert-image 
Azulene can also be synthesized via a [Diels Alder](/source/Diels%E2%80%93Alder_reaction) and [retro-Diels Alder reaction](/source/Retro-Diels%E2%80%93Alder_reaction):<ref name=":0" />
center|frameless|upright=1.5|class=skin-invert-image 
The starting material of the above reaction can be generated through the [flash vacuum pyrolysis](/source/flash_vacuum_pyrolysis) of phenyl propiolate.

==Organometallic complexes==
In [organometallic chemistry](/source/organometallic_chemistry), azulene serves as a ligand for low-valent metal centers. Illustrative complexes are (azulene)Mo<sub>2</sub>(CO)<sub>6</sub> and (azulene)Fe<sub>2</sub>(CO)<sub>5</sub>.<ref>{{Cite book | doi=10.1002/9780470166123.ch2| chapter=Transition Metal Complexes of Azulene and Related Ligands| title=Progress in Inorganic Chemistry| pages=53–98| year=2007| last1=Churchill| first1=Melvyn R.| volume=11| isbn=978-0-470-16612-3}}</ref>

==Derivatives==

1-Hydroxyazulene is an unstable green oil and it does not show [keto–enol tautomerism](/source/keto%E2%80%93enol_tautomerism).<ref>{{cite journal | last =  Asao | first = Toyonobu | author2 = Shunji Ito | author3 =Noboru Morita  |title = 1-Hydroxyazulene and 3-hydroxyguaiazulene: Synthesis and their properties | journal = [Tetrahedron Letters](/source/Tetrahedron_Letters) | year = 1989 | volume = 30 | issue = 48 | pages = 6693–6696 | doi = 10.1016/S0040-4039(00)70653-8 }}</ref>  2-Hydroxyazulene is obtained by hydrolysis of 2-methoxyazulene with hydrobromic acid. It is stable and does show keto–enol tautomerism.<ref name="2- and 6- hydroxyazulene">{{cite journal | last = Takase | first = Kahei | author2 = Toyonobu Asao | author3 =Yoshikazu Takagi | author4 = Tetsuo Nozoe |title = Syntheses and some properties of 2- and 6-hydroxyazulenes | journal = [Chemical Communications](/source/Chemical_Communications) | year = 1968 | issue = 7| pages = 368b–370 | doi = 10.1039/C1968000368B }}</ref> The [p''K''<sub>a</sub>](/source/pKa) of 2-hydroxyazulene in water is 8.71. It is more acidic than [phenol](/source/phenol) or [naphthol](/source/naphthol).  The p''K''<sub>a</sub> of 6-hydroxyazulenes in water is 7.38 making it also more acidic than phenol or naphthol.<ref name="2- and 6- hydroxyazulene" />

In naphth[''a'']azulene, a [naphthalene](/source/naphthalene) ring is condensed at the 1,2-positions of azulene. In one such system<ref>{{cite journal | doi = 10.1021/jo051409f| pmid = 16238325| title = Novel Synthesis of Benzalacetone Analogues of Naphth[''a'']azulenes by Intramolecular Tropylium Ion-Mediated Furan Ring-Opening Reaction and X-ray Investigation of a Naphth[1,2-''a'']azulene Derivative| journal = [The Journal of Organic Chemistry](/source/The_Journal_of_Organic_Chemistry)| volume = 70| issue = 22| pages = 8902–6| year = 2005| last1 = Yamamura| first1 = Kimiaki| last2 = Kawabata| first2 = Shizuka| last3 = Kimura| first3 = Takatomo| last4 = Eda| first4 = Kazuo| last5 = Hashimoto| first5 = Masao}}</ref> deviation from planarity is found, similar to that of [tetrahelicene](/source/helicene).

[Guaiazulene](/source/Guaiazulene) (1,4-dimethyl-7-isopropylazulene) is an alkylated derivative of azulene with an almost identical intensely blue colour. It is commercially available to the cosmetics industry where it functions as a skin conditioning agent.

== References ==
{{reflist|30em}}

== External links ==
* [MSDS](/source/MSDS) [http://physchem.ox.ac.uk/MSDS/AZ/azulene.html Website] {{Webarchive|url=https://web.archive.org/web/20071018223113/http://physchem.ox.ac.uk/MSDS/AZ/azulene.html |date=18 October 2007 }}

{{Hydrocarbons}}
{{PAHs}}
{{Authority control}}

Category:Cosmetics chemicals
Category:Azulenes
Category:Polycyclic aromatic hydrocarbons

---
Adapted from the Wikipedia article [Azulene](https://en.wikipedia.org/wiki/Azulene) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Azulene?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.