View source for Azulene

{{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]] |date=2014 |title=Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013 |publisher=[[Royal Society of Chemistry|The 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 = 
| 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]] |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]] |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]] and an [[Valence isomer|isomer]] of [[naphthalene]]. Naphthalene is colourless, whereas azulene is dark blue. The compound is named after its colour, as "azul" is [[Spanish language|Spanish]] for blue. Two terpenoids, [[vetivazulene]] (4,8-dimethyl-2-isopropylazulene) and [[guaiazulene]] (1,4-dimethyl-7-isopropylazulene), that feature the azulene skeleton are found in nature as constituents of pigments in mushrooms, [[Oil of guaiac|guaiac wood oil]], and some marine invertebrates.

Azulene has a long history, dating back to the 15th century as the azure-blue [[chromophore]] obtained by [[steam distillation]] of [[Matricaria recutita|German chamomile]]. The chromophore was discovered in [[yarrow]] and [[Artemisia (genus)|wormwood]] and named in 1863 by Septimus Piesse. Its structure was first reported by [[Lavoslav Ružička]], followed by its [[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]]'': (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]] |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]]'': 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]] and [[cycloheptatriene]] rings. Like naphthalene and [[cyclodecapentaene]], it is a 10 [[pi electron]] system. It exhibits [[aromatic]] properties: (i) the [[wikt:periphery|peripheral]] bonds have similar lengths and (ii) it undergoes [[Friedel-Crafts reaction|Friedel-Crafts]]-like substitutions. The stability gain from aromaticity is estimated to be half that of naphthalene.

Its [[Molecular dipole moment|dipole moment]] is {{val|1.08|u=[[debye|D]]}},<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]] |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 [[Pi bond|π-electron]] [[cyclopentadienyl anion]] and a 6 π-electron [[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]]. Reactivity studies confirm that seven-membered ring is electrophilic and the five-membered ring is [[nucleophilic]].

[[File:Azulene resonance.svg|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 (journal)|Tetrahedron]] |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]] 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]] | 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]] |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]]|volume = 22|pages = 202–208|year = 1939|last1 = St. Pfau|first1 = Alexander|last2 = Plattner|first2 = Pl. A.}}</ref> starting from [[indane]] and [[ethyl diazoacetate]].

An efficient [[one-pot synthesis|one-pot]] route entails [[annulation]] of [[cyclopentadiene]] with unsaturated C<sub>5</sub>-[[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]] | volume= 62| page= 134| year= 1984}}</ref> The alternative approach from [[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]]| 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]] |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>

[[File:Azulenesynthesis.svg|center|frameless|upright=2.8|class=skin-invert-image]]

Procedure:
# [[cycloheptatriene]] 2+2 [[cycloaddition]] with dichloro [[ketene]]
# [[diazomethane]] [[insertion reaction]]
# [[dehydrohalogenation]] reaction with [[dimethylformamide|DMF]]
# [[Luche reduction]] to alcohol with [[sodium borohydride]]
# [[elimination reaction]] with [[Burgess reagent]]
# [[organic oxidation|oxidation]] with [[p-chloranil]]
# [[dehalogenation]] with [[polymethylhydrosiloxane]], [[palladium(II) acetate]], potassium [[phosphate]] and the [[DPDB ligand]] (BINAP)
Another synthesis route starts by treating [[pyridinium]] or [[pyrylium]] salts with [[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>
[[File:Azulene synthesis 1.png|center|frameless|upright=2|class=skin-invert-image]] 
Azulene can also be synthesized via a [[Diels–Alder reaction|Diels Alder]] and [[Retro-Diels–Alder reaction|retro-Diels Alder reaction]]:<ref name=":0" />
[[File:Azulene synthesis 2.png|center|frameless|upright=1.5|class=skin-invert-image]] 
The starting material of the above reaction can be generated through the [[flash vacuum pyrolysis]] of phenyl propiolate.

==Organometallic complexes==
In [[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]].<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]] | 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]] | year = 1968 | issue = 7| pages = 368b–370 | doi = 10.1039/C1968000368B }}</ref> The [[pKa|p''K''<sub>a</sub>]] of 2-hydroxyazulene in water is 8.71. It is more acidic than [[phenol]] or [[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]] 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]]| 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 [[helicene|tetrahelicene]].

[[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]] [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 }}
* [[MSDS]] [http://www.chemicalland21.com/lifescience/foco/AZULENE.htm Website]

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