Azulene

Source: Wikipedia, the free encyclopedia.

Azulene
Names
Preferred IUPAC name
Azulene[1]
Systematic IUPAC name
Bicyclo[5.3.0]decapentaene
Identifiers
3D model (
JSmol
)
ChEBI
ChemSpider
ECHA InfoCard
 100.005.449 Edit this at Wikidata
KEGG
UNII
  • InChI=1S/C10H8/c1-2-5-9-7-4-8-10(9)6-3-1/h1-8H checkY
    Key: CUFNKYGDVFVPHO-UHFFFAOYSA-N checkY
  • InChI=1/C10H8/c1-2-5-9-7-4-8-10(9)6-3-1/h1-8H
    Key: CUFNKYGDVFVPHO-UHFFFAOYAT
  • c1cccc2cccc2c1
Properties
C10H8
Molar mass 128.174 g·mol−1
Melting point 99 to 100 °C (210 to 212 °F; 372 to 373 K)
Boiling point 242 °C (468 °F; 515 K)
-98.5·10−6 cm3/mol

g/L[2]

Thermochemistry
Std enthalpy of
combustion
(ΔcH⦵298)
 −1266.5 kcal/mol[3]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Azulene is an aromatic organic compound and an isomer of naphthalene. Naphthalene is colourless, whereas azulene is dark blue. The compound is named after its colour, as "azul" is 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, guaiac wood oil, and some marine invertebrates.

Azulene has a long history, dating back to the 15th century as the azure-blue

Lavoslav Ružička, followed by its organic synthesis
in 1937 by Placidus Plattner.

Structure and bonding

The blue color of the mushroom Lactarius indigo is due to the azulene derivative (7-isopropenyl-4-methylazulen-1-yl)methyl stearate.[4]
The blue color of the mushroom Entoloma hochstetteri is also identified as another kind of azulene derivative: 7-acetyl-1,4-dimethylazulene.[5]

Azulene is usually viewed as resulting from fusion of

Friedel-Crafts
-like substitutions. The stability gain from aromaticity is estimated to be half that of naphthalene.

Its

nucleophilic
.

The dipolar nature of the ground state is reflected in its deep colour, which is unusual for small unsaturated aromatic compounds.[7] Another notable feature of azulene is that it violates Kasha's rule by exhibiting fluorescence from an upper-excited state (S2 → S0).[8]

Organic synthesis

Synthetic routes to azulene have long been of interest because of its unusual structure.[9] In 1939 the first method was reported by St. Pfau and Plattner [10] starting from indane and ethyl diazoacetate.

An efficient one-pot route entails annulation of cyclopentadiene with unsaturated C5-synthons.[11] The alternative approach from cycloheptatriene has long been known, one illustrative method being shown below.[12][13]

Procedure:

  1. cycloheptatriene 2+2 cycloaddition with dichloro ketene
  2. diazomethane insertion reaction
  3. dehydrohalogenation reaction with DMF
  4. Luche reduction to alcohol with sodium borohydride
  5. elimination reaction with Burgess reagent
  6. p-chloranil
  7. DPDB ligand

Organometallic complexes

In organometallic chemistry, azulene serves as a ligand for low-valent metal centers. Illustrative complexes are (azulene)Mo2(CO)6 and (azulene)Fe2(CO)5.[14]

Derivatives

1-Hydroxyazulene is an unstable green oil and it does not show

pKa of 2-hydroxyazulene in water is 8.71. It is more acidic than phenol or naphthol. The pKa of 6-hydroxyazulenes in water is 7.38 making it also more acidic than phenol or naphthol.[16]

In naphth[a]azulene, a naphthalene ring is condensed at the 1,2-positions of azulene. In one such system[17] deviation from planarity is found, similar to that of 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

  1. ISBN 978-0-85404-182-4
    .
  2. PMID 8975804
    .
  3. doi:10.1021/ed075p1341
    .
  4. S2CID 21207966
    .
  5. doi:10.26021/9162
    .
  6. doi:10.1021/ja01527a046
    .
  7. doi:10.1016/0040-4020(76)87002-0
    .
  8. doi:10.1021/jp984407q
    .
  9. doi:10.1021/cr60155a004
    .
  10. doi:10.1002/hlca.19390220126
    .
  11. doi:10.15227/orgsyn.062.0134
    .
  12. PMID 16013070
    .
  13. doi:10.1021/ed065p923
    .
  14. ISBN 9780470166123
    .
  15. doi:10.1016/S0040-4039(00)70653-8
    .
  16. ^
    doi:10.1039/C1968000368B
    .
  17. PMID 16238325
    .

External links
Retrieved from "https://en.wikipedia.org/w/index.php?title=Azulene&oldid=1190608554"