Azulene is an organic compound and a monoterpene. It is an isomer of naphthalene but its physical properties are quite different. Naphthalene is a white crystalline solid whereas azulene, whose name is derived from the Spanish word azul, meaning "blue", is a dark blue crystalline solid used in many cosmetics. Azulene has a long history dating back to the 15th century as the azure-blue distillate obtained by steam distillation of Chamomile. The compound was discovered and named in 1863 by Septimus Piesse in azure-blue distillates from other sources such as yarrow and wormwood. Lavoslav Ružička solved the structure for this compound and the first organic synthesis followed in 1937 by Placidus Plattner.
or 4,8-dimethyl-2-isopropylazulene is obtained from vetiver
or 1,4-dimethyl-7-isopropylazulene occurs naturally as a constituent of guaiac wood oil.
Azulene consists of a fused cyclopentadiene
ring and a cycloheptatriene
ring and is an isomer of naphthalene
. Azulene is 10 pi electron
system just like cyclodecapentaene
and does have aromatic
properties even though it is not a single ring system like benzene
. The peripheral
bonds have similar lengths and only the shared bond in the middle is a single bond. The stability gain from aromaticity is half of that of naphthalene. The molecule can be considered a fusion product of a 6 pi electron cyclopentadienyl anion
which is aromatic and the likewise aromatic 6 pi electron tropylium cation
. The observed dipole moment
of 0.8 Debye
(very different from naphthalene, which has zero dipole moment) is consistent with this picture.
For many years not many synthetic routes existed to azulene and the compound was therefore expensive. A recent contribution takes cycloheptatriene
as starting material .
In naphth[a]azulenes, a naphthalene ring is condensed with the 1,2-position of an azulene ring. In one such system deformation from planarity is found similar to that of tetrahelicene.