Azulene

Azulene is an organic compound whose molecules contain 10 carbons and 8 hydrogens and consist of a five-membered ring fused to a seven-membered ring. It is 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.

Derivatives
Vetivazulene or 4,8-dimethyl-2-isopropylazulene is obtained from vetiver oil. Guaiazulene or 1,4-dimethyl-7-isopropylazulene occurs naturally as a constituent of guaiac wood oil.

Structure
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 1.0 Debye is consistent with this picture.

Organic synthesis
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.