Azulene

Azulene is a blue, nonbenzenoid aromatic hydrocarbon with fused five- and seven-membered rings and molecular formula C₁₀H₈. Its unusual color and electronic structure distinguish it from its isomer naphthalene.

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Aromaticity: A stabilization associated with cyclic, conjugated electron systems containing characteristic numbers of π electrons. Azulene is aromatic despite lacking a benzenoid ring system.
Nonbenzenoid aromatic compound: An aromatic compound whose conjugated ring system does not consist solely of benzene-like rings. Azulene is a classic example of aromaticity outside benzenoid structures.
Guaiazulene: A naturally occurring azulene derivative with alkyl substituents and an intense blue color. This substituted azulene is used as a colorant and in topical formulations.
Sergei Reformatsky: A Russian chemist known for the Reformatsky reaction and early studies of organic compounds. Reformatsky and collaborators isolated azulene from essential oils in the nineteenth century.
Resonance structure: A Lewis representation that contributes to a molecule’s delocalized electronic structure. Charge-separated resonance forms explain azulene’s polarized five- and seven-membered rings.
Fused aromatic ring: An arrangement in which two or more rings share adjacent atoms and a bond within a conjugated system. Azulene’s five- and seven-membered rings share a fused bond.
Cosmetics: Products applied to the body for cleansing, appearance, fragrance, or protection. Azulene derivatives appear in some cosmetic formulations, often valued for their color and soothing associations.
Essential oil: A concentrated mixture of volatile compounds produced by plants and responsible for characteristic aromas. Azulene was first encountered in plant-derived essential oils.
Dipole moment: A vector measure of the separation of positive and negative charge in a system. Azulene has a substantial permanent dipole for a hydrocarbon because its rings are polarized.
Naphthalene: A colorless aromatic hydrocarbon composed of two fused six-membered rings, with formula C₁₀H₈. Naphthalene shares azulene’s formula but differs in ring sizes, polarity, and color.