Aromaticity of Azulene
Azulene is a bicyclic non-benzenoid aromatic hydrocarbon exhibits aromaticity due to its unique structure, consisting of a fused seven-membered and five-membered ring system. It is an isomer of naphthalene.
Azulene is aromatic compound because it is a cyclic and planar structure, each atoms are sp2 hybridized and follows Huckel's rule.
Azulene is a bicyclic molecule with two fused rings, one seven and one five membered ring.
Azulene is a planar molecule, which allows the effective overlap of its p-orbitals.
Every atom in the rings of azulene is sp2 hybridized, that means delocalization of electrons occurs continuously via overlapping of p-orbitals across both rings.
Huckel's Rule:
(4n + 2) π-electrons
where n = 0,1,2,3...
Azulene has 10 π-electrons from five double bonds.
[(4 x 2) + 2] = 10 π-electrons.
Thus, azulene satisfies Hückel's rule.
Azulene exhibits a dipole moment (1.08 D). The dipole moment of azulene indicates the existence of charge separation (electron flow from the seven-membered ring to the five-membered ring) in the molecule and that each ring approximately contains six π-electron systems.
Therefore, Azulene is aromatic as it follows all the necessary conditions for a molecule to be an aromatic.
The aromaticity of azulene is also supported by its chemical properties, such as its ability to undergo electrophilic substitution reactions similar to benzene. It does not get polymerized and under normal conditions, it does not act as a diene in Diels-Alder reactions.
Also read Synthesis of Azulene
Test Your Knowledge
Aromaticity of Azulene MCQs
Azulene is considered as aromatic due to?
A. It contains a benzene ring.
B. It has a conjugated system of 10 π-electrons satisfying Hückel's rule.
C. It is non-cyclic and non-planar.
D. It contains only single bonds.
View Answer
Azulene is aromatic because it has a conjugated system of 10 π-electrons, which satisfies Hückel's rule for aromaticity.
Which statement about the electron distribution in azulene is correct?
A. Both rings have equal electron density and reactivity.
B. The seven-membered ring is nucleophilic, and the five-membered ring is electrophilic.
C. The five-membered ring is nucleophilic, and the seven-membered ring is electrophilic.
D. Both rings are non-reactive.
View Answer
In azulene, the five-membered ring is nucleophilic and the seven-membered ring is electrophilic due to the electron distribution that gives both rings aromatic stability.
Which statement is true regarding the reactivity of azulene in electrophilic substitution reactions?
A. Azulene is less reactive than benzene due to lack of aromaticity.
B. Substitution occurs at the 1-position of the five-membered ring.
C. Substitution occurs only at the seven-membered ring.
D. Azulene does not undergo electrophilic substitution.
View Answer
Azulene undergoes electrophilic substitution at the 1- and 3-position of the five-membered ring.
Which property of azulene is a direct result of its aromaticity?
A. High reactivity with electrophiles
B. Blue color due to π-electron delocalization
C. Non-polar nature
D. Lack of resonance stabilization
View Answer
Blue color due to π-electron delocalization.
How many π-electrons azulene possess to follow the Hückel's rule for aromaticity?
A. 6 π-electrons
B. 8 π-electrons
C. 10 π-electrons
D. 12 π-electrons
View Answer
10 π-electrons azulene possess to follow the Hückel's rule for aromaticity
(4n + 2) π-electrons = 10 π-electrons