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0:00 ORGANIC CHEMISTRY- Nature of Bonding and Stereochemistry
0:14 Six membered cyclic species
1:32 Seven membered cyclic species
3:32 Fused polynuclear hydrocarbons
4:10 Substituted cyclic species
7:58 MCQ About Aromaticity
Seven membered cyclic species:
As earlier, one can clearly understand that cycloheptatriene is non-aromatic, cycloheptatrienyl anion is anti-aromatic but cyclheptatrienyl cation (tropilium ion) is aromatic as shown below.
Note that cycloheptatriene is cyclic and planar but one of the carbons is saturated and doesn't possess a p-orbital.
Fused polynuclear hydrocarbons:
Naphthalene and azulene have 10 𝜋 electrons each, Anthracene has 14 𝜋 electrons. All of them satisfy both the conditions and hence are aromatic.
Substituted cyclic species:
Don’t get carried away by the conjugations present as open chain substituents on the cyclic ring while counting the number of 𝜋 electrons. Only count those 𝜋 electrons which are continuously delocalized in a cyclic manner.
For example, in the following two cases, both fulfill the first condition.
Ethenyl-cyclobutadiene (I) is anti-aromatic as it has 4 𝜋 electrons which are continuously delocalized in a cyclic manner (and not 6 𝜋 electrons).
On the other hand, ethenyl-benzene(II) is aromatic as it has 6 𝜋 electrons which are continuously delocalized in a cyclic manner (and not 8 𝜋 electrons).
This molecule seems to be aromatic as it is cyclic, seems to be planar, conjugated system and has 10 𝜋 electrons.
The reason is that if the molecule acquires planar geometry (I), there will be angular strain in the molecule as each angle will try to be 120o (due to sp2 hybridisation) but cannot do so, also it will lead to five cis double bonds.
In order to become stable, it tries to exist as (II) where there are three cis and two trans double bonds.
Looking closely we find that the two H atoms on the two trans double bonds come very close to each other and thus the molecule actually exists in a manner where the ring becomes puckered or non-planar so that the two half-cyclic rings are tilted in order to make these two hydrogen atoms slightly away from each other (III).