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Planar monocyclic rings containing (4n + 2) π-electrons (where n = 0, 1, 2, 3, and so on) should be aromatic. This is known as Huckel’s rule for aromaticity. Applying this rule, benzene and its derivatives have six delocalized electrons and n = 1. Thus to be aromatic, a molecule must have 2(n = 0), 6(n = 0), 6(n = 1), 10(n = 2), etc., π-electrons in a closed π-orbital loop. Another requirement for aromaticity is planarity of the ring. If the ring is not planar, overlap of p-orbitals is diminished or prevented, and then the compound will not be aromatic.
Derivatives of benzene are aromatic as they fulfill both the conditions. Molecules having two or more benzene rings fused together are called polycyclic benzenoid aromatic hydrocarbons.
For example, naphthalene and anthracene.
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Non-benzenoid aromatic compounds are those which contain a ring that is not six membered.
For example, cyclopentadienyl anion, cyclopropeniumcation, and tropyliumcation.
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Cyclopentadiene is not aromatic as it does not obey (4n + 2) Huckel rule, but when a proton is removed from this carbon atom, it becomes sp2 hybridized and thus delocalization is permitted. Thus, this anion becomes aromatic in nature. Cyloheptatriene has six π-electrons but the CH2 group prevents delocalization. When a hydride ion is removed from this carbon atom, a vacant p-orbital is created, which makes the carbon atom sp2 hybridized and thus cation becomes aromatic in nature.
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Cyclic conjugated compounds which possess only 4n electrons do not satisfy the Huckel rule and are anti-aromatic in nature.
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Pyrrole, furan, thiophene, and pyridine are all aromatic.

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