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Aromatic

Last updated

Feb 28, 2022
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- Arenium Ion
- Aromatic Amine

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Valence bond theory uses Lewis diagrams to depict structure and bonding of covalent entities, such as molecules and polyatomic ions, henceforth molecules. The Lewis diagram of many a molecule, however, is not consistent with the observed properties of the molecule. The Lewis diagram of some molecules suggests a ring bearing a fully conjugated $\pi$ -electron system, or loop of $\pi$ electrons, provided each atom in the ring is sp²- or sp-hybridized.

cyclobutadiene (1)

benzene (2)

cyclooctatetraene (3)

pyridine (4)

pyrrole (5)

cyclopentadienyl cation (6)

cyclopentadienyl anion (7)

As evident from the stability of $\pi$ electrons however, only some such rings actually have a loop of $\pi$ electrons. Of the above examples, only the rings in 2, 4, 5, and 7 have a loop of $\pi$ electrons. Resonance theory can predict that the rings in 2, 4, 5, and 7 have a loop of $\pi$ electrons.

According to resonance theory, however, the rings in 1, 3, and 6 also should have a loop of $\pi$ electrons.

Evidently, resonance theory is no more reliable a tool than Lewis diagrams to be used in predicting if a ring has a loop of $\pi$ electrons. Rings whose Lewis diagram implies a loop of $\pi$ electrons can be classified into three classes:

aromatic rings
antiaromatic rings
nonaromatic rings

To find whether a ring is aromatic, antiaromatic, nonaromatic, or none of the above, use the following flow chart.

Figure 1.1.1: Flowchart for determining aromatic behavior in cyclic molecules.

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