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Aromaticity

Aromaticity is a property associated with the extra stability of certain types of π systems due to the nature of the molecular orbitals. It is not restricted to substituted benzenes, 6-membered rings or just hydrocarbons.

Based on the properties of compounds, there are FOUR criteria about the π system that need to be met in order for the "special" aromatic stabilisation to be observed:

  1. Conjugated (there needs to one "p" orbital from each atom in the cycle, so each atom must be either sp2 or sp hybridised)
  2. Cyclic (Linear systems are not aromatic)
  3. Planar (so there is good overlap / interaction between the "p" orbitals.... not always easy to consider)
  4. The Huckel Rule..... 4n+2 π electrons ( this is equivalent to an odd number of π -electrons pairs) in the cyclic conjugated system.
Study Tip : One way to think about the first 3 criteria is to compare the p orbitals of the system to fence posts surrounding a field. The posts need to be regularly spaced, all around the field with all the posts standing upright in order to have an effective fence.

In order for a compound to be aromatic, all FOUR of these criteria must be met.

The most important and well known aromatic system is benzene:

Benzene
6 pi electron system so 4n+2 where n = 1
benzene pi system

To aid in counting the electrons the following factors may help:

  1. Each atom in the conjugated, cyclic system can only contribute a maximum of one pair of electrons (or one orbital)
  2. "Refining" your "predicted" hybridisation will allow lone pairs to be part of the π system.
  3. Carbocations and boron atoms (C+ & B) are sp2 hybridised and have empty "p" orbitals. This allows them to be part of the conjugated system but they contribute NO electrons.

organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary