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,
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:
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
need to be regularly spaced, all around the field with all the posts
upright in order to have an effective fence.
- Conjugated (there needs to one "p"
orbital from each atom in the
cycle, so each atom must be either sp2 or sp hybridised)
- Cyclic (Linear
systems are not aromatic)
- Planar (so
there is good overlap /
interaction between the "p" orbitals....
not always easy to consider)
- The Huckel Rule.....
4n+2 π electrons
( this is equivalent to an odd number of π -electrons
pairs) in the cyclic conjugated system.
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:
To aid in counting the electrons the
following factors may help:
- Each atom in the
conjugated, cyclic system can only contribute a
of one pair of electrons (or one orbital)
- "Refining" your
"predicted" hybridisation will allow lone pairs to be
of the π system.
- Carbocations and boron
atoms (C+ & B) are sp2 hybridised and have empty "p" orbitals. This allows them to be
of the conjugated system but they contribute NO electrons.
||© Dr. Ian Hunt, Department of Chemistry, University