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Chapter 11 : Arenes and Aromaticity |
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Aromaticity
Aromaticity is a property associated with the extra stability of certain
types of p 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 p system that need to be met in order for
the "special" aromatic stabilisation to be observed:
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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 p electrons
( this is equivalent to an odd number of p-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:
To aid in counting the electrons the following factors may help:
-
Each atom in the conjugated, cyclic system can only contribute a maximum
of one pair of electrons (or one orbital)
-
"Refining" your "predicted" hybridisation will allow lone pairs to be part
of the p system.
-
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.
