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Chapter 11 : Arenes and Aromaticity |
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Aromaticity Criteria
Qu: How can we recognise systems that are aromatic ?
Ans: By applying the criteria for aromaticity outlined below.
Based on the properties of aromatic compounds, there are FOUR criteria
about
the p system that need to be met in order
for the "special" aromatic stabilization to be observed:
-
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 (note that 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 (i.e.
no gaps), 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 (i.e. one p orbital)
-
"Refining" your "predicted" hybridization for heteroatoms (esp. O, N and
S) 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.
