[narrator] What are the criteria for classifying organic compounds as aromatic, non-aromatic,
or anti-aromatic?
Well, aromatic compounds are cyclic molecules that contain a continuous array of p-orbitals,
so everything is sp2 or sp hybridized, and have a Huckel number of electrons.
A Huckel number means [4n+2].
For benzene, the classic example, n is 1 and we have six pi electrons.
In our cycle the molecule is planar.
But, we also can hybridize a lone pair into a p-orbital, say with cyclopentadiene anion
we can hybridize a lone pair into the cycle in order to achieve six pi electrons.
Two, four, six--two from the lone pair and four from the double bonds of cyclopentadienide.
We can also invoke a carbocation as one of our p-orbitals in our cyclic array.
If n equals 0 the two pi--or cyclopropenyl cation is also aromatic.
Non-aromatic compounds violate the rules a or b, above.
They either violate the cyclic requirements, so like cyclo--or, excuse me, hexatriene.
You might see that the molecule is not planar and if we look at cyclooctatetraene--one,
two, three, four double bonds.
Eight membered ring.
It’s like a stop sign.
This compound is not anti-aromatic, it’s really non, because it’s not planar.
It bends out of plane to relieve the anti-aromaticity.
It doesn’t want to be anti-aromatic, so in fact it becomes non-aromatic by violating
rule a for aromatic compounds.
You can also have an interruption in the cycle such as the compound cyclopentadiene.
That sp3 hybridized carbon, here, would break up the cyclic array of p-orbitals.
For anti-aromatic compounds we’re going to violate the Huckel number rule only.
These are very, very rare.
If you’ve got to guess non-aromatic is probably the way to go, but if we force planarity with
a very small ring such as a five membered ring here with the cation, we have four pi
electrons so we have 4n.
We violate the Huckel number with 4n and four pi electrons, we force planarity, we know
that all atoms of these two double bonds are in the same plane as one another because of
the planarity of double bonds.
We know that we’re planar and we have a non-Huckel number.
This is anti-aromatic.
Also, cyclobutadiene; four pi electrons.
Fused rings can also be anti-aromatic, such as the following compound.
We force planarity with two rings fused together by a bridge.
A four membered bridge.
If we count; two, four, six, eight, ten, twelve pi electrons, as four times three is 4n.
We force planarity, we have a Huckel number, and we have the cyclic array of p-orbitals.