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Aromaticity
Aromatic compounds were originally named for their unique odors, but now aromatic and aromaticity have special meanings related to their unique reactivity and stability.
Comparing the heats of hydrogenation of cyclohexene with benzene is a classic way to illustrate aromatic stabilization.
The Molecular Orbitals of Benzene
These diagrams depict the molecular orbitals of benzene and illustrate why the pi electrons in the benzene molecule are so unreactive compared to other alkenes. Need an orbital refresher?
There’s a rule about making molecular orbitals that says for a combination of n atomic orbitals there must be n new molecular orbitals. In benzene there are six 2p orbitals, and these combine to make six new molecular orbitals at four different energies.
In some cases predicting what the new molecular orbitals look like is fairly easy, as in the case of, say, an allyl radical. However, in benzene things are already looking intimidating. The most important point to note about the molecular orbitals of benzene is that the lowest energy orbital (the ‘bagel’ orbital) is ONE orbital over all SIX atoms. This orbital looks totally unlike anything we’ve seen before! It is NOT three individual pi bonds as benzene is drawn above at the upper left. The big bagel orbital is very low in energy and quite unreactive toward things like Br2, which a regular pi bond (such as ethylene) would react with. Below are some more images of the (benzene-bagel) orbital at different perspectives:
So, how do we predict whether something will be aromatic
without having to draw lots of complicated orbital diagram? Hückel's Rules.
1.
Be a ring or cyclic
compound
2.
Each atom in the ring
must have a p orbital available
a)
whether empty or full of
electrons
b)
carbon or any heteroatom
is OK
3.
The ring must be flat
4.
the number of electrons
in the loop must be (4n + 2) where n = an integer greater than or
equal to zero.
The key thing not to forget here is Hückel’s (4n + 2) rule.
Electrostatic
Potential Images of Other Aromatic Compounds
In the above structures each atom in the ring is sp2 hybridized (click here).
Sources include http://www.cm.utexas.edu/academic/courses/Spring2001/CH610B/Iverson/index.html