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Pictures of the Day 4-16-07
The acidity of phenol - charge distribution in an aromatic ring
Phenol is relatively acidic, with a pKa near 9.95. Compare this to a normal alcohol in which the pKa is closer to 16. Phenol is more acidic because the negative charge of the anion is delocalized around the aromatic ring as described by the 4 contributing structures above. Note that most of the charge is still on oxygen, but enough is delocalized in the aromatic pi system to make a substantial difference. The interesting prediction of these resonance contributing structures is that the negative charge is not distributed evenly around the ring, but remains primarily in the ortho and para positions. This is shown in the electrostatic potential surface in which the negative charge (red color) is primarily ortho and para to the oxygen atom.
Benzene
Above are representations of benzene. The "flower" representation on the top right is an electrostatic potential energy surface that answers the most important question in chemistry, namely where are the electrons in benzene. The red color indicates the region of high electron density that is focused on the pi system. The blue color indicates the area of lowest electron density. This picture makes sense since the pi electrons, depicted in the lower two pictures, are exposed to solution, not surrounded by atomic nuclei like electrons in a sigma bond. Thus, these electrons provide for the red color, and they are able to interact with extremely strong (wicked strong) electrophiles. Benzene should be considered a very weak nucleophile.