Pictures of the Day 4-17-06

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.
Arenium ion
Shown above are models of the arenium ion produced during the chlorination of benzene. The chlorine atom is brown. As described by the electrostatic potential energy surface on the right, the positive charge (blue color) of the arenium ion is localized at the positions ortho and para to where the electrophile is attached. You should be able to draw resonance forms that predict the ortho and para distribution of the positive charge. This ortho and para distribution of positive charge is important because substituents already on an aromatic ring must interact with this charge distribution, and the result of that interaction is a modulation of rate and orientation of reaction. By understanding these interactions, you will be able to predict accurately the products of electrophilic substitution reactions that take place on aromatic rings containing substituents.