Pictures of the Day 320N

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.

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.