Pictures of the Day 4-11-08

 
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.