MU - Chemistry 210
IAE on Organic Acids
First, you need to go back and review the very basics of the
acid dissociation and equilibration. You should be able to write down
the acid dissociation equilibrium, the formula for the equilibrium
constant Ka, and know the definition of the pKa. You can review the
basics in any book on general chemistry. You can also review this
material using ChemTutor's site on Acids and Bases.
Now, let's get a few basic ideas about magnitudes right. For simplicity
always consider a 1M solution of an acid HA.
If one in ten molecules dissociates, then Ka = (0.1 x 0.1) / 0.9 = 0.011.
If we approximate 0.9 as 1.0, then the approximate pKa = 2 results.
If one in one hundred molecules dissociates, then Ka = (0.01 x 0.01) /
0.99 = 1.01E-4. If we approximate 0.99 as 1.0, then the approximate pKa =
If one in one thousand molecules dissociates, then Ka = (0.001 x 0.001) /
0.999. If we approximate 0.999 as 1.0, then the approximate pKa = 6
Keep playing this game. For a pKa = 20, typical for many organic
compounds, what is the ratio between undissociated and dissociated acid?
What for pKa = 40?
An incredible wealth of acidity data of organic compounds has been
measured by the group of Professor Bordwell of Northwestern University.
Table for a while to get an idea as to what kinds of things are known.
Come back to this table in future as we proceed to discuss the various