Structure and Acidity

Acid/Base Equilibrium

The acidity of a molecules is directly related to the structure of the molecule.  Its most convenient to discuss acidity by doing comparisons with other structures, hence the reason why organic chemists have developed the pKa scale.

You can think of every reaction as being an equilibrium reaction especially acid-base reactions.  For example consider some arbitrary acid H-A₁ .  If it were to spontaneously loose a proton it would form the conjugate base A₁^-.  Notice that the conjugate base has a charge of -1 less than the acid.  It has an over abundance of electrons.

If A₁^- is very stable then the equilibrium would be to the right side and the equilibrium constant would be very large.  We would have a strong acid because as a consequence of the equilibrium being on the right there are more H⁺ ion.  If A₁^- is not very stable then the equilibrium would be to the left side and the equilibrium constant would be very small.  We would have a weak acid.

What makes a conjugate base stable?

It all comes down to the conjugate base being able to handle the increased charge or electrons it must deal with.  Charge is best handled if it is located on electronegative atoms and/or of it can be "smeared out" or distributed amongst other atoms by resonance.

Example 1

Methane has a pKa=60, while methanol has a pKa of 15.  Explain?

You need to examine and compare the structures of the conjugate bases of each.  The conjugate base of methane (methyl anion) is has the negative charge (over abundance of electrons) on a C atom while the conjugate base of methanol (methoxide anion) has the negative charge on an O atom.  Since O is more electronegative than C, the O atom can handle the negative charge better making it more stable.  The equilibrium for methanol would be further to the right since its conjugate base is more stable.  

Example 2

Explain why acetic acid is about 10¹¹ times more acidic than methanol.

As before we begin by writing out the conjugate bases.  In this case the negative charge resides on O atoms in each.  However you should quickly recognize the three-atom resonance grouping as discussed earlier.  The conjugate base of acetic acid, the acetate anion has another resonance structure.  The negative charge is "smeared" or delocalized over each O atom.  Thus since the conjugate base of acetic acid is more stable than that for methanol, acetic acid is more acidic.

Armed with these two examples you should be able to rationalize most any structures acidity or lack thereof.