Is acetate a weak base due to its resonance structure?

I just read that acetic acid is an exception to the general idea that the conjugate base to a weak acid is a strong base. The example says that the conjugate base of a weak acid will only be strong if the acid is a weaker acid than water.

What I'm curious about though, is if acetate is a weak base due to its resonance structure. Does the resonance cause acetate to be more stable than hydrogen acetate?

Is this true in the more general case where some substituent can bond to the atoms involved in the resonance?

Thanks.

Answer

Consider this generic acid-base reaction in water: $$\ce{HA <=>[K_\text{a}] A- + H+ }$$

The (thermodynamic) dissociation constant $K_\text{a}$ depends mainly on two things:

• The stability of the product $\ce{A-}$


• The instability of the starting material $\ce{HA}$

The higher one of those factors is, the higher the dissociation constant. So in effect, what a chemist states is the following:

The carboxylate anion is the conjugated base of the carboxylic acid, whose strength is determined by the stability of said carboxylate ion and the instability of the acid. It may be reasoned that carboxylic acids would be much less strong if it was not for the additional stability via conjugation.