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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.

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  • $\begingroup$ Please be more specific in the "general idea that the conjugate base to a weak acid is a strong base". How general it is in your opinion? All carboxylic acids, e.g.? $\endgroup$
    – ssavec
    Commented Jan 27, 2014 at 12:44
  • $\begingroup$ @ssavec Well, it's true in some cases, such as with $\mathrm{NH_3}$ and its conjugate base $\mathrm{NH_2^-}$, but it's not always true such as in the case of hydrogen acetate and acetate. I'm just simply curious about how much resonance affects how basic a given compound is. $\endgroup$
    – Alex Petzke
    Commented Jan 27, 2014 at 14:27
  • $\begingroup$ Maybe there is some confusion on what the weak acid means. Just calculate the equilibrium concentration of the species involved and guess yourself, what is weak and what is strong. To call NH3 a weak acid is technically possible, but nonsense, similar to call a boling water "weakly cold". For sure, it is cold compared to e.g. lava, but for common use, it is damn hot. And this acid/base scale is even muuuuch wider, than temperature. $\endgroup$
    – ssavec
    Commented Jan 27, 2014 at 16:07
  • $\begingroup$ @ssavec I'm completely aware that the idea of a strong/weak acid is completely relative, but it's a well-established notion. In Carey's Organic Chemistry text, 7th ed., he offers as a definition that a strong acid is an acid stronger than $\mathrm{H_3O^+}$ and a weak acid as an acid weaker than $\mathrm{H_3O^+}$. And you're really missing the point - I just want to know how the basicity of a species is affected by the presence of a resonance structure. $\endgroup$
    – Alex Petzke
    Commented Jan 27, 2014 at 19:58
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    $\begingroup$ I can agree with the cited definition. Find one also for the basicity. And I am not missing the point, I am openly ignoring it. Whatever stabilizing effect on acid or it's conjugated base affects the pKa and pKb of the reverse reaction by the very same amount, but in opposing sense. Thermodynamics. $\endgroup$
    – ssavec
    Commented Jan 28, 2014 at 21:11

1 Answer 1

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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.

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