My book says

To compare the acidities of two compounds, compare the stability of the anions formed by loss of a proton with respect to the original compound.

Suppose we have acetic acid and ethyl alcohol. Now on losing a proton, they form the respective anions:

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My book again says

The acetate anion is resonance stabilized whereas in the ethoxide ion the charge is localized on the oxygen atom. Hence, acetate ion is more stable as compared to the ethoxide ion and hence, acetic acid is the stronger acid.

The problem is we have only compared the stabilities of the anions with respect to each other and not with respect to the original compounds. So how do we know that it is correct?

  • $\begingroup$ Comparing basicities does give you information about related acidities. $\endgroup$ – Mithoron Mar 17 at 21:08

Once you understand that the acetate ion is more stable the the ethoxide ion, we understand the fact that after releasing The acidic hydrogen the final product formed is more stable for acetic acid as compared to ethanol. This would lead to the conclusion that acetic acid is a better acid as compared to ethanol.

While comparing the acidity of two species, only the comparative study of conjugate base is enough, since its simply the comparison between compounds.

Comparing the stability with the original compounds will lead you to the conclusion that whether the compound itself has a tendency to act as an acid or base.
Alternately one could compare the stability of the original compound and its conjugate(Which is the acid dissociation constant) and compare its value with the acid dissociation constant of other compunds to compare its acidity.

In any case comparing the stability of the conjugate ions will tell you which one of them is comparatively more acidic/basic than others. It will not let you conclude whether its the nature of the compound to naturally act as an acid/base or not. For that one should compare the stability with original compund or just check its dissociation constant.

  • 2
    $\begingroup$ Just sthg that I would like to quip here: The answerer has not explicitly mentioned why comparing stabilities of an acid and it's conjugate base gives it's acidic/basic nature. This is because we consider acidity-basicity when a dynamic equilibrium exists b/w an acid and it's Brönsted base... And here,the reaction is driven towards that side where lower Gibbs free energy is available, or which side is more thermodynamically "stable". The arguments stated above would not work at any other reaction coordinate where kinetics would be the governing factor. Otherwise, +1 $\endgroup$ – YUSUF HASAN Mar 17 at 9:35

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