Why is Cl- ion more stable than Acetate ion?

Question

Background : I was reading about leaving groups... and then I did a question which was about comparing the leaving tendency of Chloride and Acetate ions:-

Which is a better leaving group?
(A) $Cl^{-}$ (B) $CH_3COO^{-}$

Since I am preparing for competitive exams, my chemistry professor had provided with a trick , and that was to check the stronger conjugate acid out of both.

So technically we will obtain HCl as conjugate acid from the first ion and acetic acid from the second ion , and obviously $Cl^{-}$ will be the better leaving group. Obviously this is only a trick to get the question right, but I have a conceptual question.

Main Question

I have been told that $Cl^{-}$ is a better leaving group compared to the acetate ion. That would mean that $Cl^{-}$ is more stable than Acetate ion.

But the question is , why would chloride ion be more stable than acetate ion?

See , in Acetate ion , equivalent resonance takes place because of which the negative charge is distributed equally between two oxygen atoms as shown:-

Now since equivalent resonance is (I have been told the same) a very high quality resonance, (as both the resonance structures are having equal energies) , It should be quite stable..

Where am I wrong?

---

Answer 1

The chloride ion is larger than the oxygen ion, so it is able to diffuse a negative charge to a greater extent. It’s quite hard to definitively prove this without calculating the orbital energies, so it might be best to stick with your professor’s trick although it may be frustrating.