# Why are weak acids not considered good leaving groups?

So today in my first year lecture my professor said that weak bases are good leaving groups. I understand why they are good leaving groups in comparison to unstable strong bases but why are weak acids not included? Acids are electron pair acceptors, shouldn't they be better able to take on the donated electron pair? Furthermore, when we say weak bases, weak in comparison to what? To be substrate the solvent or to other potential leaving groups?

• You should think Brønsted acids/bases rather than Lewis acids/bases in this context. – Jan Nov 18 '16 at 22:25
• I figured, that's the way that it would work according to the weak bases good leaving groups rule, but my instinct was Lewis because of the electrons moving toward the leaving group. Why is Lewis incorrect here? – cgug123 Nov 18 '16 at 22:27
• Well, the thing that’s leaving will always be some kind of base — it’s taking its electron pair with it. The only thing you can still decide is whether it is the base of a weak or a strong Brønsted acid. The Lewis theory would require you to look at the basic properties only (The Lewis acid being the electrophile that is attacked nucleophilicly) so you can’t argue with acidic strength. – Jan Nov 18 '16 at 22:29
• My original thought was that it should be a weak acid because they can handle the extra electrons as they are the electron acceptors. However the thought that the base is taking them back also makes sense. – cgug123 Nov 18 '16 at 22:34

Leaving group ability is tied to the stability of the leaving group. Acidity is directly related to the stability of the conjugate base. If you're a strong acid, the conjugate base is relatively stable in solution. Less so, if you're a weak acid. So the stability of the conjugate base is directly proportional to both acidity and leaving group ability.

Edit:

$$\ce{HA->H+ + A-}$$

$\ce{A-}$ is the species we're evaluating as the leaving group. Basically, it's a good leaving group if it's stabilized. If that's the case, the acid dissociation equilibrium also shifts toward products, making $\ce{HA}$ a stronger acid. You should be careful to differentiate between $\ce{HA}$ and $\ce{A-}$ when you're thinking about these concepts.

• So, essentially, acids are not good leaving groups because they have unstable conjugate bases, but bases are good leaving groups because they do have stable conjugate acids? – cgug123 Nov 18 '16 at 19:59
• I'm not sure how you got to that conclusion from my answer, so I guess my answer was bad... Let me edit it... – Zhe Nov 18 '16 at 20:02
• Oh!!! Okay thank you yes you were right I was thinking about HA and not A- I think I understand now! – cgug123 Nov 18 '16 at 20:07