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?
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:

$$\ce{HA-&gt;H+ + A-}$$

$\ce{A-}$ 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.