Question

Why are sulfate ion and p‐toluenesulfonate (tosylate ion) good leaving groups?

Answer 1

The methyl sulfate and p‐toluenesulfonate ions are good leaving groups because they are weak bases and are stabilized by resonance.

Explanation:

Good leaving groups are weak bases. They don't want to share their electrons.

For example, the `"p"K_"a"` of p‐toluenesulfonic acid is -2.8. The `"p"K_"b"` of its conjugate base, p‐toluenesulfonate ion, is 16.8 — a very weak base.

These ions are stabilized by resonance, and that also makes them good leaving groups.