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

1 Answer
Jan 15, 2015


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


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.