What makes a good leaving group?

by James

This one is pretty simple.

Good leaving groups are weak bases. They’re happy and stable on their own.

Some examples of weak bases: halide ions (I-, Br-, Cl-) water (OH2), and sulfonates such as p-toluenesulfonate (OTs) and methanesulfonate (OMs).

The weaker the base, the better the leaving group. 

On the other hand, strong bases are bad leaving groups. This is why alcohols don’t participate in SN2 reactions very much! the hydroxyl group (HO-) is a terrible leaving group.

So the lesson for today is look for the good leaving group in a molecule. This is where the action is! It’s the place where a bond is going to form and break.

Here’s a table that might be helpful.

Tomorrow: let’s talk about one of the key mechanisms for nucleophilic substitution reactions.

Thanks for reading! James

P.S.Further reading:
What makes a good leaving group?
Reagent Friday: Tosyl chloride (TsCl)