What is a good “leaving group?”

Posted on July 10th, 2012

In organic chemistry we use the term leaving group to refer to a portion of a reactant that will depart the rest of the molecule as a result of reacting with another reagent. This occurs often in organic chemistry reactions. Notably in substitution and elimination reactions.

For example in the following SN2 reaction:
The leaving group is the Br. This is easy to notice because it has “left” the main reactant: CH3CH2Br                                                                           Now OH is in the place of the Br hence this is a substitution reaction.

Common questions and answers:

So what makes a good leaving group “good”?
Answer: weak bases make good leaving groups

What are some examples of good leaving groups?
-OTS (tosylate)