A leaving group , LG, is an atom (or a group of atoms) that is displaced as stable species taking with it the electrons in the bond that were connecting it to the rest of the molecule. Typically the leaving group is an anion (e.g. Cl-) or a neutral molecule (e.g. H2O).
There are several factors that contribute to the ability of a group to function as a leaving group. This includes properties of the C-LG bond (including it's polarizability and strength) and the stability of the leaving group after it leaves. The better the leaving group, the more likely it is to depart.
A "good" leaving group can be recognized as being the conjugate base of a strong acid.
What do we mean by this? First we should write the chemical equations for the two processes:
These equations represent Bronsted acid dissociation (top) and loss of a leaving group in a SN1 or E1 type reaction (bottom).
Note the similarity of the two equations: both show heterolytic (both electrons go to one side) cleavage of a single bond to create an anion and a cation.
For acidity, the more stable A- is, then the more the equilibrium will favor dissociation, and release of protons meaning that HA is more acidic. For the leaving group, the more stable LG- is, the more it favors "leaving".
Hence factors that stabilize A- also apply to the stablization of a LG-.
Here is a table classifying some common leaving groups that we will eventually meet......
||HO-, NH2-, RO-
Note, once again, that HO- is a poor leaving group (remember from the reactions of alcohols?).... after all its the conjugate base of water.... and when we turn on a tap in the kitchen, we aren't usually trying to get a strong acid to drink!
But water itself, H2O, is a good leaving group, since it is the conjugate base of H3O+, which is a strong acid.