Leaving group ability of nucleophile a in nucleophilic substitution reactions

Leaving Groups

A leaving group , LG, is an atom (or a group of atoms) that is displaced as stable species taking with it the bonding electrons. 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 polarisability and strength) and the stability of the displaced group. The better the leaving group, the more likely it is to depart.

A "good" leaving group can be recognised 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 two equations represent Bronsted acid dissociation and loss of a leaving group in a S_N1 type reaction. 

Note the similarity of the two equations: both show heterolytic cleavage of a sinlge bond to create an anion and a cation.

For acidity, the more stable A^- is, then the more the equilibrium will favour dissociation, and release of protons meaning that HA is more acidic.

For the leaving group, the more stable LG^- is, the more it favours "leaving".

Hence factors that stabilise A^- also apply to the stablisation of a  LG^-.

Here is a table classifying some common leaving groups that we will eventually meet......

Excellent	TsO-, NH3
Very Good	I-, H2O
Good	Br-
Fair	Cl-
Poor	F-
Very Poor	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.