|Chapter 8: Nucleophilic Substitution|
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).
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 SN1 type reaction.
Note the similarity of the two equations: both show heterolytic cleavage of a σ 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......
|Very Good||I-, H2O|
|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.
|© Dr. Ian Hunt, Department of Chemistry|