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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).

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:

Bronsted acid dissociation
loss of a leaving group in an S<sub>N</sub>1

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......

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.

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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary