Leaving Groups or Leaving Nucleophiles
A leaving group (LG) is an atom or a group of atoms that breaks away from a molecule as stable species taking with its bonding electrons during a chemical reaction. Leaving groups may be neutral, anions, or cations. halides, cyanides, water, sulfonic acid are common examples of leaving group. Good leaving groups are weak bases that are stable and can stabilize the negative charge of the product and transition state.
The leaving group must have the following three characteristics
1. A leaving group is electron widthdrawing so that it creats a partial positive charge on the carbon atom.
2. The laeving group should be stable after leaving with the bonding pair of electrons. In general, good leaving groups should be weak bases and therefore, they are the conjugate bases of strong acids.
3. The ability of a species to act as a good leaving group depends on its polarizibility; that is its ability to continue to bonding with a carbon atom while it is leaving. This bonding stabilities the transition state, minimizing the activation energy.
The leaving power of some nucleophilic groups are given below-
