It's Carbon's nature to form new bonds, and in its quest, it breaks away from the old ones.
The atom or group of atoms that leaves a carbonyl carbon or a saturated/unsaturated carbon to make way for the new bond is called the leaving group(s).
The leaving group may leave before or after the incoming atom approaches during the bond formation reaction. Therefore, it may happen that the leaving group first leaves then the incoming groups comes in to form the bond. Or the entry of the incoming groups and the exit of the leaving group may happen simultaneously.
But, before it leaves, the leaving group may take away the shared electron pair through a process known as heterolytic bond cleavage. At times, the atoms leave behind their electrons and go away with nothing. These decisions depend on the nature of the leaving group.
Nature of the Leaving group
If the leaving group chooses to walk away with the bond electrons, then it is called a nucleofuge. A nucleofuge can be negatively charged or neutral in nature.
For example, the chloride is displaced as Cl- by the hydroxide ion (OH-) on forming a new bond with the carbon. Or the hydroxide displaces a neutral molecule of water.
When the incoming group is a nucleophile, the leaving group is always a nucleofuge.
The nucleofuge is also displaced from carbonyl-containing substrates.
However, if the leaving group leaves behind the bond electrons and goes without them, it is called an electrofuge. The most common atom that leaves behind its electron and goes away naked is Hydrogen as H+.
The loss of Hydrogen as H+ is a common occurrence in aromatic electrophilic substitution reactions, where incoming electrophile displaces another electrophile (H+) as a leaving group.
For example, in the nitration reaction of the benzene ring, the incoming electrophile is a NO2+ group, and the H+ is the leaving group.
However, apart from proton (H+), the cationic leaving groups are uncommon. The most common leaving groups are anionic in nature.
Presently, the usage of the term- ‘electrofuge or nucleofuge’ is not so prevalent and they are combinedly referred to as the ‘leaving group’.
The role of a leaving group is of the highest importance to the substrate to transform further by attaching various functional groups. The leaving group should enable a faster reaction and facilitate a smoother transition.
Good and Bad leaving groups
A good leaving group is those atoms that take away the bond electron pair and does not share it with anyone else, basically hoarding it. It tries to house those electrons by giving them maximum stability. If Lewis defined bases as good (or strong) electron donors, then the leaving groups try not to be good Lewis bases. They are, therefore, called weak bases and are good leaving groups.
However, bad leaving groups are those atoms that refuse to leave the substrate and tend to hold on. Even if they depart, they try to partner up easily by transacting their electrons, acting as good (strong) Lewis bases. So, these bad leaving groups show less stability on being solo and subsequently also display high reactivity.
Hence, how well the leaving group stabilizes the extra electrons it achieved post-bond breaking determines whether it is a good or a bad leaving group.
Some examples of good and bad leaving groups are-