Loss of Leaving Group

The third fundamental mechanistic pattern is Loss of a Leaving Group, which involves heterolytic bond cleavage. In this process, an atom or a group departs from a molecule, taking the shared electron pair of the broken bond with it.

Key Principle of Leaving Group Departure:

• The leaving group takes the electrons of the bond with it. For a group to be a "good" leaving group, its conjugate base must be stable, meaning it can effectively accommodate the negative charge it acquires upon departure.

In this straightforward example, the bromine atom, acting as a leaving group, departs from t-butyl bromide, taking the bonding electrons with it to form a bromide ion and a t-butyl carbocation.

Loss of a Leaving Group - Multiple Arrow Scenarios:

More than one curved arrow may be needed to depict the loss of a leaving group, especially when coupled with other electron-pushing events like resonance.

In this more complex scenario, while only one arrow directly shows the chloride atom leaving, other arrows are simultaneously involved in depicting resonance. Here, electron density is pushed from the nitro group (−NO₂) to facilitate the departure of the leaving group, stabilizing the intermediate that forms.