Why do leaving groups detach from molecules?

[Runei]

Hi,

My question is short and very simple: Is the loss of a leaving group primarily a random event?
What is the actual mechanism that initiates that a specific leaving group.. leaves?

Thanks in advance :)

 

[TeethWhitener]

This depends on the mechanism. If you're familiar with elimination/nucleophilic substitution mechanisms, then the leaving group in the E1/S_N1 pathways are either attached or detached from the parent molecule in an equilibrium:
$$R-X\leftrightharpoons R^+ + X^-$$
Which side of the equilibrium a given molecule lies on at any particular moment is largely random in solution. For E2/S_N2 mechanisms, the transition state is bimolecular. This means that the substituting/abstracting group needs to be present to effect the loss of the leaving group. For other substitutions, like electrophilic aromatic substitution, a base usually abstracts a proton from the arenium intermediate. More complex mechanisms than this require more complex answers.

 

[Runei]

Thank you, so much for your answer !