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The SN1 is a two-step mechanism which begins with heterolysis of the C-X bond. What triggers this? Are there external causes that prompt the leaving group to simply leave?
This question is more conceptual than anything, but I can't seem to find answers anywhere.
Recall that SN1 heavily favors protic solvents (polar/H+). In such cases, the nucleophile to-be-leaving-group (to be Nu-) is going to be attracted to the positively charged molecules of the protic solvent (...H+) and away from the molecule it is attached to. The power of the attraction comes from the dipole between C-Nu, where Nu has the negative bit of the dipole.
At the end of this step, when the nucleophile attached group leaves, you often see "Nu-" hanging around, but really it's with the solvent, e.g. H3O+Br- , and it's fairly common to omit the solvent here and in other similar ionic situations, if I'm not mistaken.
Examples that invoke hydrogen bonding make this more apparent.
F-C...(tertiary) + H2O
The dipole here looks like (δ-)F-C...(δ+) because F is more electronegative
The F is strongly attracted to the closer H: (δ+)H-(δ-)O-H(δ+)(δ-)F-C...(δ+) (pretend the HOH is bent :)
Greater nucleophilicity of the attacking group than the nucleophilicity of the leaving group causes SN1 reactions. The cleavage of the C-X bond keeps occuring always, both the reactant and the product remaining in the solution. The equilibrium shifts towards the final product.
