In SN1 reactions, the nature of the leaving group is a critical factor. A good leaving group is typically a weak base, which means it is the conjugate base of a strong acid. Weak bases can stabilize the negative charge they carry after departure.
The classic example is the halide series:
Additional stabilization mechanisms include resonance delocalization (e.g., in tosylates or mesylates) and high electronegativity (which allows the leaving group to better accommodate the negative charge).
The rate-determining step of an SN1 reaction is the departure of the leaving group to form the carbocation. As such, a better leaving group directly increases the reaction rate.
For example, a compound with iodide as the leaving group will typically undergo SN1 much faster than one with chloride.
Tosylates (TsO−) and mesylates (MsO−) are commonly used synthetic leaving groups because their negative charge is extensively delocalized by resonance, making them exceptionally stable.
When the leaving group is expelled, it possesses a negative charge. Therefore, leaving groups that can handle this charge react the fastest. Good leaving groups are weak bases and stabilized anions. The following leaving group order can be expected.
TsO- > NH3 > I- >H2O > Br- > Cl- > F- >> HO- > NH2-
Can you explain why tosylates are good leaving groups? Can you draw resonance structures for it?