When the leaving group is expelled, it possess a negative charge. Therefore leaving goups 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?
SN2 reactions experience the same leaving group trends as in SN1 reaction.
TsO- > NH3 > I- >H2O > Br- > Cl- > F- >> HO- > NH2-