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Leaving Group Effects

SN1 - Leaving group effect

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?

SN2 - Leaving group effect

SN2 reactions experience the same leaving group trends as in SN1 reaction.

TsO- > NH3 > I- >H2O > Br- > Cl- > F- >> HO- > NH2-

 

 

Take Note

A good leaving group is the conjugate base of a strong acid (i.e. a weak base).  Typically, the best leaving groups are the conjugate base of an acid with a pKa < 0.