1. Steric Effects

2. Effect of Nucleophile Strength on Reaction Rate
Across a period:
CH3- > NH2- > OH- > F-
Nucleophilicity: Strongest Weakest
Basicity (Bronsted): Strongest Weakest
Down a group:
F- > Cl- > Br- > I- aprotic solvent
I- > Br- > Cl- > F- protic solvent (H bonds)
Nucleophilicity: Strongest Weakest