In polar APROTIC solvents (DMSO, DMF, acetone): Nucleophilicity follows basicity: F- > Cl- > Br- > I- (more basic = more nucleophilic)

In polar PROTIC solvents (H2O, MeOH, EtOH): Nucleophilicity is REVERSED: I- > Br- > Cl- > F- (smaller ions are more heavily solvated, reducing their nucleophilicity)

Key nucleophilicity trends:

• RS- > RO- (sulfur is more polarizable, better Nu)
• CN- is a great nucleophile and weak base (favors SN2 over E2)
• I- is a great nucleophile (large, polarizable) and very weak base (favors SN2)
• KOtBu is a strong bulky base but moderate nucleophile (favors E2)