Rule: The weaker the base, the better the leaving group.

Order: I- > Br- > Cl- > F- (for alkyl halides — correlates with bond weakness) Equivalent: TsO- (tosylate) ≈ Br- (excellent LG, often used to activate alcohols)

For haloarenes (SNAr): The order REVERSES: F > Cl > Br > I. In SNAr, the addition step (forming Meisenheimer complex) is rate-determining, NOT the LG departure. F is most electronegative → stabilizes the Meisenheimer complex best → fastest reaction.

Common LG conversions:

• OH is a terrible LG (OH- is a strong base) → convert to -OTs, -OMs, or protonate to -OH2+
• NH2 is a terrible LG → convert to -N2+ (diazonium) — excellent LG