A good leaving group is one that is stable as an anion — a weak base, the conjugate base of a strong acid. The lower its conjugate acid's pKa, the better it leaves.
In every SN1/SN2/E1/E2 reaction a bond to carbon breaks and one group leaves — so leaving-group stability decides whether the reaction goes at all.
The whole story in one row: stable, weak-base anions leave; strong-base anions do not.
1. A leaving group departs with the bonding electrons
The C–LG bond breaks heterolytically — both electrons leave with the group, so it must hold the negative charge on its own.
Iodide leaves easily because it is a stable, weak base; run it backward and hydroxide refuses to leave.
2. The best leaving groups are the weakest bases
Every leaving group is a conjugate base, so you can rank leaving-group ability straight off a pKa table.
Lower pKa of the conjugate acid → weaker base → better leaving group.
HI, HBr, and HCl are strong acids, so those halides leave well; water's pKa of 15.7 marks hydroxide a strong base and a dreadful leaving group.
3. Down the halogens, leaving-group ability improves: I⁻ > Br⁻ > Cl⁻ ≫ F⁻
The trend follows size: big, soft iodide spreads its charge and leaves best, while tiny, hard fluoride holds its charge tightly and barely leaves.
Bond polarity is a red herring — what matters is anion stability, which is why fluoroethane essentially does not undergo SN2.
4. Sulfonates — tosylate and mesylate — are among the best leaving groups
A departed sulfonate spreads its charge over three SO₃ oxygens by resonance, making it a very weak base (TsOH pKa ≈ −2.8) that rivals iodide.
Their strategic value: esterify an inert alcohol –OH as a tosylate and that carbon is instantly ready to react — without disturbing its stereochemistry.
5. Hydroxide is terrible — so alcohols must be activated first
An alcohol won't do SN2 directly because its leaving group would be hydroxide; two standard fixes solve this.
Fix A — protonate it. Strong acid turns –OH into –OH₂⁺, so neutral water (H₃O⁺ pKa −1.7) leaves instead.
Protonation converts a bad LG (OH⁻) into a good one (neutral H₂O).
Fix B — convert it to a tosylate or halide. TsCl (or SOCl₂/PBr₃) swaps the –OH for a first-rate leaving group before the substitution.
The same logic explains the "bad" list — OH⁻, OR⁻, NH₂⁻, H⁻, and F⁻ are all strong bases that won't leave voluntarily.
6. Summary
Stable anion = weak base = conjugate base of a strong acid · lower conjugate-acid pKa → better LG · I⁻ > Br⁻ > Cl⁻ · sulfonates top-tier · water fine, OH⁻/OR⁻/NH₂⁻/H⁻/F⁻ too basic · activate alcohols by protonation or tosylate/halide.
Quiz yourself
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Stability as it leaves — i.e. being a weak base (a stable anion). The best quantitative predictor is a low pKa for its conjugate acid.
I⁻ > Br⁻ > Cl⁻ ≫ F⁻. Larger, more polarizable halides spread their charge and are weaker bases (their conjugate acids HI, HBr, HCl are strong acids). Fluoride is small, hard, and a stronger base, so it barely leaves.
Its leaving group would be hydroxide, a strong base (H₂O pKa 15.7) that won't leave. Fix it by (A) protonating the –OH so neutral water leaves, or (B) converting it to a tosylate/mesylate or a halide first.
The departed sulfonate delocalizes its negative charge over three S–O oxygens by resonance, making it a very weak base (TsOH pKa ≈ −2.8). They also let you activate an alcohol without disturbing the stereocenter.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.