better leaving groups
Summary
A better leaving group departs more easily with the bonding electron pair because it is more stable afterward. Weak bases, the conjugate bases of strong acids (I⁻, Br⁻, tosylate), are generally good leaving groups, while strong bases like OH⁻ and NH₂⁻ are poor.
Detail
Meaning: A leaving group is the atom or group that departs with the electron pair from the carbon it was bonded to in SN1, SN2, E1 and E2 reactions. The better it stabilizes the extra negative charge, the lower the transition state energy and the faster the reaction. Stability comes from electronegativity, large size (polarizability) and resonance, so good leaving groups are typically weak bases.
Equation: Leaving group ability increases as pKa of HX decreases. HX is the conjugate acid of the leaving group X⁻, and pKa = −log Ka is dimensionless.
On the MCAT: You rank leaving groups or predict which substrate reacts faster. Expect I⁻ > Br⁻ > Cl⁻ > F⁻, with tosylate and mesylate excellent. A poor leaving group such as OH⁻ can be improved by protonating it with strong acid, which lets H₂O leave, or by converting the alcohol to a tosylate. Better leaving groups speed SN1 (where departure is the rate-determining step) and SN2.
Common confusion: Leaving group ability is not the same as nucleophilicity, and a strong base is not a good leaving group. Fluoride is the poorest halide leaving group because HF has the highest pKa of the hydrogen halides, so F⁻ is the most basic halide.
Sources
- Organic Chemistry (Clayden)
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.