Leaving Group Order
For common organic chemistry leaving groups, the order of leaving-group ability is:
I⁻ > Br⁻ > Cl⁻ > F⁻
So:
RI > RBr > RCl > RF
for SN1/SN2 reactions, generally.
Common leaving groups — good → poor
OTf⁻ > OTs⁻ ≈ OMs⁻ > I⁻ > Br⁻ > Cl⁻ > OAc> F⁻>OH–> NH2->H-
- OTf = triflate
- OTs = tosylate
- OMs = mesylate
Important trick
“The weaker the base, the better the leaving group.”
For halide ions, basicity increases:
F⁻ > Cl⁻ > Br⁻ > I⁻
Therefore leaving-group ability is the reverse:
I⁻ > Br⁻ > Cl⁻ > F⁻.
