Leaving group
A
leaving group is an
atom or
molecule that detaches from an organic molecule, which, after detachment, is called the residual or main part. The ability for a leaving to leave is
lability. Leaving group affects the intrinsic reactivity, not the nucleophilic discrimination factors.
The lower the pKa of the conjugate acid, the better the leaving group, because then they can easily stabilize the developing negative charge. SN1 reactionss prefer halide ions as leaving groups.
In room-temperate water, the sequence of lability is:
- Weak, unstable, great lability
- NH2-
- CH3O-
- HO-
- CH3COO-
- F
- water
- Cl
- Br
- I
- N3
- SCN
- NO2
- CN
- Strong, stable
- NO3 is also weaker than F.
During the S
N2 nucleophilic attack, a partial negative charge forms on the leaving group. During S
N1 reactions, the leaving group anionizes and leaves.
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In the non-mechanistic transformations, leaving group is the actual substituent group present in the substrate and product.