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 functional group to leave is called lability. Leaving groups affect 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. Conversely, a strong base is a poor leaving group. SN1 reactions prefer halide ions as leaving groups.
In room temperature water, the sequence of lability is:
- Weak, unstable, great lability
- amine NH2-
- methoxy CH3O-
- hydroxyl HO-
- carboxylate CH3COO-
- F
- water
- Cl
- Br
- I
- azide N3
- thiocyanate SCN
- nitro NO2
- cyanide CN
- Strong, stable
- NO3 is also weaker than F.
During the SN2 nucleophilic attack, a partial negative charge forms on the leaving group. During SN1 reactions, the leaving group anionizes and leaves.
In the non-mechanistic transformations, leaving group is the actual substituent group present in the substrate and product.
External links
- Strength (http://www.madsci.org/posts/archives/feb2002/1014145631.Ch.r.html): Bluffton College