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 groups affect the intrinsic reactivity, not the nucleophilic discrimination factors.

The lower the pK_a 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. S_N1 reactions 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.

External links

• Strength (http://www.madsci.org/posts/archives/feb2002/1014145631.Ch.r.html): Bluffton College

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In the non-mechanistic transformations, leaving group is the actual substituent group present in the substrate and product.