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.

Related Topics:
Atom - Molecule - Organic molecule - Functional group - Intrinsic reactivity - Nucleophilic discrimination factor

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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.

Related Topics:
PK_a - Conjugate acid - Charge - Base - S_N1 reaction - Halide

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In room temperature water, the sequence of lability is:

Related Topics:
Room temperature - Water

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• 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.

Related Topics:
S_N2 - Nucleophilic attack - Ion

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

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