Leaving group

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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 pK_a of the conjugate acid, the better the leaving group, because then they can easily stabilize the developing negative charge. Without stabilization, a leaving group will become a nucleophile due to its negative charge, and if this happens, the reaction goes in circles. This is why a strong base is a poor leaving group. S_N1 reactions prefer halide ions as leaving groups.

In room temperature water, the sequence of lability is:

Less lability
amine NH₂^-
methoxy CH₃O^-
hydroxyl HO^-
carboxylate CH₃COO^-
F^-
water
Cl^-
Br^-
I^-
azide N₃^-
thiocyanate SCN^-
nitro NO₂
cyanide CN^-
Greater Lability
: NO₃ is also weaker than F^-.

A direct application of the difference in leaving group stability is in the carboxylic acid derivatives. The worse the leaving group, the more stable the chemical, which makes these form a hierarchy that can be separated in the lab. Amines, which are the worst leaving group, are the most stable compound, and attacking any carboxylic acid derivative with an amine produces an amide. Esters are the second most stable followed by anhydrides and finally halides.

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. In general, amine, methoxy and hydroxyl groups never act as leaving groups in a substitution reaction.

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

External links

[Strength]: Bluffton College

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Leaving group

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