Leaving group

A Leaving group is a term in the field of organic chemistry that refers to an atom or group of atoms that departs with a pair of electrons in heterolytic bond cleavage. Leaving groups can be anions or neutral molecules, but in either case it is crucial that the leaving group be able to stabilize the additional electron density that results from bond heterolysis.

Characteristics of Good Leaving Groups

Good leaving groups are those that can stabilize the extra electron density that results from bond cleavage. This can be achieved through resonance, inductive effects, and by the presence of a full or partial positive charge.

Examples of Leaving Groups

Common leaving groups include halides such as Cl-, Br-, and I-, as well as sulfonate esters such as tosylate (OTs-) and mesylate (OMs-). Water (H2O) and alcohols (ROH) can also act as leaving groups under certain conditions.

Leaving Group Ability

The ability of a group to act as a leaving group can be influenced by a variety of factors, including the nature of the substrate, the solvent, and the specific reaction conditions. In general, the best leaving groups are those that are the weakest bases.

Leaving Group in Substitution and Elimination Reactions

In both substitution reactions and elimination reactions, the leaving group plays a crucial role. In a substitution reaction, the leaving group is replaced by a nucleophile, while in an elimination reaction, the leaving group departs along with a proton to form a pi bond.

See Also

• Nucleophile
• Electrophile
• Substitution reaction
• Elimination reaction

References

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  Three types of reactions involving leaving groups
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  Ionization reaction
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  Reaction of propyl halide with toluene thiolate
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  Reaction of ethyl halide with ethoxide
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  Activation of leaving group by Lewis acid complexation
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  E1cB reaction
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  Elimination continuum
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  Reductive elimination involving leaving group