Leaving group

In organic chemistry, a leaving group typically means a molecular fragment that departs with an electron pair during a reaction step with heterolytic bond cleavage. In this usage, a leaving group is a less formal but more commonly used synonym of the term nucleofuge; although IUPAC gives the term a broader definition.

A species' ability to serve as a leaving group can affect whether a reaction is viable, as well as what mechanism the reaction takes.

Leaving group ability depends strongly on context, but often correlates with ability to stabilize additional electron density from bond heterolysis. Common anionic leaving groups are Cl−, Br− and I− halides and sulfonate esters such as tosylate (TsO−). Water (H2O), alcohols (R−OH), and amines (R3N) are common neutral leaving groups, although they often require activating catalysts. Some moieties, such as hydride (H−) serve as leaving groups only extremely rarely.

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