Leaving group

A leaving group is an atom or group of atoms that departs from a molecule with the bonding electron pair during a chemical reaction. Its ability to depart depends on how stable it is after separation.

Nucleophilic substitution: A reaction in which a nucleophile replaces a leaving group at an electrophilic atom. The leaving group exits as the incoming nucleophile forms a bond.
Electronegativity: A measure of an atom's tendency to attract shared electrons in a chemical bond. It helps explain which atom receives the bonding pair when a group departs.
Tosylate: The anion derived from p-toluenesulfonic acid, commonly attached to oxygen as an organic sulfonate ester. Tosylate converts an alcohol's poor hydroxyl leaving group into a useful one.
Nucleophile: An electron-pair donor that forms a bond to an electrophile. A nucleophile enters the reaction center as the leaving group exits.
SN1 reaction: A nucleophilic substitution that proceeds through a carbocation intermediate in a unimolecular rate-determining step. Its rate-determining step is the leaving group's departure to form a carbocation.
Conjugate acid: The species formed when a base accepts a proton. A leaving group's conjugate acid helps predict how stable the group is after departure.
Triflate: The trifluoromethanesulfonate group, a weakly coordinating anion and highly effective leaving group in organic reactions. Its stability after departure supports substitutions and cross-coupling reactions.
Base: A species that accepts a proton or donates an electron pair, depending on the definition used. In elimination, the base removes a proton while the leaving group departs.
SN2 reaction: A concerted nucleophilic substitution in which bond formation and bond breaking occur in one step. The nucleophile bonds as the leaving group departs from the same reaction center.
Resonance stabilization: The lowering of a species' energy through electron delocalization across multiple atoms. Delocalized charge can stabilize an anionic leaving group.

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