Nucleophilic substitution

A reaction in which a nucleophile replaces a leaving group attached to an atom, usually carbon. It proceeds through concerted displacement or a stepwise pathway involving a carbocation.

SN2 reaction: A bimolecular nucleophilic substitution in which bond formation and leaving-group departure occur in one concerted step. This pathway replaces the leaving group directly, with inversion at the reacting stereocenter.
Nucleophile: An electron-pair donor that forms a bond to an electron-poor atom. It supplies the electron pair that replaces the departing group.
Williamson ether synthesis: A reaction that forms an ether by SN2 attack of an alkoxide on an alkyl electrophile. It is a widely used synthetic application of nucleophilic substitution.
Elimination reaction: A reaction that removes atoms or groups from a molecule to form a multiple bond. Elimination competes with substitution when a nucleophile or base meets an alkyl substrate.
SN1 reaction: A unimolecular nucleophilic substitution that proceeds through a carbocation intermediate. This stepwise pathway separates leaving-group departure from nucleophile attack.
Leaving group: An atom or group that departs from a molecule with the bonding electron pair. Its ability to depart strongly affects substitution rate and feasibility.
Gabriel synthesis: A method for preparing primary amines using phthalimide alkylation followed by release of the amine. Its alkylation step uses nucleophilic substitution to install an alkyl group.
Addition reaction: A reaction in which atoms or groups add across a multiple bond, reducing its bond order. Addition consumes a multiple bond rather than replacing a group at an electrophilic center.
Transition state: A high-energy molecular arrangement at the boundary between reactants and products along a reaction pathway. Its structure distinguishes the concerted SN2 pathway from the carbocation-forming SN1 pathway.
Electrophile: An electron-pair acceptor that forms a bond with a nucleophile. The electrophilic atom attached to the leaving group is the site of attack.

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