additionelimination

Addition–elimination is a two-step reaction mechanism in which a nucleophile adds to an electrophilic center to form a transient intermediate, which then eliminates a leaving group to give the product. The process effectively substitutes part of the substrate via a temporary addition, rather than occurring in a single concerted step.

Two common contexts are nucleophilic acyl substitution and nucleophilic aromatic substitution (S_NAr). In acyl substitution, nucleophiles such as alcohols, amines, or alkoxides attack the carbonyl carbon of an acid chloride, anhydride, or activated ester to form a tetrahedral intermediate. Collapse of this intermediate ejects a leaving group (for example chloride, alkoxide, or amide anion) and yields a substituted carbonyl compound (an ester, amide, or anhydride). In S_NAr, nucleophiles add to electron-deficient aromatic rings at a carbon bearing a leaving group, creating a Meisenheimer complex; loss of the leaving group restores aromaticity and delivers the substituted arene.

Kinetics and scope: The reaction favors substrates with good leaving groups and, for S_NAr, electron-withdrawing substituents on the ring to stabilize the intermediate. The rate-determining step is typically the initial addition to the substrate, with elimination occurring rapidly thereafter. Solvent, temperature, and nucleophile strength influence both steps.

Limitations and alternatives: Addition–elimination is not universal; in many substrates, direct substitution may proceed via concerted S_N2 pathways or via radical mechanisms. The choice of mechanism depends on substrate structure, leaving-group ability, and reaction conditions.