Nucleophilic substitution is a substitution in which an electron-rich nucleophile forms a bond to an electrophilic carbon, displacing a leaving group that departs with the bonding electron pair. It matters because it is one of the most important reactions for converting one functional group into another, especially on alkyl halides and protonated alcohols. It proceeds by one of two mechanisms, the one-step SN2 or the two-step SN1, depending on the substrate and conditions.

亲核取代是一类取代反应,其中富电子的亲核试剂与亲电碳成键,并把带着成键电子对离去的离去基团取代下来。它之所以重要,是因为它是把一种官能团转化为另一种官能团最重要的反应之一,尤其适用于卤代烷和质子化的醇。它按两种机理之一进行——一步的 SN2 或两步的 SN1——视底物和条件而定。