A type of substitution reaction in which a nucleophile displaces another group or atom from a compound. For example, in CR3Cl+OH − → CR3OH+Cl − the nucleophile is the OH − ion. There are two possible mechanisms of nucleophilic substitution. In SN1 reactions, a positive carbonium ion is first formed:CR3Cl → CR3++Cl − This then reacts with the nucleophile CR3++OH − → CR3OH The CR3+ ion is planar and the OH − ion can attack from either side. Consequently, if the original molecule is optically active (the three R groups are different) then a racemic mixture of products results.
CR3Cl+OH − → CR3OH+Cl −
CR3Cl → CR3++Cl −
CR3++OH − → CR3OH
The alternative mechanism, the SN2 reaction, is a concerted reaction in which the nucleophile approaches from the side of the R groups as the other group (Cl in the example) leaves. In this case the configuration of the molecule is inverted. If the original molecule is optically active, the product has the opposite activity, an effect known as Walden inversion. The notations SN1 and SN2 refer to the kinetics of the reactions. In the SN1 mechanism, the slow step is the first one, which is unimolecular (and first order in CR3Cl). In the SN2 reaction, the process is bimolecular (and second order overall).