The S2 reaction (also known as bimolecular nucleophilic substitution) is a substitution reaction in organic chemistry. It is a type of nucleophilic substitution, where a lone pair from a nucleophile attacks an electron deficient electrophilic center and bonds to it. This expels another group called a "leaving group". So, the incoming group replaces the leaving group in one step. Since two reacting species are involved in the slow, rate-determining step of the reaction, this leads to the name "bimolecular nucleophilic substitution", or "S2". Among inorganic chemists, the S2 reaction is often known as the "interchange mechanism".