The S_N2 reaction, or bimolecular nucleophilic substitution, is a type of chemical reaction where a nucleophile attacks an electrophile, resulting in the substitution of one group for another. In this process, the nucleophile approaches the electrophile from the opposite side of the leaving group, leading to a transition state where both the nucleophile and the leaving group are partially bonded to the electrophile. This results in a concerted reaction mechanism, meaning that bond formation and bond breaking occur simultaneously. S_N2 reactions are characterized by their second-order kinetics, which means the reaction rate depends on the concentration of both the nucleophile and the substrate. Typically, primary and secondary alkyl halides are more reactive in S_N2 reactions, while tertiary alkyl halides are less favorable due to steric hindrance. Common nucleophiles include hydroxide ions and alkoxide ions, while halides often serve as leaving groups.