Final answer:
The SN2 rate refers to the speed of a substitution nucleophilic bimolecular reaction, which is second-order and occurs in a concerted fashion. The occurrence of an SN2 reaction is influenced by factors such as substrate structure, nature of the nucleophile and solvent, with less steric hindrance favoring SN2.
Step-by-step explanation:
The rate of an SN2 reaction is defined as the speed at which the reaction takes place and is measured by the change in concentration of the reactants or products over time. SN2 stands for substitution nucleophilic bimolecular, indicating that it involves two molecules in the rate-determining step, the incoming nucleophile and the substrate. The reaction order for SN2 is second-order because the rate is proportional to the concentration of both the substrate and the nucleophile. SN2 reactions occur in a concerted fashion, meaning that the bond formation and bond breaking steps happen simultaneously in a single step without any intermediates.
Several factors influence whether a reaction will proceed via SN2, such as the nature of the nucleophile, the leaving group, and the solvent. SN2 reactions are favored in less hindered substrates (methyl or primary) and with strong nucleophiles in aprotic solvents. Steric hindrance in tertiary substrates prevents SN2 reactions, as a result, tertiary substrates often undergo elimination or other substitution reactions like E2, SN1, or E1.