An SN2 reaction is a nucleophilic substitution reaction in which the rate-determining step involves two components. The reaction name derives from S standing for 'substitition', N for 'nucleophilic' and the 2 denoting the kinetic order of the reaction - or simply the number of reaction components involved in the rate-determining step.
An SN2 reaction arises from the combination of a good nucleophile and a substrate with an electrophilic reaction center attached to a good leaving group. A good example of this is the carbon-halogen (C-X) bond you'd find in an alkyl halide.
Figure 1: General SN2 reaction mechanism. L is the leaving group. Nu is the nucleophile. The partially positive carbon is an electrophile.
SN2 reactions are one-step bimolecular reactions with concerted - or simultaneous - bond-breaking and bond-making steps. Since SN2 reactions proceed in one step, a defining characteristic of this substitution is that the mechanism does not proceed via a reaction intermediate. Instead, the nucleophile coordinates to the reaction center to form a bond at the same time the C-X bond breaks simultaneously. This results in inversion of configuration at the reaction stereocenter.
Factors affecting an SN2 reaction:
Nucleophile: The best nucleophiles are strong, negatively charged, and they are high energy. Since the nucleophile is a reactant, in an energy diagram a good nucleophile would raise the energy of the reactants. By raising the energy of the reactants, the activation energy is lowered. The activation energy is the energy difference between the reactants and the transition state.
Solvent: The rate of an SN2 reaction is significantly influenced by the solvent in which the reaction takes place. Protic solvents (e.g. water or alcohols with hydrogen-bond donating ability) decrease the power of the nucleophile due to a solvation effect. Protic solvents lower the energy of the nucleophile, therefore, the energy of the reactants is lower and the activation energy is increased. Strong hydrogen-bond interactions between solvent protons and the highly reactive lone pairs on the nucleophile form a 'shell' or 'cage' that prevents the nucleophile from reacting. SN2 reactions are faster in polar, aprotic solvents (e.g. acetone) that lack hydrogen-bonding capability.
Leaving group: The best leaving group lowers the energy of the transition state, therefore, the activation energy is reduced.
Steric effects: Since SN2 reactions rely heavily on easy access to the reaction center, steric effects are one important factor that could impede reaction. A bulky alkyl halide increases the energy of the transition state, therefore, the activation energy is increased. By selecting a less sterically hindered alkyl halide and a strong nucleophile, it's possible to favor the SN2 reaction over potentially competing reactions such as the SN1 reaction or elimination reactions.