Final answer:
The correct mechanism for an SN₂ reaction is the SN₂ mechanism, which involves a bimolecular interaction between the nucleophile and the electrophilic substrate, and is influenced by substrate structure, solvent, and base/nucleophile strength.
Step-by-step explanation:
The correct mechanism for the given SN₂ reaction is SN₂ mechanism. The substrate structure and the reaction conditions determine whether a reaction will proceed via SN2, E2, SN1, or E1 mechanisms. SN2 reactions are characterized by a bimolecular rate-determining step involving the nucleophile and the electrophilic substrate. This is in contrast to SN1 and E1 reactions, which have a unimolecular rate-determining step due to the formation of a carbocation intermediate. E2 reactions also involve the removal of a proton adjacent to the leaving group, and, like SN2, they tend to occur with strong bases or nucleophiles.
For an SN2 reaction, the substrate preference is methyl > primary > secondary > tertiary, with tertiary substrates typically not reacting due to steric hindrance. In contrast, E2 reactions are favored with more substituted, sterically hindered substrates. SN1 and E1 reactions are more likely to occur with secondary and tertiary substrates where a stable carbocation can form. Solvent and nucleophile/base strength also play critical roles, with polar aprotic solvents favoring SN2 and strong bases favoring E2.