Final answer:
SN2 reactions typically occur at methyl and primary alkyl halides with the nucleophilic attack causing inversion of stereochemistry at the carbon atom. Secondary alkyl halides may undergo SN2 or E2 mechanisms, while tertiary are unreactive in SN2 due to steric hindrance.
Step-by-step explanation:
SN2 reactions occur with alkyl halides, specifically with methyl and primary substrates. During an SN2 reaction, there is a single, concerted step where a nucleophile directly attacks an electrophilic carbon, causing a leaving group (usually a halide) to depart. This occurs with simultaneous inversion of stereochemistry, meaning the product will have the opposite configuration compared to the starting material. This mechanism is characterized by its bimolecularity, involving the nucleophile and substrate in its rate-determining step.
While SN2 is preferred for methyl and primary alkyl halides due to minimal steric hindrance, secondary alkyl halides may undergo either SN2 or E2 mechanisms, and tertiary alkyl halides are usually unreactive in SN2 due to significant steric hindrance that prevents the nucleophile from approaching the electrophilic carbon. Instead, tertiary alkyl halides typically undergo SN1 or E1 mechanisms.
SN2 mechanisms are sensitive to the choice of solvent and nucleophile. Polar aprotic solvents are conducive to SN2 reactions, as they do not solvate anions as strongly, allowing the nucleophile to be more reactive. Moreover, the strength and steric bulk of the nucleophile influence whether a reaction proceeds via the SN2 pathway.