Final answer:
Whether an internal or external nucleophile is favored in a reaction depends on the relative strength of the incoming nucleophile compared to the leaving group. Poor nucleophiles favor reactants, good nucleophiles favor products, and similar nucleophilic strengths lead to an equal mixture. The SN2 mechanism involves a concerted bimolecular process with inversion of stereochemistry.
Step-by-step explanation:
The choice between using an internal or external nucleophile in a reaction depends on the relative nucleophilic strength of the incoming nucleophile (HNu) compared to the leaving group (HLv). If HNu is a poor nucleophile relative to HLv, the reactants are favored because HNu is less likely to displace HLv efficiently. Conversely, if HNu is a good nucleophile relative to HLv, the products are favored as HNu will readily replace HLv. An approximately equal mixture of reactants and products occurs when the nucleophilicity is similar for both HNu and HLv.
Different factors such as solvent, steric hindrance, and the stability of intermediates determine whether a reaction proceeds via SN2, E2, SN1, or E1 mechanism. The SN2 mechanism is characterized by a bimolecular, concerted process where the incoming nucleophile attacks the electrophilic carbon from the opposite side to the leaving group, resulting in inversion of stereochemistry. Factors such as a strong nucleophile, less steric hindrance, and a primary alkyl halide favor the SN2 pathway.