The document discusses four key factors that affect nucleophilic substitution reactions: 1) the nature of the substrate, 2) the nature of the nucleophile, 3) the nature of the solvent, and 4) the nature of the leaving group. It provides details on how each factor influences whether a reaction proceeds by an SN1 or SN2 mechanism. For the nature of the substrate, it explains that tertiary substrates favor SN1 and primary substrates favor SN2 due to steric effects. For the nucleophile, it states that small, less electronegative, stronger bases react faster in SN2. The document also discusses how solvent polarity and ionization potential influence SN1 vs SN2 mechanisms. Finally, it