Final answer:
In SN1 reactions, the reactivity of a molecule is determined by the stability of the carbocation intermediate formed in the rate-determining step.
Step-by-step explanation:
The reactivity of a molecule in SN1 reactions is determined by B) the stability of the carbocation intermediate. In SN1 mechanisms, the rate-determining step involves the formation of a carbocation intermediate after the leaving group has left the molecule. The stability of this intermediate is crucial as it will affect the overall rate of the reaction.
In contrast to SN2 reactions, where both the substrate concentration and the nucleophile concentration are important, SN1 reactions are unimolecular—meaning the rate depends on the concentration of only one reactant, the substrate. As a result, the stability of the carbocation intermediate plays a more significant role in determining the reactivity and rate of SN1 reactions.
Furthermore, the structure of the substrate affects the stability of the carbocation; for example, a tertiary carbocation is more stable than a secondary or primary carbocation due to hyperconjugation and inductive effects. Solvents that are more polar and can stabilize the carbocation will also favor SN1 reactions. Thus, factors like the substrate's structure and the solvent used are essential for the stability of the carbocation and therefore the reactivity in SN1 reactions.