SN1 Reaction Rate and Unimolecular Step

SN1 reaction rate depends on the substrate, not the nucleophile. This pattern was identified by Sir Christopher Ingold and Edward D. Hughes while they studied nucleophilic substitution reactions of tertiary alkyl halides. A tertiary alkyl halide can still undergo substitution even when the nucleophile is weak.

Recall that the rate of an S_N2 reaction depends on the concentration of both the nucleophile and the substrate. While increased nucleophile basicity increases the rate of S_N2 reactions, the increased steric hindrance of the substrate decreases the reaction rate.

This explains why sterically hindered tertiary halides,...