The Effect of Solvation Sphere Composition on the Nucleophilic Substitution Mechanism: The Hunt for the Elusive Carbocation Intermediate

Abstract

Despite its ubiquity, the SN1 carbocation intermediate and its reaction profile have remained computationally elusive, owing to the sensitivity to the number of solvating molecules and the flat potential energy surface. The ionization step for solvolysis of tert-butyl chloride in water is a classic textbook example of a SN1 reaction. Although this system has been extensively studied computationally, only the concerted Walden (SN2) pathway has been identified. Here, we provide a rigorous and detailed description of the carbocation intermediate and the SN1 pathway using both implicit solvation and explicit water molecules. This approach stabilizes the carbocation intermediate and reveals the long-sought SN1 profile. The calculated free energy barriers show remarkable agreement with experimental data, thereby validating the theoretical framework employed. We found that the carbocation stabilization is governed by two key factors: the bulk solvent effect and σC–H → pz hyperconjugation between C–H bonds and the vacant pz orbital of the carbocation. Strikingly, when the solvation shell contains fewer than 12 water molecules the computational mechanism shifts to an SN2-type pathway, underscoring the critical role of explicit solvation in dictating the reaction mechanism. Finally, we explored the origin of the inversion and racemization reported in chiral substrates.