Exploring borderline SN1–SN2 mechanisms: the role of explicit solvation protocols in the DFT investigation of isopropyl chloride

Abstract

Nucleophilic substitution at saturated carbon is a crucial class of organic reactions, playing a pivotal role in various chemical transformations that yield valuable compounds for society. Despite the well-established S_N1 and S_N2 mechanisms, secondary substrates, particularly in solvolysis reactions, often exhibit a borderline pathway. A molecular-level understanding of these processes is fundamental for developing more efficient chemical transformations. Typically, quantum-chemical simulations of the solvent medium combine explicit and implicit solvation methods. The configuration of explicit molecules can be defined through top-down approaches, such as Monte Carlo (MC) calculations for generating initial configurations, and bottom-up methods that involve user-dependent protocols to add solvent molecules around the substrate. Herein, we investigated the borderline mechanism of the hydrolysis of a secondary substrate, isopropyl chloride (ⁱPrCl), at DFT-M06-2X/aug-cc-pVDZ level, employing explicit and explicit + implicit protocols. Top-down and bottom-up approaches were employed to generate substrate–solvent complexes of varying number (n = 1, 3, 5, 7, 9, and 12) and configurations of H₂O molecules. Our findings consistently reveal that regardless of the solvation approach, the hydrolysis of ⁱPrCl follows a loose-S_N2-like mechanism with nucleophilic solvent assistance. Increasing the water cluster around the substrate in most cases led to reaction barriers of ΔH^‡ ≈ 21 kcal mol⁻¹, with nine water molecules from MC configurations sufficient to describe the reaction. The More O'Ferrall–Jencks plot demonstrates an S_N1-like character for all transition state structures, showing a clear merged profile. The fragmentation activation strain analyses indicate that energy barriers are predominantly controlled by solvent–substrate interactions, supported by the leaving group stabilization assessed through CHELPG atomic charges.