Nucleophilic identity substitution reactions. The reaction between water and protonated alcohols

Abstract

The potential energy surfaces for the reaction between H₂O and the protonated alcohols MeOH₂⁺, EtOH₂⁺, PrⁱOH₂⁺, and Bu^tOH₂⁺ have been explored by means of high level ab initio theoretical methods. Both nucleophilic substitution (S_N2) and elimination (E2) pathways have been investigated. Front side (S_NF) and the familiar back side (S_NB) Walden inversion attack of the nucleophile have been found to be competing for the H₂O–Bu^tOH₂⁺ system. In contradiction with the customary relationship between so-called “steric effects” and barrier heights—more alkyl-substituted S_N2 reaction centres have higher S_N2 reaction barriers—the S_N2 reaction barriers are found to be Et > Me > Prⁱ > Bu^t. This result is in excellent agreement with available experimental data.