A theoretical study on the mechanism of the base-promoted decomposition of N-chloro,N-methylethanolamine

Abstract

The first step of the base-promoted decomposition of N-chloro,N-methylethanolamine in aqueous solution (CH₃N(Cl)CH₂CH₂OH + HO⁻→ imine + Cl⁻ + H₂O (+ CH₂O) → amine + aldehyde) is investigated at the MP2/6-31++G(d,p) computing level. Solvation is included by using both a microsolvated model, in which two explicit water molecules simulate the specific solvent effects, and a hybrid cluster-continuum model, by applying a polarized continuum on the previous results, to account for the bulk effect of the solvent. Four alternative pathways (bimolecular fragmentation, Hofmann, Zaitsev and intramolecular eliminations) are possible for the rate-limiting step of this base-promoted decomposition. These reactive processes are bimolecular asynchronous concerted reactions. The common feature of the four pathways is the proton transfer to HO⁻ being more advanced than all other molecular events, whereas imine formation is delayed. Non-reactive cyclic arrangements involving one of the explicit water molecules are found at transition structures of Hofmann and Zaitsev eliminations, such water molecule acting both as H⁺ donor and acceptor. Although MP2 calculations misjudge the absolute activation Gibbs free energy values, this computational level adequately predicts the enhancement in the decomposition rate due to the presence of the -OH group.