Isotope Effects, Dynamics, and the Mechanism of Solvolysis of Aryldiazonium Cations in Water

Abstract

The mechanism of the heterolytic solvolysis of p-tolyldiazonium cation in water was studied by a combination of kinetic isotope effects, theoretical calculations, and dynamics trajectories. Significant (13)C kinetic isotope effects were observed at the ipso (k(12)C/k(13)C = 1.024), ortho (1.017), and meta (1.013) carbons, indicative of substantial weakening of the C(2)-C(3) and C(5)-C(6) bonds at the transition state. This is qualitatively consistent with a transition state forming an aryl cation, but on a quantitative basis, simple S(N)1 heterolysis does not account best for the isotope effects. Theoretical S(N)2Ar transition structures for concerted displacement of N(2) by a single water molecule lead to poor predictions of the experimental isotope effects. The best predictions of the (13)C isotope effects arose from transition structures for the heterolytic process solvated by clusters of water molecules. These structures, formally saddle points for concerted displacements on the potential energy surface, may be described as transition structures for solvent reorganization around the aryl cation. Quasiclassical dynamics trajectories starting from these transition structures afforded products very slowly, compared to a similar S(N)2 displacement, and the trajectories often afforded long-lived aryl cation intermediates. Critical prior evidence for aryl cation intermediates is reconsidered with the aid of DFT calculations. Overall, the nucleophilic displacement process for aryldiazonium ions in water is at the boundary between S(N)2Ar and S(N)1 mechanisms, and an accurate view of the reaction mechanism requires consideration of dynamic effects.

The Study of Reactive Intermediates in Condensed Phases

“…From the combined experimental results and simulations, they found that the energy flow within the alkyl side-chain could be reasonably well represented by a first-order process having a rate coefficient of ~4×10 11 s -1 . In their second study, 124 Singleton's group again combined theory and experiment to model a solution-phase reaction. The reactant in this case was an aryldiazonium cation, which was undergoing nucleophilic substitution of the nitrogen by water.…”

A Mechanistic Spectrum of Chemical Reactions

“…A mechanism at the S N 2Ar/S N 1 boundary was proposed for the nucleophilic substitution reaction of aryldiazonium ions in water [86].…”

Evidence for Nonstatistical Dynamics in the Wolff Rearrangement of a Carbene

“…2 Thus far, the intermediates known to exhibit such behavior are singlet biradicals, 3 radical cations, 4 radicals, 5 and carbocations. 6 In the present study, we sought to investigate whether carbene chemistry might also be subject to these dynamical effects.…”