Imaging dynamic fingerprints of competing E2 and SN2 reactions

Abstract

The competition between bimolecular nucleophilic substitution and base-induced elimination is of fundamental importance for the synthesis of pure samples in organic chemistry. Many factors that influence this competition have been identified over the years, but the underlying atomistic dynamics have remained difficult to observe. We present product velocity distributions for a series of reactive collisions of the type X− + RY with X and Y denoting the halogen atoms fluorine, chlorine and iodine. By increasing the size of the residue R from methyl to tert-butyl in several steps, we find that the dynamics drastically change from backward to dominant forward scattering of the leaving ion relative to the reactant RY velocity. This characteristic fingerprint is also confirmed by direct dynamics simulations for ethyl as residue and attributed to the dynamics of elimination reactions. This work opens the door to a detailed atomistic understanding of transformation reactions in even larger systems.

Nucleophilic Substitution (SN2): Dependence on Nucleophile, Leaving Group, Central Atom, Substituents, and Solvent

“…This was later rationalized by Hennig and Schmatz in terms of a quantum‐mechanical study . Very recently, Wester and co‐workers quantified the atomic dynamics associated with S N 2 vs E2 reactions and analyzed product velocity distributions for reactive collisions, X − +RY (where X=Y=F, Cl, I and R=methyl, iso ‐propyl, and tert ‐butyl) . They established the dynamics of the reactivity preference shift, from substitution to elimination, as the degree of methylation increases.…”

Imaging the dynamics of ion–molecule reactions

“…The scattering into the forward hemisphere is observed in all reactions with tert-butyl halides, irrespective of the characteristics of nucleophile or leaving group. 205 Given the fact that E2 is assumed to be increasingly favored with methyl substitution on the central carbon atom, we ascribe the observed forward scattered events as direct fingerprints of base induced elimination dynamics. Fig.…”

“…126,203,204 Recently, our group has exploited the capabilities of crossed beam velocity map imaging to disentangle the reaction mechanisms of E2 and S N 2 reactions by monitoring the scattering angle and velocity distribution of product Y À and correlating them with the respective channels. 205 Several nucleophileleaving group combinations at stepwise methylated alkyl halides have been investigated,…”

“…16 Snapshots along the most representative reaction trajectories for direct anti-E2 (A), direct syn-E2 (B) and direct backward S N 2 (C) reactions. 205 The animations of the trajectories can be found at (http://hase-group.ttu. edu/animations.html).…”

QM- and ONIOM-Molecular Dynamics Studies of the S_N2 Reaction. How Does the Rare Event Take Place?

“…For example, we want to know how does the reaction start, how does the reaction go over the energy barrier and so on. Although the various examinations concerning the dynamics of the chemical reaction have recently provided the new findings [3][4][5][6][7][8], the problem how does the rare event of the chemical reaction take place remains unsolved. It is still a challenging subject to understand the process of the chemical reaction from the dynamical point of view.…”