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.
Highlights
The competition between bimolecular nucleophilic substitution and base-induced elimination is of fundamental importance for the synthesis of pure samples in organic chemistry
For each of these combinations reactive scattering experiments have been performed on reactions with four increasingly methylated neutral species (CH3Y, C2H5Y, iC3H7Y and tC4H9Y) that represent the transition from primary to tertiary alkyl halides
Strong similarities between the reactions of different anions with tert-butyl halides clearly indicate the presence of a mechanism inherent to E2 reactions that leads to forward scattered products
Summary
The competition between bimolecular nucleophilic substitution and base-induced elimination is of fundamental importance for the synthesis of pure samples in organic chemistry. The addition of bulky substituents is supposed to hinder the well-known Walden inversion and promote the E2 reaction Such an elimination process is assumed to occur via an anionic attack on a β-carbon-bonded hydrogen atom in periplanar orientation with respect to the α-carbon-Y bond (see Fig. 1). Many studies have dealt with the influence of the degree of methyl-substitution on the height of the potential energy barriers for E2 and SN2 reactions as an indirect estimation of the preference for a specific reaction[27, 28] These calculations cannot account for dynamical effects such as the influence of the impact parameter, the role of reactant reorientation during the collision, the specific atomistic reaction mechanisms, or the energy partitioning[16]. Many investigations have provided indirect mass spectrometric estimations of the branching ratio between SN2 and E2 for a variety of systems: deuterium kinetic isotope effects have been employed as a qualitative measure of the tendency of a specific reaction towards substitution or elimination[24, 31]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
