Abstract
Nucleophilic substitution (SN2) and base-induced elimination (E2), two indispensable reactions in organic synthesis, are commonly assumed to proceed under stereospecific conditions. Understanding the way in which the reactants pre-orient in these reactions, that is its stereodynamics, is essential in order to achieve a detailed atomistic picture and control over such processes. Using crossed beam velocity map imaging, we study the effect of steric hindrance in reactions of Cl- and CN- with increasingly methylated alkyl iodides by monitoring the product ion energy and scattering angle. For both attacking anions the rebound mechanism, indicative of a direct SN2 pathway, is found to contribute to the reaction at high relative collision energies despite being increasingly hindered. An additional forward scattering mechanism, ascribed to a direct E2 reaction, also contributes at these energies. Inspection of the product energy distributions confirms the direct and fast character of both mechanisms as opposed to an indirect reaction mechanism which leads to statistical energy redistribution in the reaction complex. This work demonstrates that nonstatistical dynamics and energetics govern SN2 and E2 pathways even in sterically hindered exchange reaction systems.
Highlights
One of the central characteristics that make bimolecular nucleophilic substitutions (SN2) central in organic synthesis is its stereospeci c character.[1]
We report on a series of gas phase angle- and energy differential cross section measurements of the ve reactions listed in Table 1 and show that direct substitution and elimination dynamics persist even when complex neutral partners are involved
Reactions of both ClÀ and CNÀ with methylated alkyl iodides show no direct rebound at 0.4 eV, whereas the contribution is considerable at this energy in ClÀ + CH3I and CNÀ + CH3I
Summary
One of the central characteristics that make bimolecular nucleophilic substitutions (SN2) central in organic synthesis is its stereospeci c character.[1] In the simplest approximation, the SN2 reaction is assumed to proceed in a collinear fashion, following a three step mechanism: rst, the attacking anion approaches the neutral molecule through a long-range attractive ion–dipole interaction. The second step is the formation of an intermediate complex which follows the well known Walden inversion. The simplest gas phase nucleophilic substitution reactions between a halide ion and a methyl halide (XÀ + CH3Y) present a minimum energy path resembling a double minimum structure separated by a transition state barrier, which is usually submerged with respect to the reactants' energy.[2] According to classical statistical models, such as Rice–Ramsperger–Kassel–Marcus (RRKM)
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