The Rates of S_N2 Reactions and Their Relation to Molecular and Solvent Properties

Abstract

The energy barriers of symmetrical methyl exchanges in the gas phase have been calculated with the reaction path of the intersecting/interacting‐state model (ISM). Reactive bond lengths increase down a column of the Periodic Table and compensate for the decrease in the force constants, which explains the near constancy of the intrinsic barriers in the following series of nucleophiles: F−≈Cl−≈Br−≈I−. This compensation is absent along the rows of the Periodic Table and the trend in the reactivity is dominated by the increase in the electrophilicity index of the nucleophile in the series C<N<O<F. Solvent effects have been quantitatively incorporated into the ISM model through a correlation between electrophilicity and the solvent acceptor number. This correlation is transferable between nucleophiles and solvents and allows the methyl transfer rate constants in solution to be calculated with remarkable simplicity and accuracy. The relationship between the SN2 and electron‐transfer mechanisms is clarified and it is shown that smaller solvent static effects should be expected for electron transfer in the absence of a thermodynamic driving force.

Understanding Chemical Reactivity: The Case for Atom, Proton and Methyl Transfers

“…[38] The line is the ideal correlation. Data and references to the literature are available as Supporting Information.…”

“…[34] lozenges: methyl transfers from Figure 6 of ref. [38]; *: enzyme catalyzed H-atom transfers from ref. [40].…”

A chemical understanding for the enhanced hydrogen tunnelling in hydroperoxidation of linoleic acid catalysed by soybean lipoxygenase‐1

“…This is not expected to affect meaningfully our calculations because minor changes in bond lengths tend to be compensated by opposing changes in vibrational frequencies (or force constants). [19] In these calculations we also replaced the ionization potential of the tert-butyl peroxyl radical by that of the hydroperoxy radical, but this is immaterial because the calculation a Gas phase data; boldface letters indicate where the radical is centred after the bond to the hydrogen atom is broken; data from ref [35] and http://webbook.nist.gov or http://srdata.nist.gov/cccbdb/, except where noted. b Ref.…”

“…We have recently shown that the Intersecting/Interacting-State Model (ISM) associated with semiclassical TST provides a reliable way to calculate absolute rates of elementary chemical reaction, notably hydrogen-atom transfers, proton transfers and methyl transfers, [9,[17][18][19][20] and offers a chemical understanding of reactivity in polyatomic systems. ISM deals in a compact manner with the chemical bonds that are broken and formed in a chemical reaction, characterizing them by their Morse potentials and equilibrium bond lengths, and associates their interactions along a unidimensional reaction coordinate with the reaction energy and an electronic parameter, the electrophilicity index proposed by Parr to measure the saturation of the electron inflow between interacting atoms.…”

Organocatalysis: Fundamentals and Comparisons to Metal and Enzyme Catalysis

“…Shaik and Pross, and others, have described the SN2 reaction in terms of valence bond theory [235][236][237][238]. This led to the transition state model (Figure 12) for the SN2 reactions of type (c) in Scheme 35 [239][240][241][242]. The overall rate constant k obs for Y: − + RX X: − + RY is given by Equation (3) when the system is treated according to the steady state approximation.…”