S_N2 reactions in the gas phase. Nucleophilicity effects

Abstract

Abstract-The heats and entropies of gas phase association of chloride, bromide and iodide with the respective methyl halides are reported. Comparison of these results with results published for S,2 reactions in solution suggests that the solvent is the dominant factor in determining relative halide nucleophilicities for the reactions in solution.RATES of SN2 reactions in solution are subject to considerable variation depending upon the nature of the attacking nucleophile. These rate variations have been interpreted in terms of the HSAB principle,l and charge and frontier control of reactivity.2 Both the HSAB and the charge and frontier models include solvation effects in predictions of reactivity; until recently, however, it was not possible to determine experimentally the magnitude of the effect of solvation in reactivity. The data which follows suggests that nucleophilicity differences between the halides chloride, bromide and iodide in Finkelstein substitution reactions, are largely due to changes in the relative solvation of the reactants and transition states for the exchange processes. Table 1 lists the enthalpies and entropies obtained for a series of gas phase equilibria at constant pressure using the methods of the previous paper.3 The mixed equilibria (cJ (4) in Table 1) were studied by introducing two gases into the CI source at approximately equal pressures. The heat of association of [XI-with the methyl halides in Table 1 should reflect the relative stabilities of the transition states for the respective SK2 reactions. At present we have no experimental method for estimating the height of the barrier in the quartic potential for X. CH,. .Y-in the gas phase; hence, we have directly used the gas phase heats of association in Table 1 to estimate the relative desolvation energies of activation for the six reactions. The activation energy for the reaction is given by the sum of the energy of formation of the transition state in the gas phase, AE,*, which we have assumed to be equal to the heat of association of [CH3XY]-and the amount of energy required to desolvate the nucleophile so that it can form the transition state, AE&:The values for AEL, in Table 1 are simply the difference between the AE,*(AH,) and AE* values for each reaction. Two points are clear from this data: (1) the values of AEA, are large when compared to either AE* or AHo and (ii) the order of nucleophilic reactivity, or the stability of the respective transition states in solution is very largely controlled by the solvent.It has been known for many years that the reactivity of hard nucleophiles like F-or C1-is subject to a substantial solvent effect. The evidence above suggests that solvation may exert even more control on HSAB interactions than had been *

Modified Gaussian‐2 level investigation of the identity ion‐pair SN2 reactions of lithium halide and methyl halide with inversion and retention mechanisms

“…The calculated central barrier heights for the reactions X − + CH 3 X with backside or frontside attack at the present level of the G2M(+) theory are also close to the available experimental data34–37 or other high levels of theory13, 26 (see Table 3).…”

Definitive Ab Initio Studies of Model SN2 Reactions CH3X+F− (X=F, Cl, CN, OH, SH, NH2, PH2)

“…The introduction of powerful digital computers, coupled with sophisticated techniques of electronic structure theory, also allowed computational chemists to research this class of reactions, confirming the double‐well feature. As it stands currently, gas‐phase S N 2 reactions have been widely investigated by kinetic experiments,3, 4, 7–14 ab initio quantum and semiclassical dynamical methods and trajectory simulations,15–22 statistical mechanical studies,5, 23–29 ab initio and density functional structural analyses,30–42 and electron transfer studies 43–48. There has been a clear effort to understand the distinctions between reactions in the gas phase and in solution, to more clearly expose intrinsic versus solvent effects.…”

The Rates of SN2 Reactions and Their Relation to Molecular and Solvent Properties

“…For the reaction illustrated above, the complexation energies for Cl − ⋅⋅⋅CH 3 Br and Br − ⋅⋅⋅CH 3 Cl are −12.5 and −10.9 kcal mol −1 , respectively,55 and the energetic separation between the complexes is only 1.6 kcal mol −1 less exothermic than Δ H r ⊖. When the difference between the complexation energies is small, as in the case of alkyl halide⋅⋅⋅halide ions,55, 56 it is legitimate to expect that the global free energy of the reaction is reflected by the central barrier and that the reactivity follows a free‐energy relationship. This is approximately the case for the cross‐reactions between halide ions, such as Cl − /CH 3 Br, F − /CH 3 Cl and F − /CH 3 Br, for which the reaction exothermicity, calculated as the energy difference between separated products and reactants, increases from −6.0 to −35.6 to −41.6 kcal mol −1 .…”