Gas-phase measurements of the influence of stepwise solvation on the kinetics of nucleophilic displacement reactions with chloromethane and bromomethane at room temperature

Abstract

The absolute influence of stepwise solvation on nucleophilicity is explored with flowing afterglow measurements of rate constants in the gas phase at room temperature for nucleophilic displacement reactions of CH3C1 and CH3Br with the solvated homoconjugate anions A-*(AH),, with AH = H20, D20, CH30H, C2H50H, CH3COCH3, HCOOH, and CH3COOH. The specific rates are followed as a function of stepwise solvation for additions of up to three molecules of solvent (from n = 0-3). Comparisons are made with the kinetic behavior of the analogous reactions proceeding in solution. The observed trends in specific rate with solvation are discussed in terms of overall energetics and models for the intermediate potential energy profile.Nucleophilic displacement (SN2) reactions between anions and methyl halides are known to be extremely sensitive to the medium in which they proceed. For example, solution measurements have shown that nucleophilic displacement can be as much as a million times faster in dipolar aprotic than in protic solvents.' These same reactions have now also been executed in the gas phase where they have been found to proceed in the total absence of solvent with specific rates as much as 20 orders of magnitude higher than those in solution.2 The gas-phase experiments have provided, for the first time, a direct measure of the absolute influence of bulk solvent on the kinetics and efficiencies of these reactions.In addition to the solvent-free reactions, solvated nulceophilic displacement reactions of the type in (1) can also be executed in the gas phase, and their kinetics can be followed as a function A-4, + CH3B -B-6, + [ ( n -m)S + CH,A] (1) of stepwise solvation of the nucleophilic as indicated in Figure 1. For example, reactions of the type in (2) were executed in the gas phase in our early gas-phase studies of solvated nucleophilic Cl-.(ROH), + [ ( n -m)ROH + ROCH3] (2) displacement for R = CH,, C2H5, (CH3),CH, and (CH3)3C and for values of n equal to 0 and A significant decrease in reactivity was observed for all alkyl groups with the addition of just the one molecule of solvent to the nucleophile. More recently we have reported in brief a quantitative study for reactions of the type in (3) between hydrated hydroxide ions and methyl bromide OH-.(H,O), + CH3Br -Br-.(H,O), + [ ( n -m)H20 + C H 3 0 H ] (3)for additions of up to three molecules of water.4 Here we report these results in more detail as part of an extended study of the influence of solvation on nucleophilic displacement in the gas phase.With our previous measurements of intrinsic reactivities as a benchmark,, we have followed as a function of stepwise solvation the kinetics of reactions of homoconjugate anions A-.(AH),, with both CH$l and CH3Br. The measurements were carried out at room temperature with additions of up to three solvent molecules to the nucleophile. While such amounts of solvation scarcely correspond to the solvent environment found in solution, they do begin to bridge the gap between the totally unsolvated gas phase and solution. A...

Accelerated C–N Bond Formation in Dropcast Thin Films on Ambient Surfaces

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Reactions of Microsolvated Organic Compounds at Ambient Surfaces: Droplet Velocity, Charge State, and Solvent Effects

“…The selected reactions have characteristic pathways under bulk solution-phase conditions, and the general goal of this study was to examine the reactivity-including possible new pathways-of the microsolvated species in the open air, and to interpret the results in the light of solvent effects. Experimental [16][17][18][19] and theoretical [20] studies on gasphase ion/molecule reactions of microsolvated ions have included the investigation of solvent effects [21,22]. The experimentally observed trend is a reduction in the reaction efficiency, which starts with monosolvation.…”

Thermochemistry and structures of solvated SN2 complexes and transition states in the gas phase: experiment and theory

“…(3)) using a variety of experimental techniques [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. By performing these kinds of experiments it is hoped to gain additional insight into the intrinsic contribution of the solvent molecules to the progress [25] of these reactions.…”

“…For most mono-solvated reactions only a small fraction of the leaving groups are solvated (Eq. (4)) [11][12][13][14][15][16][19][20][21][22]24,25], even though this reaction pathway is more exothermic than formation of the non-solvated leaving group (Eq. (5)).…”

“…Tachikawa showed that the reaction between F − (H 2 O) and CH 3 Cl can proceed via three pathways (Eqs. (12)- (14)), and that the branching ratios are dependent on the centre-of-mass collision energy [30,31].…”