Effect of higher order solvation and temperature on SN2 and E2 reactivity

• Increasing solvation of a reactant decreases the overall reactivity of the anion.
• Overall sensitivity to temperature increases with increasing solvation.
• Negative temperature dependences are found for all reactions.
• Substitution mechanisms dominate methyl bromide reactions.
• Elimination mechanisms occur for larger alkyl bromide reactions.
• Unsolvated ions dominate the products.

The reactivity of microsolvated fluoride ions, F−(CH3OH)0–2, with methyl, ethyl, n-propyl, and t-butyl bromide is evaluated over a broad range of temperatures. Significant decreases in reactivity are observed as either solvation or temperature increases. Increasing solvation increases sensitivity to the reaction barrier as revealed by a larger temperature dependence. These reactions are dominated by an SN2 mechanism for the methyl bromide reaction, while the SN2 and E2 mechanisms compete for the reactions with ethyl and n-propyl bromide reactions. The elimination mechanism, with some association, dominates the t-butyl bromide reactions. In all cases the unsolvated bromide ion is the primary ionic product. Branching ratios are discussed in both qualitative and quantitative terms for all reactions at 300 K.

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