The potential energy versus reaction coordinate profiles for the S_ N 2 reaction of methyl bromide with four different nucleophiles ( SH⁻ , OH⁻ , CH₃COO⁻ , and H₂O ) in a polar protic solvent are plotted. The profiles show four distinct activation energy barriers labeled E_ a1 , E_ a2 , E_ a3 , and E_ a4 , such that E_ a1 Which of the following nucleophiles corresponds to the reaction pathway with the activation ener

Step-by-step solution

The activation energy of an S_ N 2 reaction is inversely related to the reactivity of the nucleophile. The reaction with the lowest activation energy ( E_ a1 ) corresponds to the fastest reaction, which requires the strongest nucleophile. Comparing the given species: SH⁻ versus OH⁻ : In a polar protic solvent, sulfur is larger, more polarizable, and less heavily solvated than oxygen. This makes SH⁻ a significantly stronger nucleophile than OH⁻ . OH⁻ versus CH₃COO⁻ : The negative charge on the hydroxide ion is local