The kinetic isotope effect () is approximately for most electrophilic aromatic substitutions, indica — Aromatic Hydrocarbons Chemistry Question
Question
The kinetic isotope effect ($k_H / k_D$) is approximately $1$ for most electrophilic aromatic substitutions, indicating $C-H$ bond cleavage is not the rate-determining step. However, which of the following reactions display a significant kinetic isotope effect ($k_H / k_D > 1$), indicating a shift in the rate-determining step?
Answer: B,C
💡 Solution & Explanation
Sulfonation and iodination are unique because they are readily reversible under standard reaction conditions. The energy barrier for the deprotonation step becomes significant and comparable to the attack step. Breaking a stronger $C-D$ bond (compared to $C-H$) slows down the reverse step, leading to a measurable kinetic isotope effect in the overall observed rate.
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