The kinetic isotope effect () is approximately for most electrophilic aromatic substitutions, indicating bond cleavage is not the rate-determining step. However, which of the following reactions displ

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?

Prashant Kotian · Accepted Answer

Correct answer: B,C. 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 bond (compared to ) slows down the reverse step, leading to a measurable kinetic isotope effect in the overall observed rate.

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