What is the kinetic isotope effect and what does it reveal about the rate-determining step?

• A. Replacing H with D speeds up the reaction if C–H bond cleavage is in RDS; indicates bond formation is in RDS
• B. Replacing H with D has no effect
• C. Replacing H with D alters the solvent polarity
• D. Replacing H with D slows the reaction if C–H bond cleavage is in RDS; indicates bond breaking is in RDS

Answer: (D)

The kinetic isotope effect shows whether a bond to the isotope is involved in the rate-determining step. Replacing hydrogen with deuterium changes the bond’s zero-point energy and vibrational characteristics, so if the bond to hydrogen must be broken in the rate-determining step, using the heavier deuterium raises the activation energy and slows the reaction. This primary KIE (a noticeable rate decrease with deuteration) tells you that C–H bond breaking is part of the slowest step. If the H–D substitution doesn’t affect the rate, then that bond isn’t involved in the RDS. Thus, the scenario where deuteration slows the reaction due to C–H bond cleavage in the RDS correctly reveals that bond breaking is in the rate-determining step.